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+Gadget Box
+
+The gadget box is a 5"x3"x2" aluminum minibox containing level-converter
+and modem circuitry and intended for use with host time servers
+supporting the Network Time Protocol. It includes two subcircuits. One
+of these converts a TTL positive edge into a fixed-width pulse at EIA
+levels and is for use with a timecode receiver or oscillator including a
+TTL one-pulse-per-second (1-pps) output. The other converts the timecode
+modulation broadcast by Canadian time/frequency standard station CHU
+into a 300-bps serial character stream at EIA levels and is for use with
+the clk_chu.c or clk_chu_STREAMS.c modules in the xntp3 distribution.
+
+This archive contains complete construction details for the gadget box,
+including schematic, parts list and artwork for a two-sided, printed-
+circuit board. All files are in PostScript, with the exception of this
+file and an information file, which are in ASCII. The artwork is in the
+1:1 scale and is suitable for direct printing on photographic resist for
+each side of the board. While a plated-through-holes process is most
+convenient, it is possible to bridge the two sides using soldered wires
+where necessary.
+
+Following is a brief functional description of the device. See the
+schematic diagram gadget.s01 for reference. The audio output of a
+shortwave radio tuned to CHU at 3330, 7335 or 14670 kHz is connected to
+J2. A level of at least 30 mV peak-peak is required, such as provided by
+the recorder output on many receivers. The input level is adjusted by
+potentiometer R8 so that the timecode modulation broadcast at 31-39
+seconds past the minute reliably lights green LED1, but the signals
+broadcast during other seconds of the minute do not.
+
+Opamp U4A provides low-impedance drive for the bridged-tee bandpass
+filter U4B. The filter has a bandpass of about 600 Hz at the 6-dB points
+and a center frequency of about 2150 Hz. It is designed to avoid
+aliasing effects with receivers of relatively wide bandpass
+characteristics. The modem itself is implemented by U2 and its
+associated circuitry. Resistors R4 and R1 are a 40-dB pad which matches
+the filter output to the modem input. U2 is a TTL/EIA level converter
+with integral power supply for bipolar signals. The modem output is
+available at pin 3 (receive data) of DB25 connector J1.
+
+The TTL 1-pps signal is connected via J3 to a retriggerable one-shot
+U3A, which generates a TTL pulse of width determined by potentiometer
+R7. The pulse width is determined by the bit rate of the attached serial
+port. In the common case the width is one bit-time, such as 26 us for
+38.4 kbps, for example. This appears to the port as a single start bit
+of zero followed by eight bits of ones and a stop bit of one. The second
+one-shot U3B generates a 200-ms pulse suitable for driving the amber
+LED3 as a visual monitor. The output of U3A is converted to EIA levels
+by U1 and appears at pin 12 (secondary receive data) of J1.
+
+If only the 1-pps circuit is required, U2 and U4 can be deleted and the
+gadget box powered from the EIA modem-control signal at pin 20 (terminal
+ready) of J1, assuming this signal is placed in the on (positive
+voltage) condition by the computer program. J1 is wired to keep most
+finicky UARTs and terminal-driver programs happy. If the CHU circuit is
+required, an external 12-volt AC transformer or 9-12-volt DC supply
+connected to J4 is required. Red LED2 indicates power is supplied to the
+box.
+
+Following is a list of files included in this archive. All files are in
+PostScript, except this one (README) and the information file
+(gadget.lst), which are in ASCII. The files gadget.s01, gadget.s02 and
+gadget.lst were generated using the Schema schematic-capture program
+from Omation. The printed-circuit files *.lpr were generated using
+Schema-PCB, also from Omation.
+
+Files
+----------------------------------------
+README          this file
+gadget.s01      circuit schematic
+gadget.s02      minibox assembly drawing
+gadget.lst      net list, pin list, parts list, etc.
+gen0102.lpr     pcb x-ray diagram
+art01.lpr       pcb artword side 1
+art02.lpr       pcb artwork side 2
+adt0127.lpr     pcb assembly drawing
+dd0124.lpr      pcb drill drawing
+sm0228.lpr      pcb solder mask (side 2)
+sst0126.lpr     pcb silkscreen mask (side 1)
+
+Dave Mills
+University of Delaware
+6 July 1992
+Revised 21 August 1992