diff options
author | jkh <jkh@FreeBSD.org> | 1996-02-16 13:28:22 +0000 |
---|---|---|
committer | jkh <jkh@FreeBSD.org> | 1996-02-16 13:28:22 +0000 |
commit | b15e80264b328f9db043194fb40d0a7a9a0b8f0d (patch) | |
tree | 0c81e97c4721a03bb28c9b60aaaa5409c1a736c3 /share | |
parent | 86ea6f43bc345760df043fefe5563f5d75fb0f99 (diff) | |
download | FreeBSD-src-b15e80264b328f9db043194fb40d0a7a9a0b8f0d.zip FreeBSD-src-b15e80264b328f9db043194fb40d0a7a9a0b8f0d.tar.gz |
Three part commit:
1. Import UART technical description from Frank Durda IV
<uhclem@nemesis.lonestar.org>.
2. Push existing sio chapter out of hw.sgml and make a separate
document out of it.
3. Make hw.sgml include the first two under serial devices section.
Submitted by: Frank Durda IV <uhclem@nemesis.lonestar.org> & I
Diffstat (limited to 'share')
-rw-r--r-- | share/doc/handbook/Makefile | 6 | ||||
-rw-r--r-- | share/doc/handbook/hw.sgml | 219 | ||||
-rw-r--r-- | share/doc/handbook/sections.sgml | 5 | ||||
-rw-r--r-- | share/doc/handbook/sio.sgml | 207 | ||||
-rw-r--r-- | share/doc/handbook/uart.sgml | 1107 |
5 files changed, 1344 insertions, 200 deletions
diff --git a/share/doc/handbook/Makefile b/share/doc/handbook/Makefile index 749d7ac..044d6d3 100644 --- a/share/doc/handbook/Makefile +++ b/share/doc/handbook/Makefile @@ -1,4 +1,4 @@ -# $Id: Makefile,v 1.7 1995/12/07 13:22:12 jkh Exp $ +# $Id: Makefile,v 1.8 1996/02/11 00:16:06 jkh Exp $ SRCS= authors.sgml basics.sgml bibliography.sgml boothelp.sgml SRCS+= booting.sgml contrib.sgml crypt.sgml ctm.sgml current.sgml dialup.sgml @@ -8,8 +8,8 @@ SRCS+= handbook.sgml history.sgml hw.sgml install.sgml kerberos.sgml SRCS+= kernelconfig.sgml kerneldebug.sgml memoryuse.sgml SRCS+= mirrors.sgml nfs.sgml nutshell.sgml SRCS+= porting.sgml ports.sgml ppp.sgml printing.sgml relnotes.sgml -SRCS+= routing.sgml scsi.sgml sections.sgml +SRCS+= routing.sgml scsi.sgml sections.sgml sio.sgml SRCS+= skey.sgml slipc.sgml slips.sgml stable.sgml submitters.sgml sup.sgml -SRCS+= synching.sgml troubleshooting.sgml userppp.sgml +SRCS+= synching.sgml troubleshooting.sgml userppp.sgml uart.sgml .include <bsd.sgml.mk> diff --git a/share/doc/handbook/hw.sgml b/share/doc/handbook/hw.sgml index e568d01..f7674f6 100644 --- a/share/doc/handbook/hw.sgml +++ b/share/doc/handbook/hw.sgml @@ -1,4 +1,4 @@ -<!-- $Id: hw.sgml,v 1.14 1996/02/06 09:02:07 jkh Exp $ --> +<!-- $Id: hw.sgml,v 1.15 1996/02/06 10:48:50 jkh Exp $ --> <!-- The FreeBSD Documentation Project --> <!-- @@ -87,6 +87,27 @@ Slippery when wet. Beware of dog. you're putting together an NFS or NEWS server that will be doing a lot of multiuser disk I/O. + <sect2><heading>CDROM drives</heading> + <p>My SCSI preferences extend to SCSI CDROM drives as well, and the + <htmlurl url="http://www.toshiba.com" name="Toshiba"> XM-3501B (now + released in a caddy-less model called the XM-5401B) drive has always + performed well for me. Generally speaking, most SCSI CDROM drives I've + seen have been of pretty solid construction (probably because they don't + occupy the lower end of the market, due to their higher price) and you + probably won't go wrong with an HP or NEC SCSI CDROM drive either. + + <sect2><heading>Tape drives</heading> + <p>I've had pretty good luck with both + <htmlurl url="http://www.Exabyte.COM:80/Products/8mm/8505XL/Rfeatures.html" + name="8mm drives"> from <htmlurl url="http://www.exabyte.com" + name="Exabyte"> and + <htmlurl url="http://www-dmo.external.hp.com:80/tape/_cpb0001.htm" + name="4mm (DAT)"> drives from <htmlurl url="http://www.hp.com" name="HP">. + + <p>For backup purposes, I'd have to give the higher recommendation to the + Exabyte due to the more robust nature (and higher storage capacity) of + 8mm tape. + <sect2><heading>Video Cards</heading> <p>If you can also afford to buy a commercial X server for $99 from <htmlurl url="http://www.xinside.com/" name="X Inside"> then I @@ -219,200 +240,8 @@ Slippery when wet. Beware of dog. <sect1><heading>* Sound cards</heading> <sect1><heading>Serial ports and multiport cards</heading> - <p>The <tt>sio</tt> driver provides support for NS8250-, - NS16450-, NS16550 and NS16550A-based EIA RS-232C (CCITT - V.24) communications interfaces. Several multiport - cards are supported as well. See the <tt>sio(4)</tt> - manual page for detailed technical documentation. - -<sect2><heading>Digiboard PC/8</heading> - - <p><em>Contributed by &a.awebster;.<newline>26 August - 1995.</em> - - Here is a config snippet from a machine with - digiboard PC/8 with 16550. It has 8 modems connected - to these 8 lines, and they work just great. Do not - forget to add <tt>options "COM_MULTIPORT"</tt> or it - will not work very well! - -<tscreen><verb> -device sio4 at isa? port 0x100 tty flags 0xb05 -device sio5 at isa? port 0x108 tty flags 0xb05 -device sio6 at isa? port 0x110 tty flags 0xb05 -device sio7 at isa? port 0x118 tty flags 0xb05 -device sio8 at isa? port 0x120 tty flags 0xb05 -device sio9 at isa? port 0x128 tty flags 0xb05 -device sio10 at isa? port 0x130 tty flags 0xb05 -device sio11 at isa? port 0x138 tty flags 0xb05 irq 9 vector siointr -</verb></tscreen> - - The trick in setting this up is that the MSB of the - flags represent the last SIO port, in this case 11 so - flags are 0xb05. - -<sect2><heading>Boca 16</heading> - - <p><em>Contributed by &a.whiteside;.<newline>26 August - 1995.</em> - - The procedures to make a Boca 16 pord board with - FreeBSD are pretty straightforward, but you will need - a couple things to make it work: - - <enum> - <item>You either need the kernel sources installed - so you can recompile the necessary options or - you will need someone else to compile it for you. - The 2.0.5 default kernel does <bf>not</bf> come with - multiport support enabled and you will need to add - a device entry for each port anyways. - </item> - <item>Two, you will need to know the interrupt and IO - setting for your Boca Board so you can set these - options properly in the kernel.</item> - </enum> - - One important note - the actual UART chips for the - Boca 16 are in the connector box, not on the internal - board itself. So if you have it unplugged, probes of - those ports will fail. I have never tested booting with - the box unplugged and plugging it back in, and I - suggest you do not either. - - If you do not already have a custom kernel - configuration file set up, refer to <ref - id="kernelconfig" name="Kernel Configuration"> for - general procedures. The following are the specifics - for the Boca 16 board and assume you are using the - kernel name MYKERNEL and editing with vi. - - <enum> - <item>Add the line -<tscreen><verb> -options "COM_MULTIPORT" -</verb></tscreen> -to the config file. -</item> - - <item>Where the current <tt>device sio - <em>xxx</em></tt> lines are, you will need to add - 16 more devices. <em>Only the last device - includes the interrupt vector for the - board</em>. (See the <tt>sio(4)</tt> manual page - for detail as to why.) - - The following example is for a Boca Board with an - interrupt of 3, and a base IO address 100h. The - IO address for Each port is +8 hexadecimal from - the previous port, thus the 100h, 108h, 110h... - addresses. - -<tscreen><verb> -device sio1 at isa? port 0x100 tty flags 0x1005 -device sio2 at isa? port 0x108 tty flags 0x1005 -device sio3 at isa? port 0x110 tty flags 0x1005 -device sio4 at isa? port 0x118 tty flags 0x1005 -[...] -device sio15 at isa? port 0x170 tty flags 0x1005 -device sio16 at isa? port 0x178 tty flags 0x1005 irq 3 vector siointr -</verb></tscreen> - - The flags entry <em>must</em> be changed from - this example unless you are using the exact same - sio assignments. Flags are set according to - 0x<em>MYY</em> where <em>M</em> indicates the - minor number of the master port (the last port on - a Boca 16) and <em>YY</em> indicates if FIFO is - enabled or disabled(enabled), IRQ sharing is - used(yes) and if there is an AST/4 compatible IRQ - control register(no). - - In this example, -<tscreen><verb> -flags 0x1005 -</verb></tscreen> - - indicates that the master port is sio16. If I - added another board and assigned sio17 through - sio28, the flags for all 16 ports on - <em>that</em> board would be 0x1C05, where 1C - indicates the minor number of the master port. - Do not change the 05 setting.</item> - - <item>Save and complete the kernel configuration, - recompile, install and reboot. - - Presuming you have successfully installed the - recompiled kernel and have it set to the correct - address and IRQ, your boot message should - indicate the successful probe of the Boca ports - as follows: (obviously the sio numbers, IO and - IRQ could be different) - -<tscreen><verb> -sio1 at 0x100-0x107 flags 0x1005 on isa -sio1: type 16550A (multiport) -sio2 at 0x108-0x10f flags 0x1005 on isa -sio2: type 16550A (multiport) -sio3 at 0x110-0x117 flags 0x1005 on isa -sio3: type 16550A (multiport) -sio4 at 0x118-0x11f flags 0x1005 on isa -sio4: type 16550A (multiport) -sio5 at 0x120-0x127 flags 0x1005 on isa -sio5: type 16550A (multiport) -sio6 at 0x128-0x12f flags 0x1005 on isa -sio6: type 16550A (multiport) -sio7 at 0x130-0x137 flags 0x1005 on isa -sio7: type 16550A (multiport) -sio8 at 0x138-0x13f flags 0x1005 on isa -sio8: type 16550A (multiport) -sio9 at 0x140-0x147 flags 0x1005 on isa -sio9: type 16550A (multiport) -sio10 at 0x148-0x14f flags 0x1005 on isa -sio10: type 16550A (multiport) -sio11 at 0x150-0x157 flags 0x1005 on isa -sio11: type 16550A (multiport) -sio12 at 0x158-0x15f flags 0x1005 on isa -sio12: type 16550A (multiport) -sio13 at 0x160-0x167 flags 0x1005 on isa -sio13: type 16550A (multiport) -sio14 at 0x168-0x16f flags 0x1005 on isa -sio14: type 16550A (multiport) -sio15 at 0x170-0x177 flags 0x1005 on isa -sio15: type 16550A (multiport) -sio16 at 0x178-0x17f irq 3 flags 0x1005 on isa -sio16: type 16550A (multiport master) -</verb></tscreen> - - If the messages go by too fast to see, <tt>dmesg - > more</tt> will show you the boot - messages.</item> - - <item>Next, appropriate entries in <tt>/dev</tt> for the devices - must be made using the <tt>/dev/MAKEDEV</tt> - script. After becoming root: -<tscreen> -# cd /dev<newline> -# ./MAKEDEV tty1<newline> -# ./MAKEDEV cua1<newline> -<em>(everything in between)</em><newline> -# ./MAKEDEV ttyg<newline> -# ./MAKEDEV cuag -</tscreen> - - If you do not want or need callout devices for some - reason, you can dispense with making the <tt>cua*</tt> - devices.</item> - - <item>If you want a quick and sloppy way to make - sure the devices are working, you can simply plug - a modem into each port and (as root) <tt>echo at - > ttyd*</tt> for each device you have - made. You <em>should</em> see the RX lights flash - for each working port.</item> - </enum> - + &uart; + &sio; <sect1><heading>* Parallel ports</heading> <sect1><heading>* Modems</heading> diff --git a/share/doc/handbook/sections.sgml b/share/doc/handbook/sections.sgml index 59a76e7..945d7ea 100644 --- a/share/doc/handbook/sections.sgml +++ b/share/doc/handbook/sections.sgml @@ -1,4 +1,4 @@ -<!-- $Id: sections.sgml,v 1.7 1995/12/07 13:22:17 jkh Exp $ --> +<!-- $Id: sections.sgml,v 1.8 1996/02/11 00:16:15 jkh Exp $ --> <!-- The FreeBSD Documentation Project --> <!-- Entities containing all the pieces of the handbook are --> @@ -37,6 +37,7 @@ <!ENTITY relnotes SYSTEM "relnotes.sgml"> <!ENTITY routing SYSTEM "routing.sgml"> <!ENTITY scsi SYSTEM "scsi.sgml"> +<!ENTITY sio SYSTEM "sio.sgml"> <!ENTITY skey SYSTEM "skey.sgml"> <!ENTITY slipc SYSTEM "slipc.sgml"> <!ENTITY slips SYSTEM "slips.sgml"> @@ -44,5 +45,5 @@ <!ENTITY sup SYSTEM "sup.sgml"> <!ENTITY synching SYSTEM "synching.sgml"> <!ENTITY troubleshooting SYSTEM "troubleshooting.sgml"> +<!ENTITY uart SYSTEM "uart.sgml"> <!ENTITY userppp SYSTEM "userppp.sgml"> - diff --git a/share/doc/handbook/sio.sgml b/share/doc/handbook/sio.sgml new file mode 100644 index 0000000..221fb6e --- /dev/null +++ b/share/doc/handbook/sio.sgml @@ -0,0 +1,207 @@ +<!-- $Id$ --> +<!-- The FreeBSD Documentation Project --> + +<!-- +<!DOCTYPE linuxdoc PUBLIC "-//FreeBSD//DTD linuxdoc//EN" [ + +<!ENTITY % authors SYSTEM "authors.sgml"> +%authors; + +]> +--> +<sect2><heading>Configuring the <tt>sio</tt> driver<label id="sio"></heading> + + <p>The <tt>sio</tt> driver provides support for NS8250-, + NS16450-, NS16550 and NS16550A-based EIA RS-232C (CCITT + V.24) communications interfaces. Several multiport + cards are supported as well. See the <tt>sio(4)</tt> + manual page for detailed technical documentation. + +<sect3><heading>Digiboard PC/8</heading> + + <p><em>Contributed by &a.awebster;.<newline>26 August + 1995.</em> + + Here is a config snippet from a machine with + digiboard PC/8 with 16550. It has 8 modems connected + to these 8 lines, and they work just great. Do not + forget to add <tt>options "COM_MULTIPORT"</tt> or it + will not work very well! + +<tscreen><verb> +device sio4 at isa? port 0x100 tty flags 0xb05 +device sio5 at isa? port 0x108 tty flags 0xb05 +device sio6 at isa? port 0x110 tty flags 0xb05 +device sio7 at isa? port 0x118 tty flags 0xb05 +device sio8 at isa? port 0x120 tty flags 0xb05 +device sio9 at isa? port 0x128 tty flags 0xb05 +device sio10 at isa? port 0x130 tty flags 0xb05 +device sio11 at isa? port 0x138 tty flags 0xb05 irq 9 vector siointr +</verb></tscreen> + + The trick in setting this up is that the MSB of the + flags represent the last SIO port, in this case 11 so + flags are 0xb05. + +<sect3><heading>Boca 16</heading> + + <p><em>Contributed by &a.whiteside;.<newline>26 August + 1995.</em> + + The procedures to make a Boca 16 pord board with + FreeBSD are pretty straightforward, but you will need + a couple things to make it work: + + <enum> + <item>You either need the kernel sources installed + so you can recompile the necessary options or + you will need someone else to compile it for you. + The 2.0.5 default kernel does <bf>not</bf> come with + multiport support enabled and you will need to add + a device entry for each port anyways. + </item> + <item>Two, you will need to know the interrupt and IO + setting for your Boca Board so you can set these + options properly in the kernel.</item> + </enum> + + One important note - the actual UART chips for the + Boca 16 are in the connector box, not on the internal + board itself. So if you have it unplugged, probes of + those ports will fail. I have never tested booting with + the box unplugged and plugging it back in, and I + suggest you do not either. + + If you do not already have a custom kernel + configuration file set up, refer to <ref + id="kernelconfig" name="Kernel Configuration"> for + general procedures. The following are the specifics + for the Boca 16 board and assume you are using the + kernel name MYKERNEL and editing with vi. + + <enum> + <item>Add the line +<tscreen><verb> +options "COM_MULTIPORT" +</verb></tscreen> +to the config file. +</item> + + <item>Where the current <tt>device sio + <em>xxx</em></tt> lines are, you will need to add + 16 more devices. <em>Only the last device + includes the interrupt vector for the + board</em>. (See the <tt>sio(4)</tt> manual page + for detail as to why.) + + The following example is for a Boca Board with an + interrupt of 3, and a base IO address 100h. The + IO address for Each port is +8 hexadecimal from + the previous port, thus the 100h, 108h, 110h... + addresses. + +<tscreen><verb> +device sio1 at isa? port 0x100 tty flags 0x1005 +device sio2 at isa? port 0x108 tty flags 0x1005 +device sio3 at isa? port 0x110 tty flags 0x1005 +device sio4 at isa? port 0x118 tty flags 0x1005 +[...] +device sio15 at isa? port 0x170 tty flags 0x1005 +device sio16 at isa? port 0x178 tty flags 0x1005 irq 3 vector siointr +</verb></tscreen> + + The flags entry <em>must</em> be changed from + this example unless you are using the exact same + sio assignments. Flags are set according to + 0x<em>MYY</em> where <em>M</em> indicates the + minor number of the master port (the last port on + a Boca 16) and <em>YY</em> indicates if FIFO is + enabled or disabled(enabled), IRQ sharing is + used(yes) and if there is an AST/4 compatible IRQ + control register(no). + + In this example, +<tscreen><verb> +flags 0x1005 +</verb></tscreen> + + indicates that the master port is sio16. If I + added another board and assigned sio17 through + sio28, the flags for all 16 ports on + <em>that</em> board would be 0x1C05, where 1C + indicates the minor number of the master port. + Do not change the 05 setting.</item> + + <item>Save and complete the kernel configuration, + recompile, install and reboot. + + Presuming you have successfully installed the + recompiled kernel and have it set to the correct + address and IRQ, your boot message should + indicate the successful probe of the Boca ports + as follows: (obviously the sio numbers, IO and + IRQ could be different) + +<tscreen><verb> +sio1 at 0x100-0x107 flags 0x1005 on isa +sio1: type 16550A (multiport) +sio2 at 0x108-0x10f flags 0x1005 on isa +sio2: type 16550A (multiport) +sio3 at 0x110-0x117 flags 0x1005 on isa +sio3: type 16550A (multiport) +sio4 at 0x118-0x11f flags 0x1005 on isa +sio4: type 16550A (multiport) +sio5 at 0x120-0x127 flags 0x1005 on isa +sio5: type 16550A (multiport) +sio6 at 0x128-0x12f flags 0x1005 on isa +sio6: type 16550A (multiport) +sio7 at 0x130-0x137 flags 0x1005 on isa +sio7: type 16550A (multiport) +sio8 at 0x138-0x13f flags 0x1005 on isa +sio8: type 16550A (multiport) +sio9 at 0x140-0x147 flags 0x1005 on isa +sio9: type 16550A (multiport) +sio10 at 0x148-0x14f flags 0x1005 on isa +sio10: type 16550A (multiport) +sio11 at 0x150-0x157 flags 0x1005 on isa +sio11: type 16550A (multiport) +sio12 at 0x158-0x15f flags 0x1005 on isa +sio12: type 16550A (multiport) +sio13 at 0x160-0x167 flags 0x1005 on isa +sio13: type 16550A (multiport) +sio14 at 0x168-0x16f flags 0x1005 on isa +sio14: type 16550A (multiport) +sio15 at 0x170-0x177 flags 0x1005 on isa +sio15: type 16550A (multiport) +sio16 at 0x178-0x17f irq 3 flags 0x1005 on isa +sio16: type 16550A (multiport master) +</verb></tscreen> + + If the messages go by too fast to see, <tt>dmesg + > more</tt> will show you the boot + messages.</item> + + <item>Next, appropriate entries in <tt>/dev</tt> for the devices + must be made using the <tt>/dev/MAKEDEV</tt> + script. After becoming root: +<tscreen> +# cd /dev<newline> +# ./MAKEDEV tty1<newline> +# ./MAKEDEV cua1<newline> +<em>(everything in between)</em><newline> +# ./MAKEDEV ttyg<newline> +# ./MAKEDEV cuag +</tscreen> + + If you do not want or need callout devices for some + reason, you can dispense with making the <tt>cua*</tt> + devices.</item> + + <item>If you want a quick and sloppy way to make + sure the devices are working, you can simply plug + a modem into each port and (as root) <tt>echo at + > ttyd*</tt> for each device you have + made. You <em>should</em> see the RX lights flash + for each working port.</item> + </enum> + diff --git a/share/doc/handbook/uart.sgml b/share/doc/handbook/uart.sgml new file mode 100644 index 0000000..bc3656e --- /dev/null +++ b/share/doc/handbook/uart.sgml @@ -0,0 +1,1107 @@ +<!-- $Id: uart.sgml,v 1.1.1.1 1996/01/13 00:00:00 uhclem Exp $ --> +<!-- The FreeBSD Documentation Project --> + +<!-- +<!DOCTYPE linuxdoc PUBLIC "-//FreeBSD//DTD linuxdoc//EN" [ + +<!ENTITY % authors SYSTEM "authors.sgml"> +%authors; + +]> +--> +<sect2><heading>The UART: What it is and how it works<label id="uart"></heading> + +<p><em>Copyright © 1996 &a.uhclem;, All Rights Reserved.<newline> +13 January 1996.</em> + +<!-- Version 1(2) 13-Jan-96 --> + + The Universal Asynchronous Receiver/Transmitter (UART) controller + is the key component of the serial communications subsystem of a + computer. The UART takes bytes of data and transmits the individual + bits in a sequential fashion. At the destination, a second UART + re-assembles the bits into complete bytes. + + Serial transmission is commonly used with modems and for + non-networked communication between computers, terminals + and other devices. + + There are two primary forms of serial transmission: Synchronous and + Asynchronous. Depending on the modes that are supported by the + hardware, the name of the communication sub-system will usually + include a "A" if it supports Asynchronous communications, and a + "S" if it supports Synchronous communications. Both forms are + described below. + + Some common acryonyms are: +<quote>UART Universal Asynchronous Receiver/Transmitter</quote> +<quote>USART Universal Synchronous-Asynchronous Receiver/Transmitter</quote> + + +<sect3><heading>Synchronous Serial Transmission</heading> + + <p>Synchronous serial transmission requires that the sender and + receiver share a clock with one another, or that the sender provide + a strobe or other timing signal so that the receiver knows when to + "read" the next bit of the data. In most forms of serial + Synchronous communication, if there is no data available at a given + instant to transmit, a fill character must be sent instead so that + data is always being transmitted. Synchronous communication is + usually more efficient because only data bits are transmitted + between sender and receiver, and synchronous communication can be + more more costly if extra wiring and circuits are required to + share a clock signal between the sender and receiver. + + A form of Synchronous transmission is used with printers and + fixed disk devices in that the data is sent on one set of wires + while a clock or strobe is sent on a different wire. Printers and + fixed disk devices are not normally serial devices because most + fixed disk interface standards send an entire word of data for each + clock or strobe signal by using a separate wire for each bit of the + word. In the PC industry, these are known as Parallel devices. + + The standard serial communications hardware in the PC does not + support Synchronous operations. This mode is described here for + comparison purposes only. + + +<sect3><heading>Asynchronous Serial Transmission</heading> + + <p>Asynchronous transmission allows data to be transmitted without + the sender having to send a clock signal to the receiver. Instead, + the sender and receiver must agree on timing parameters in advance + and special bits are added to each word which are used to + synchronize the sending and receiving units. + + When a word is given to the UART for Asynchronous transmissions, + a bit called the "Start Bit" is added to the beginning of each word + that is to be transmitted. The Start Bit is used to alert the + receiver that a word of data is about to be sent, and to force the + clock in the receiver into synchronization with the clock in the + transmitter. These two clocks must be accurate enough to not + have the frequency drift by more than 10% during the transmission + of the remaining bits in the word. (This requirement was set in + the days of mechanical teleprinters and is easily met by modern + electronic equipment.) + + After the Start Bit, the individual bits of the word of data are + sent, with the Least Significant Bit (LSB) being sent first. Each + bit in the transmission is transmitted for exactly the same + amount of time as all of the other bits, and the receiver "looks" + at the wire at approximately halfway through the period assigned + to each bit to determine if the bit is a "1" or a "0". For example, + if it takes two seconds to send each bit, the receiver will examine + the signal to determine if it is a "1" or a "0" after one second + has passed, then it will wait two seconds and then examine the value + of the next bit, and so on. + + The sender does not know when the receiver has "looked" at the + value of the bit. The sender only knows when the clock says to + begin transmitting the next bit of the word. + + When the entire data word has been sent, the transmitter may add + a Parity Bit that the transmitter generates. The Parity Bit may + be used by the receiver to perform simple error checking. Then at + least one Stop Bit is sent by the transmitter. + + When the receiver has received all of the bits in the data word, + it may check for the Parity Bits (both sender and receiver must + agree on whether a Parity Bit is to be used), and then the receiver + looks for a Stop Bit. If the Stop Bit does not appear when it is + supposed to, the UART considers the entire word to be garbled and + will report a Framing Error to the host processor when the data + word is read. The usual cause of a Framing Error is that the sender + and receiver clocks were not running at the same speed, or that + the signal was interrupted. + + Regardless of whether the data was received correctly or not, the + UART automatically discards the Start, Parity and Stop bits. If the + sender and receiver are configured identically, these bits are not + passed to the host. + + If another word is ready for transmission, the Start Bit for the new + word can be sent as soon as the Stop Bit for the previous + word has been sent. + + Because asynchronous data is "self synchronizing", if there is no + data to transmit, the transmission line can be idle. + + +<sect3><heading>Other UART Functions</heading> + + <p>In addition to the basic job of converting data from parallel to + serial for transmission and from serial to parallel on reception, + a UART will usually provide additional circuits for signals that + can be used to indicate the state of the transmission media, and + to regulate the flow of data in the event that the remote device + is not prepared to accept more data. For example, when the + device connected to the UART is a modem, the modem may report the + presence of a carrier on the phone line while the computer may be + able to instruct the modem to reset itself or to not take calls + by asserting or deasserting one more more of these extra signals. + The function of each of these additional signals is defined in + the EIA RS232-C standard. + + +<sect3><heading>The RS232-C and V.24 Standards</heading> + + <p>In most computer systems, the UART is connected to circuitry that + generates signals that comply with the EIA RS232-C specification. + There is also a CCITT standard named V.24 that mirrors the + specifications included in RS232-C. + +<sect4><heading>RS232-C Bit Assignments (Marks and Spaces)</heading> + + <p>In RS232-C, a value of "1" is called a "Mark" and a value of "0" + is called a "Space". When a communication line is idle, the line + is said to be "Marking", or transmitting continuous "1" values. + + The Start bit always has a value of "0" (a Space). The Stop Bit + always has a value of "1" (a Mark). This means that there will + always be a Mark (1) to Space (0) transition on the line at the + start of every word, even when multiple word are + transmitted back to back. This guarantees that sender and + receiver can resynchronize their clocks regardless of the content + of the data bits that are being transmitted. + + The idle time between Stop and Start bits does not have + to be an exact multiple (including zero) of the bit rate of the + communication link, but most UARTs are designed this way for + simplicity. + + In RS232-C, the "Marking" signal (a "1") is represented by a voltage + between -2 VDC and -12 VDC, and a "Spacing" signal (a "0") is + represented by a voltage between 0 and +12 VDC. The transmitter + is supposed to send +12 VDC or -12 VDC, and the receiver is supposed + to allow for some voltage loss in long cables. Some transmitters + in low power devices (like portable computers) sometimes use only + +5 VDC and -5 VDC, but these values are still acceptable to a + RS232-C receiver, provided that the cable lengths are short. + + +<sect4><heading>RS232-C Break Signal</heading> + + <p>RS232-C also specifies a signal called a "Break", which is caused + by sending continuous Spacing values (no Start or Stop bits). When + there is no electricity present on the data circuit, the line is + considered to be sending "Break". + + The "Break" signal must be of a duration longer than the time + it takes to send a complete byte plus Start, Stop and Parity bits. + Most UARTs can distinguish between a Framing Error and a + Break, but if the UART cannot do this, the Framing Error detection + can be used to identify Breaks. + + In the days of teleprinters, when numerous printers around the + country were wired in series (such as news services), any unit + could cause a "Break" by temporarily opening the entire circuit + so that no current flowed. This was used to allow a location with + urgent news to interrupt some other location that was currently + sending information. + + In modern systems there are two types of Break signals. If the + Break is longer than 1.6 seconds, it is considered a "Modem Break", + and some modems can be programmed to terminate the conversation and + go on-hook or enter the modems' command mode when the modem detects + this signal. If the Break is smaller than 1.6 seconds, it signifies + a Data Break and it is up to the remote computer to respond to + this signal. Sometimes this form of Break is used as an Attention + or Interrupt signal and sometimes is accepted as a substitute for + the ASCII CONTROL-C character. + + Marks and Spaces are also equivalent to "Holes" and "No Holes" + in paper tape systems. + + Note that Breaks cannot be generated from paper tape or from any + other byte value, since bytes are always sent with Start and Stop + bit. The UART is usually capable of generating the continuous + Spacing signal in response to a special command from the host + processor. + +<sect4><heading>RS232-C DTE and DCE Devices</heading> + + <p>The RS232-C specification defines two types of equipment: the Data + Terminal Equipment (DTE) and the Data Carrier Equipment (DCE). + Usually, the DTE device is the terminal (or computer), and the DCE + is a modem. Across the phone line at the other end of a + conversation, the receiving modem is also a DCE device and the + computer that is connected to that modem is a DTE device. The DCE + device receives signals on the pins that the DTE device transmits on, + and vice versa. + + When two devices that are both DTE or both DCE must be connected + together without a modem or a similar media translater between them, + a NULL modem must be used. The NULL modem electrically re-arranges + the cabling so that the transmitter output is connected to the + receiver input on the other device, and vice versa. Similar + translations are performed on all of the control signals so that + each device will see what it thinks are DCE (or DTE) signals from + the other device. + + The number of signals generated by the DTE and DCE devices are + not symmetrical. The DTE device generates fewer signals for + the DCE device than the DTE device receives from the DCE. + +<sect4><heading>RS232-C Pin Assignments</heading> + + <p>The EIA RS232-C specification (and the ITU equivalent, V.24) calls + for a twenty-five pin connector (usually a DB25) and defines the + purpose of most of the pins in that connector. + + In the IBM Personal Computer and similar systems, a subset of + RS232-C signals are provided via nine pin connectors (DB9). + The signals that are not included on the PC connector deal mainly + with synchronous operation, and this transmission mode is not + supported by the UART that IBM selected for use in the IBM PC. + + Depending on the computer manufacturer, a DB25, a DB9, or + both types of connector may be used for RS232-C communications. + (The IBM PC also uses a DB25 connector for the parallel printer + interface which causes some confusion.) + + Below is a table of the RS232-C signal assignments in the DB25 + and DB9 connectors. + +<verb> +DB25 DB9 EIA CCITT Common Signal Description +RS232-C IBM PC Circuit Circuit Name Source +Pin Pin Symbol Symbol + +1 - AA 101 PG/FG --- Frame/Protective Ground +2 3 BA 103 TD DTE Transmit Data +3 2 BB 104 RD DCE Receive Data +4 7 CA 105 RTS DTE Request to Send +5 8 CB 106 CTS DCE Clear to Send +6 6 CC 107 DSR DCE Data Set Ready +7 5 AV 102 SG/GND --- Signal Ground +8 1 CF 109 DCD/CD DCE Data Carrier Detect +9 - - - - - Reserved for Test +10 - - - - - Reserved for Test +11 - - - - - Unassigned +12 - CI 122 SRLSD DCE Sec. Recv. Line Signal Detector +13 - SCB 121 SCTS DCE Secondary Clear To Send +14 - SBA 118 STD DTE Secondary Transmit Data +15 - DB 114 TSET DCE Trans. Sig. Element Timing +16 - SBB 119 SRD DCE Secondary Received Data +17 - DD 115 RSET DCE Receiver Signal Element Timing +18 - - 141 LOOP DTE Local Loopback +19 - SCA 120 SRS DTE Secondary Request to Send +20 4 CD 108.2 DTR DTE Data Terminal Ready +21 - - - RDL DTE Remote Digital Loopback +22 9 CE 125 RI DCE Ring Indicator +23 - CH 111 DSRS DTE Data Signal Rate Selector +24 - DA 113 TSET DTE Trans. Sig. Element Timing +25 - - 142 - DCE Test Mode +</verb> + + + <sect3><heading>Bits, Baud and Symbols</heading> + + <p>Baud is a measurment of transmission speed in asynchronous + communication. Because of advances in modem communication + technology, this term is frequently misused when describing + the data rates in newer devices. + + Traditionally, a Baud Rate represents the number of bits that are + actually being sent over the media, not the amount of data + that is actually moved from one DTE device to the other. The + Baud count includes the overhead bits Start, Stop and Parity + that are generated by the sending UART and removed by the + receiving UART. This means that seven-bit words of data + actually take 10 bits to be completely transmitted. + Therefore, a modem capable of moving 300 bits per second from one + place to another can normally only move 30 7-bit words if + Parity is used and one Start and Stop bit are present. + + If 8-bit data words are used and Parity bits are also used, the + data rate falls to 27.27 words per second, because it now + takes 11 bits to send the eight-bit words, and the modem still + only sends 300 bits per second. + + The formula for converting bytes per second into a baud rate + and vice versa was simple until error-correcting modems + came along. These modems receive the serial stream of bits + from the UART in the host computer (even when internal modems + are used the data is still frequently serialized) and converts + the bits back into bytes. These bytes are then combined into + packets and sent over the phone line using a Synchronous + transmission method. This means that the Stop, Start, and Parity + bits added by the UART in the DTE (the computer) were removed by + the modem before transmission by the sending modem. When these + bytes are received by the remote modem, the remote modem adds + Start, Stop and Parity bits to the words, converts them to a + serial format and then sends them to the receiving UART in the remote + computer, who then strips the Start, Stop and Parity bits. + + The reason all these extra conversions are done is so that the + two modems can perform error correction, which means that the + receiving modem is able to ask the sending modem to resend a + block of data that was not received with the correct checksum. + This checking is handled by the modems, and the DTE devices are + usually unaware that the process is occurring. + + By striping the Start, Stop and Parity bits, the additional bits of + data that the two modems must share between themselves to perform + error-correction are mostly concealed from the effective + transmission rate seen by the sending and receiving DTE equipment. + For example, if a modem sends ten 7-bit words to another modem + without including the Start, Stop and Parity bits, the sending + modem will be able to add 30 bits of its own information that + the receiving modem can use to do error-correction without + impacting the transmission speed of the real data. + + The use of the term Baud is further confused by modems that perform + compression. A single 8-bit word passed over the telephone + line might represent a dozen words that were transmitted to + the sending modem. The receiving modem will expand the data back + to its original content and pass that data to the receiving DTE. + + Modern modems also include buffers that allow the rate that + bits move across the phone line (DCE to DCE) to be a different speed + than the speed that the bits move between the DTE and DCE on both + ends of the conversation. Normally the speed between the DTE and + DCE is higher than the DCE to DCE speed because of the use of + compression by the modems. + + Because the number of bits needed to describe a byte varied + during the trip between the two machines plus the differing + bits-per-seconds speeds that are used present on the DTE-DCE and + DCE-DCE links, the usage of the term Baud to describe the + overall communication speed causes problems and can misrepresent + the true transmission speed. So Bits Per Second (bps) is the correct + term to use to describe the transmission rate seen at the + DCE to DCE interface and Baud or Bits Per Second are acceptable + terms to use when a connection is made between two systems with a + wired connection, or if a modem is in use that is not performing + error-correction or compression. + + Modern high speed modems (2400, 9600, 14,400, and 19,200bps) in + reality still operate at or below 2400 baud, or more accurately, + 2400 Symbols per second. High speed modem are able to encode more + bits of data into each Symbol using a technique called Constellation + Stuffing, which is why the effective bits per second rate of the + modem is higher, but the modem continues to operate within the + limited audio bandwidth that the telephone system provides. + Modems operating at 28,800 and higher speeds have variable Symbol + rates, but the technique is the same. + + <sect3><heading>The IBM Personal Computer UART</heading> + + <p>Starting with the original IBM Personal Computer, IBM selected + the National Semiconductor INS8250 UART for use in the IBM PC + Parallel/Serial Adapter. Subsequent generations of compatible + computers from IBM and other vendors continued to use the INS8250 + or improved versions of the National Semiconductor UART family. + + +<sect4><heading>National Semiconductor UART Family Tree</heading> + + <p>There have been several versions and subsequent generations of + the INS8250 UART. Each major version is described below. + +<verb> + INS8250 -> INS8250B + \ + \ + \-> INS8250A -> INS82C50A + \ + \ + \-> NS16450 -> NS16C450 + \ + \ + \-> NS16550 -> NS16550A -> PC16550D +</verb> + +<descrip> + <tag>INS8250</tag>This part was used in the original IBM PC and + IBM PC/XT. The original name for this part was the INS8250 ACE + (Asynchronous Communications Element) and it is made from NMOS + technology. + + The 8250 uses eight I/O ports and has a one-byte send and + a one-byte receive buffer. This original UART has several + race conditions and other flaws. The original IBM BIOS + includes code to work around these flaws, but this made + the BIOS dependent on the flaws being present, so subsequent + parts like the 8250A, 16450 or 16550 could not be used in + the original IBM PC or IBM PC/XT. + + <tag>INS8250-B</tag>This is the slower speed of the INS8250 made + from NMOS technology. It contains the same problems as the original + INS8250. + + <tag>INS8250A</tag>An improved version of the INS8250 using XMOS + technology with various functional flaws corrected. The INS8250A + was used initially in PC clone computers by vendors who used + "clean" BIOS designs. Because of the corrections in the chip, this + part could not be used with a BIOS compatible with the INS8250 + or INS8250B. + + <tag>INS82C50A</tag>This is a CMOS version (low power consumption) + of the INS8250A and has similar functional characteristics. + + <tag>NS16450</tag>Same as NS8250A with improvements so it can be + used with faster CPU bus designs. IBM used this part in the IBM AT + and updated the IBM BIOS to no longer rely on the bugs in the + INS8250. + + <tag>NS16C450</tag>This is a CMOS version (low power consumption) + of the NS16450. + + <tag>NS16550</tag>Same as NS16450 with a 16-byte send and receive + buffer but the buffer design was flawed and could not be reliably + be used. + + <tag>NS16550A</tag>Same as NS16550 with the buffer flaws corrected. + The 16550A and its successors have become the most popular UART + design in the PC industry, mainly due it its ability to reliably + handle higher data rates on operating systems with sluggish interrupt + response times. + + <tag>NS16C552</tag>This component consists of two NS16C550A CMOS + UARTs in a single package. + + <tag>PC16550D</tag>Same as NS16550A with subtle flaws corrected. This + is revision D of the 16550 family and is the latest design available + from National Semiconductor. +</descrip> + +<sect4><heading>The NS16550AF and the PC16550D are the same thing</heading> + + <p>National reorganized their part numbering system a few years ago, + and the NS16550AFN no longer exists by that name. (If you + have a NS16550AFN, look at the date code on the part, which is a + four digit number that usually starts with a nine. The first two + digits of the number are the year, and the last two digits are the + week in that year when the part was packaged. If you have a + NS16550AFN, it is probably a few years old.) + + The new numbers are like PC16550DV, with minor differences in the + suffix letters depending on the package material and its shape. + (A description of the numbering system can be found below.) + + It is important to understand that in some stores, you may pay + $15(US) for a NS16550AFN made in 1990 and in the next bin are the + new PC16550DN parts with minor fixes that National has made since the + AFN part was in production, the PC16550DN was probably made in the + past six months and it costs half (as low as $5(US) in volume) as + much as the NS16550AFN because they are readily available. + + As the supply of NS16550AFN chips continues to shrink, the price will + probably continue to increase until more people discover and accept + that the PC16550DN really has the same function as the old part + number. + +<sect4><heading>National Semiconductor Part Numbering System</heading> + + <p>The older NS<em>nnnnnrqp</em> part numbers are now of the + format PC<em>nnnnnrgp</em>. + + The "<em>r</em>" is the revision field. The current revision of + the 16550 from National Semiconductor is "D". + + The "<em>p</em>" is the package-type field. The types are: +<verb> "F" QFP (quad flat pack) L lead type + "N" DIP (dual inline package) through hole straight lead type + "V" LPCC (lead plastic chip carrier) J lead type</verb> + + The "<em>g</em>" is the product grade field. If an "I" preceeds + the package-type letter, it indicates an "industrial" grade part, + which has higher specs than a standard part but not as high as + Miltary Specification (Milspec) component. This is an optional field. + + So what we used to call a NS16550AFN (DIP Package) is now called a + PC16550DN or PC16550DIN. + + + <sect3><heading>Other Vendors and Similar UARTs</heading> + + <p>Over the years, the 8250, 8250A, 16450 and 16550 have been licensed + or copied by other chip vendors. In the case of the 8250, 8250A + and 16450, the exact circuit (the "megacell") was licensed to many + vendors, including Western Digital and Intel. Other vendors + reverse-engineered the part or produced emulations that had similar + behavior. + + In internal modems, the modem designer will frequently emulate the + 8250A/16450 with the modem microprocessor, and the emulated UART will + frequently have a hidden buffer consisting of several hundred bytes. + Because of the suze of the buffer, these emulations can be as + reliable as a 16550A in their ability to handle high speed data. + However, most operating systems will still report that + the UART is only a 8250A or 16450, and may not make effective use + of the extra buffering present in the emulated UART unless special + drivers are used. + + Some modem makers are driven by market forces to abandon a design + that has hundreds of bytes of buffer and instead use a 16550A UART + so that the product will compare favorably in market comparisons + even though the effective performance may be lowered by this action. + + A common misconception is that all parts with "16550A" written on + them are identical in performance. There are differences, and in + some cases, outright flaws in most of these 16550A clones. + + When the NS16550 was developed, the National Semiconductor obtained + several patents on the design and they also limited licensing, making + it harder for other vendors to provide a chip with similar features. + Because of the patents, reverse-engineered designs and emulations + had to avoid infringing the claims covered by the patents. + Subsequently, these copies almost never perform exactly the same as + the NS16550A or PC16550D, which are the parts most computer and + modem makers want to buy but are sometimes unwilling to pay the + price required to get the genuine part. + + Some of the differences in the clone 16550A parts are unimportant, + while others can prevent the device from being used at all with a + given operating system or driver. These differences may show up + when using other drivers, or when particular combinations of events + occur that were not well tested or considered in the Windows driver. + This is because most modem vendors and 16550-clone makers use the + Microsoft drivers from Windows for Workgroups 3.11 and the Microsoft + MSD utility as the primary tests for compatibility with the + NS16550A. This over-simplistic criteria means that if a different + operating system is used, problems could appear due to subtle + differences between the clones and genuine components. + + National Semiconductor has made available a program named COMTEST + that performs compatibility tests independent of any OS drivers. + It should be remembered that the purpose of this type of program is + to demonstrate the flaws in the products of the competition, so the + program will report major as well as extremely subtle differences in + behavior in the part being tested. + + In a series of tests performed by the author of this document in + 1994, components made by National Semiconductor, TI, StarTech, and + CMD as well as megacells and emulations embedded in internal modems + were tested with COMTEST. A difference count for some of these + components is listed below. Because these tests were performed in + 1994, they may not reflect the current performance of the given + product from a vendor. + + It should be noted that COMTEST normally aborts when an excessive + number or certain types of problems have been detected. As part of + this testing, COMTEST was modified so that it would not abort no + matter how many differences were encountered. + + +<verb>Vendor Part number Errors aka "differences" reported +National (PC16550DV) 0 * + +National (NS16550AFN) 0 + +National (NS16C552V) 0 * + +TI (TL16550AFN) 3 + +CMD (16C550PE) 19 + +StarTech (ST16C550J) 23 + +Rockwell reference modem + with internal 16550 or an + emulation (RC144DPi/C3000-25) 117 + +Sierra modem with an internal + 16550 (SC11951/SC11351) 91</verb> + + <p>It is important to understand that a simple count of differences + from COMTEST does not reveal a lot about what differences are + important and which are not. For example, about half of the + differences reported in the two modems listed above that have + internal UARTs were caused by the clone UARTs not supporting + five- and six-bit character modes. The real 16550, 16450, and + 8250 UARTs all support these modes and COMTEST checks the + functionality of these modes so over fifty differences are + reported. However, almost no modern modem supports five- or + six-bit characters, particularly those with error-correction + and compression capabilities. This means that the differences + related to five- and six-bit character modes can be discounted. + + Many of the differences COMTEST reports have to do with timing. In + many of the clone designs, when the host reads from one port, the + status bits in some other port may not update in the same amount + of time (some faster, some slower) as a <em>real</em> NS16550AFN + and COMTEST looks for these differences. This means that the number + of differences can be misleading in that one device may only have + one or two differences but they are extremely serious, and some + other device that updates the status registers faster or slower + than the reference part (that would probably never affect the + operation of a properly written driver) could have dozens of + differences reported. + + * To date, the author of this document has not found any non-National + parts that report zero differences using the COMTEST program. It + should also be noted that National has had five versions of the + 16550 over the years and the newest parts behave a bit differently + than the classic NS16550AFN that is considered the benchmark for + functionality. COMTEST appears to turn a blind eye to the + differences within the National product line and reports no errors + on the National parts (except for the original 16550) even when + there are official erattas that describe bugs in the A, B and C + revisions of the parts, so this bias in COMTEST must be taken into + account. + + COMTEST can be used as a screening tool to alert the administrator + to the presence of potentially incompatible components + that might cause problems or have to be handled as a special case. + + If you run COMTEST on a 16550 that is in a modem or a modem is + attached to the serial port, you need to first issue a ATE0&W + command to the modem so that the modem will not echo any of the test + characters. If you forget to do this, COMTEST will report at least + this one difference: + <quote>Error (6)...Timeout interrupt failed: IIR = c1 LSR = 61</quote> + + + <sect3><heading>8250/16450/16550 Registers</heading> + + <p>The 8250/16450/16550 UART occupies eight contiguous I/O port + addresses. In the IBM PC, there are two defined locations for + these eight ports and they are known collectively as COM1 and COM2. + The makers of PC-clones and add-on cards have created two additional + areas known as COM3 and COM4, but these extra COM ports conflict + with other hardware on some systems. The most common conflict is + with video adapters that provide IBM 8514 emulation. + +<verb> +COM1 is located from 0x3f8 to 0x3ff and normally uses IRQ 4 +COM2 is located from 0x2f8 to 0x2ff and normally uses IRQ 3 +COM3 is located from 0x3e8 to 0x3ef and has no standardized IRQ +COM4 is located from 0x2e8 to 0x2ef and has no standardized IRQ +</verb> +<p>A description of the I/O ports of the 8250/16450/16550 UART is +provided below. + +<verb> +I/O Access Description +Port Allowed + ++0x00 write Transmit Holding Register (THR) + (DLAB==0) Information written to this port are treated + as data words and will be transmitted by the + UART. + ++0x00 read Receive Buffer Register (RBR) + (DLAB==0) Any data words received by the UART from the + serial link are accessed by the host by + reading this port. + + ++0x00 write/read Divisor Latch LSB (DLL) + (DLAB==1) This value will be divided from the master + input clock (in the IBM PC, the master + clock is 1.8432MHz) and the resulting clock + will determine the baud rate of the UART. + This register holds bits 0 thru 7 of the + divisor. + + ++0x01 write/read Divisor Latch MSB (DLH) + (DLAB==1) This value will be divided from the master + input clock (in the IBM PC, the master + clock is 1.8432MHz) and the resulting clock + will determine the baud rate of the UART. + This register holds bits 8 thru 15 of the + divisor. + + ++0x01 write/read Interrupt Enable Register (IER) + (DLAB==0) The 8250/16450/16550 UART classifies events into + one of four categories. Each category can be + configured to generate an interrupt when any of + the events occurs. The 8250/16450/16550 UART + generates a single external interrupt signal + regardless of how many events in the enabled + categories have occurred. It is up to the host + processor to respond to the interrupt and then + poll the enabled interrupt categories (usually + all categories have interrupts enabled) to + determine the true cause(s) of the interrupt. + + Bit 7 Reserved, always 0. + + Bit 6 Reserved, always 0. + + Bit 5 Reserved, always 0. + + Bit 4 Reserved, always 0. + + Bit 3 Enable Modem Status Interrupt (EDSSI) + Setting this bit to "1" allows the UART + to generate an interrupt when a + change occurs on one or more of the + status lines. + + Bit 2 Enable Receiver Line Status + Interrupt (ELSI) + Setting this bit to "1" causes the UART + to generate an interrupt when the + an error (or a BREAK signal) has been + detected in the incoming data. + + Bit 1 Enable Transmitter Holding Register + Empty Interrupt (ETBEI) + Setting this bit to "1" causes the UART + to generate an interrupt when the + UART has room for one or more + additional characters that are to + be transmitted. + + Bit 0 Enable Received Data Available + Interrupt (ERBFI) + Setting this bit to "1" causes the UART + to generate an interrupt when the UART + has received enough characters to exceed + the trigger level of the FIFO, or the + FIFO timer has expired (stale data), or + a single character has been received + when the FIFO is disabled. + + ++0x02 write FIFO Control Register (FCR) + (This port does not exist on the 8250 and 16450 + UART.) + + Bit 7 Receiver Trigger Bit #1 + Bit 6 Receiver Trigger Bit #0 + These two bits control at what point the + receiver is to generate an interrupt when + the FIFO is active. + + 7 6 How many words are received + before an interrupt is generated. + 0 0 1 + + 0 1 4 + + 1 0 8 + + 1 1 14 + + Bit 5 Reserved, always 0. + + Bit 4 Reserved, always 0. + + Bit 3 DMA Mode Select + If Bit 0 is set to "1" (FIFOs enabled), + setting this bit changes the operation + of the -RXRDY and -TXRDY signals from + Mode 0 to Mode 1. + + Bit 2 Transmit FIFO Reset + When a "1" is written to this bit, + the contents of the FIFO are discarded. + Any word currently being transmitted + will be sent intact. This function is + useful in aborting transfers. + + Bit 1 Receiver FIFO Reset + When a "1" is written to this bit, + the contents of the FIFO are discarded. + Any word currently being assembled + in the shift register will be received + intact. + + Bit 0 16550 FIFO Enable + When set, both the transmit and receive + FIFOs are enabled. Any contents in the + holding register, shift registers or + FIFOs are lost when FIFOs are enabled or + disabled. + + ++0x02 read Interrupt Identification Register (IIR) + + Bit 7 FIFOs enabled. + On the 8250/16450 UART, this bit is zero. + + Bit 6 FIFOs enabled. + On the 8250/16450 UART, this bit is zero. + + Bit 5 Reserved, always 0. + + Bit 4 Reserved, always 0. + + Bit 3 Interrupt ID Bit #2 + On the 8250/16450 UART, this bit is zero. + Bit 2 Interrupt ID Bit #1 + Bit 1 Interrupt ID Bit #0 + These three bits combine to report + the category of event that caused the + interrupt that is in progress. These + categories have priorities, so if + multiple categories of events occur at + the same time, the UART will report the + more important events first and the host + must resolve the events in the order they + are reported. All events that caused the + current interrupt must be resolved before + any new interrupts will be generated. + (This is a limitation of the PC + architecture.) + + 2 1 0 Priority Description + + 0 1 1 First Receiver Error + (OE, PE, BI or FE) + + 0 1 0 Second Received Data + Available + + 1 1 0 Second Trigger level + identification + (Stale data in + receive buffer) + + 0 0 1 Third Transmitter has + room for more + words (THRE) + + 0 0 0 Fourth Modem Status + Change (-CTS, + -DSR, -RI, or + -DCD) + + Bit 0 Interrupt Pending Bit + If this bit is set to "0", then at least + one interrupt is pending. + + ++0x03 write/read Line Control Register (LCR) + + Bit 7 Divisor Latch Access Bit (DLAB) + When set, access to the data + transmit/receive register (THR/RBR) and + the Interrupt Enable Register (IER) is + disabled. Any access to these ports is + now redirected to the Divisor Latch + Registers. Setting this bit, loading + the Divisor Registers, and clearing + DLAB should be done with interrupts + disabled. + + Bit 6 Set Break + When set to "1", the transmitter begins + to transmit continuous Spacing until + this bit is set to "0". This overrides + any bits of characters that are being + transmitted. + + Bit 5 Stick Parity + When parity is enabled, setting this + bit causes parity to always be "1" or + "0", based on the value of Bit 4. + + Bit 4 Even Parity Select (EPS) + When parity is enabled and Bit 5 is "0", + setting this bit causes even parity + to be transmitted and expected. + Otherwise, odd parity is used. + + Bit 3 Parity Enable (PEN) + When set to "1", a parity bit is + inserted between the last bit of the + data and the Stop Bit. The UART will + also expect parity to be present in + the received data. + + Bit 2 Number of Stop Bits (STB) + If set to "1" and using 5-bit data words, + 1.5 Stop Bits are transmitted and + expected in each data word. For 6, 7 + and 8-bit data words, 2 Stop Bits are + transmitted and expected. When this bit + is set to "0", one Stop Bit is used on + each data word. + + Bit 1 Word Length Select Bit #1 (WLSB1) + Bit 0 Word Length Select Bit #0 (WLSB0) + Together these bits specify the number + of bits in each data word. + + 1 0 Word Length + + 0 0 5 Data Bits + 0 1 6 Data Bits + 1 0 7 Data Bits + 1 1 8 Data Bits + + ++0x04 write/read Modem Control Register (MCR) + + Bit 7 Reserved, always 0. + + Bit 6 Reserved, always 0. + + Bit 5 Reserved, always 0. + + Bit 4 Loop-Back Enable + When set to "1", the UART transmitter + and receiver are internally connected + together to allow diagnostic operations. + In addition, the UART modem control + outputs are connected to the UART modem + control inputs. CTS is connected to RTS, + DTR is connected to DSR, OUT1 is + connected to RI, and OUT 2 is connected + to DCD. + + Bit 3 OUT 2 + An auxillary output that the host + processor may set high or low. + In the IBM PC serial adapter (and most + clones), OUT 2 is used to tri-state + (disable) the interrupt signal from the + 8250/16450/16550 UART. + + Bit 2 OUT 1 + An auxillary output that the host + processor may set high or low. + This output is not used on the IBM PC + serial adapter. + + Bit 1 Request to Send (RTS) + When set to "1", the output of the UART + -RTS line is Low (Active). + + Bit 0 Data Terminal Ready (DTR) + When set to "1", the output of the UART + -DTR line is Low (Active). + + ++0x05 write/read Line Status Register (LSR) + + Bit 7 Error in Receiver FIFO + On the 8250/16450 UART, this bit is zero. + This bit is set to "1" when any of + the bytes in the FIFO have one or more + of the following error conditions: PE, + FE, or BI. + + Bit 6 Transmitter Empty (TEMT) + When set to "1", there are no words + remaining in the transmit FIFO or the + transmit shift register. The + transmitter is completely idle. + + Bit 5 Transmitter Holding Register Empty (THRE) + When set to "1", the FIFO (or holding + register) now has room for at least one + additional word to transmit. The + transmitter may still be transmitting + when this bit is set to "1". + + Bit 4 Break Interrupt (BI) + The receiver has detected a Break signal. + + Bit 3 Framing Error (FE) + A Start Bit was detected but the Stop + Bit did not appear at the expected time. + The received word is probably garbled. + + Bit 2 Parity Error (PE) + The parity bit was incorrect for the + word received. + + Bit 1 Overrun Error (OE) + A new word was received and there + was no room in the receive buffer. The + newly-arrived word in the shift + register is discarded. On 8250/16450 + UARTs, the word in the holding + register is discarded and the newly- + arrived word is put in the holding + register. + + Bit 0 Data Ready (DR) + One or more words are in the + receive FIFO that the host may read. + A word must be completely received + and moved from the shift register into + the FIFO (or holding register for + 8250/16450 designs) before this bit is + set. + + ++0x06 write/read Modem Status Register (MSR) + + Bit 7 Data Carrier Detect (DCD) + Reflects the state of the DCD line + on the UART. + + Bit 6 Ring Indicator (RI) + Reflects the state of the RI line on + the UART. + + Bit 5 Data Set Ready (DSR) + Reflects the state of the DSR line on + the UART. + + Bit 4 Clear To Send (CTS) + Reflects the state of the CTS line on + the UART. + + Bit 3 Delta Data Carrier Detect (DDCD) + Set to "1" if the -DCD line has changed + state one more more times since the last + time the MSR was read by the host. + + Bit 2 Trailing Edge Ring Indicator (TERI) + Set to "1" if the -RI line has had a + low to high transition since the last + time the MSR was read by the host. + + Bit 1 Delta Data Set Ready (DDSR) + Set to "1" if the -DSR line has changed + state one more more times since the last + time the MSR was read by the host. + + Bit 0 Delta Clear To Send (DCTS) + Set to "1" if the -CTS line has changed + state one more more times since the last + time the MSR was read by the host. + + ++0x07 write/read Scratch Register (SCR) + This register performs no function in the + UART. Any value can be written by the host to + this location and read by the host later on. +</verb> + + <sect3><heading>Beyond the 16550A UART</heading> + + <p>Although National Semiconductor has not offered any components + compatible with the 16550 that provide additional features, + various other vendors have. Some of these components are described + below. It should be understood that to effectively utilize these + improvements, drivers may have to be provided by the chip vendor + since most of the popular operating systems do not support features + beyond those provided by the 16550. + +<descrip> +<tag>ST16650</tag>By default this part is similar to the NS16550A, but an + extended 32-byte send and receive buffer can be optionally + enabled. Made by Startech. + +<tag>TIL16660</tag>By default this part behaves similar to the NS16550A, + but an extended 64-byte send and receive buffer can be + optionally enabled. Made by Texas Instruments. + +<tag>Hayes ESP</tag>This proprietary plug-in card contains a 2048-byte + send and receive buffer, and supports data rates + to 230.4Kbit/sec. Made by Hayes. +</descrip> + + <p>In addition to these "dumb" UARTs, many vendors produce + intelligent serial communication boards. This type of design + usually provides a microprocessor that interfaces with several + UARTs, processes and buffers the data, and then alerts the main + PC processor when necessary. Because the UARTs are not directly + accessed by the PC processor in this type of communication system, + it is not necessary for the vendor to use UARTs that are compatible + with the 8250, 16450, or the 16550 UART. This leaves the + designer free to components that may have better performance + characteristics. + +601131 |