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Design IdeasDecember 5, 1996 |
Rick Downs, Dallas Semiconductor, Dallas,
TXWhen debugging serial links between computers, you need to monitor the data
moving in both directions across the serial link. As long as the data is
half-duplex--only one direction "talks" at a time--the circuit in
Figure
1can capture data on both the transmit and
receive lines of the RS-232C connection and retransmit this data to a third
computer. This third computer can monitor the data using a simple terminal
program that allows you to watch the data on a screen and capture the data to a
disk file. The monitoring computer receives but does not transmit information.
The TX line of the monitoring computer provides power for the circuit, which is normally in a marking state of 5 to 12V. The circuit ties IC1's VCC to ground and ties this IC's ground pin to the negative voltage from the monitoring computer's TX line. These connections place between 5 and 7V across IC1, so the IC operates normally. However, the logic signals swing from 0 to 5V. As with IC1, the circuit also connects VCC of the two CD4011 NAND gates to ground and connects the ground pins to the negative supply. The output from these gates then sends the data back into IC1 for retransmission to the monitoring computer.
Because the maximum supply voltage for IC1 is 7V, you can only use this circuit with a monitoring computer whose RS-232C port swings no further than 7V. This level is common for most laptop computers and many desktop computers. Many computers with 12V drivers may work also, because the circuit draws about 10 mA and the loading then causes the supply voltage to drop to an acceptable range. (DI #1959)
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