I have a device that interfaces with a motor controller over RS-485 via an RJ45 socket. Currently, the port is a standard 4x2 RJ45 connector with the RS-485 using pins 2 and 7.

Eagle Circut

Currently my startup procedure tries to connect to the controller 3 times before notifying the user Device Cannot be found. Check cable.


I would like a way to electronically check if the cable is plugged in. By any chance, do 10-pin RJ45 sockets exist where two pins short if a cable is plugged in? If so, what is this feature called? If not, what is a solution used to detect if a cable is present in the system?

  • 2
    \$\begingroup\$ You could short one of the pairs at the motor controller, and then check to see at the other end if there is a short circuit \$\endgroup\$
    – TemeV
    Commented May 31, 2019 at 11:50
  • \$\begingroup\$ Yes, assuming no other solutions, that's the way I plan to go. Only issue is the controller is not easy to modify, so I would need to add a small PCB and use two Ethernet cables. \$\endgroup\$
    – GentlemanS
    Commented May 31, 2019 at 11:53
  • \$\begingroup\$ are both ends of the cable pluggable? ,... if not, then you only need to short two wires in the cable \$\endgroup\$
    – jsotola
    Commented May 31, 2019 at 17:07

1 Answer 1


Depending on the transceivers you have you might be able to go with something like these:



It seems you don't, but if you had full access to modify both sides you could also try something even easier:



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