I made a circuit that lets me monitor an RS232 circuit.
CIRCUIT DESCRIPTION
First remember that in a RS232 circuit, the idle or "1" state is -12V
And it goes to +12V to transmit a zero. This means that TX1, TX2 and TXMON are all mostly at -12V.
D1 and D2 create a -12V line. Note one could add another diode from TXMON.
This is stored on C1 100u 35V Electrolytic. Diodes Schottky 1N5817
R1, R2, R3 (all 4k7) hold the receive lines RX1 RX2 and RXMON at a -12V idle state.
If either of the TX lines TX1 or TX2 starts to transmit, then it will go to +12V and push RX MON to +12V through D5 or D6.
As shown the MON port can talk to port 2 (ie RX2). So via D7 it can force RX2 to +12V.
If you want to transmit from the monitor to port 1, then put a diode from TXMON to RX1
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