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Performing Serial Loopback Testing with Digi TransPort Products


Steps to perform serial loopback testing with Digi TransPort devices to check the integrity of the serial port:

1.  Insert a loopback plug into the serial port.  

Loopback plug pin-outs:

RS-232 DB-25 Loop Back connector plug consists of a male DB-25 connector 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)

RS-232 DB-9 Loop Back connector consists of a male DB-9 connector 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)

RS-232 RJ-45 Loop Back connector consists of a single 8 pin RJ-45 plug wired as follows:
  • Pin 1 connected to pin 8 (RTS to CTS)
  • Pin 3 connected to pin 6 (TxD to RxD)
2. Use telnet to connect directly to the serial port.  By default 4000 is the first serial port, with 4001 being the second serial port, etc..

telnet (IP_address_of_Digi_TransPort) 4000

Once CONNECTED, any data typed on the telnet session will be displayed twice.  For example, the following shows a telnet connection to the first serial port on a unit with the IP address of Once connected, the word hello is being typed:

telnet 4000


Removing the loopback plug will not display data typed in the telnet session. 

To disconnect from telnet issue Control-] Enter, then type quit at the telnet prompt.
Last updated: Jan 16, 2024

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