3. Line monitor (bidirectional, half duplex)
This simple cable will be able to monitor both directions with the help of a diode (1N4148) and a resistor (2.2kΩ). You will not be able to monitor full duplex protocols. But it will work as long as both parties doesn't talk at the same time.
25 pin D-Sub
Cable for PC with 25 pin D-Sub connector.
9 pin D-Sub
Cable for PC with 9 pin D-Sub connector.