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