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How to make an RS-232 loopback plug

Loopback plugs are very useful for initial serial port troubleshooting, to ensure the serial port in question is functional.

A DB-9 loopback plug should have a female DB-9 connector and should be wired as follows:

Pin 8 connected to pins 7 and 9 (CTS to RTS and RI)
Pin 3 connected to pin 2  (TxD to RxD)
Pin 4 connected to pins 6 and 1 (DTR to DSR and DCD)

A DB-25 loopback plug should have a female DB-25 connector and should be wired as follows: 

Pin 4 connected to pins 22 and 5 (RTS to RI and CTS)
Pin 2 connected to pin 3 (TxD to RxD)
Pin 20 connected to pins 6 and 8 (DTR to DSR and DCD) 

An 8 pin RJ-45 loopback plug should be wired as follows:

Pin 2 connected to pin 7 (RTS to CTS)
Pin 4 connected to pin 5 (TxD to RxD)
Pin 1 connected to pin 8 (DTR to DSR)

A 10 pin RJ-45 loopback plug (shipped with some Digi products) is wired as follows:

Pin 3 connected to pins 1 and 8 (RTS to RI and CTS)
Pin 5 connected to pins 6 (TxD to RxD)
Pin 9 connected to pins 2 and 10 (DTR to DSR and DCD)
Last updated: Aug 23, 2017
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