What pins should I connect when performing a loop back test? Do all pins need to be connected?

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Only the data pins (TX and RX) are necessary for a loop back test. Unless you need to test the modem control signals (RTS, CTS, DCD, DTR, DSR, and RI) there is no need to connect them. Please refer to your manual for the correct pin assignments for your serial device.

For RS-232 serial devices:

  • Connect TX to RX
  • Connect RTS to CTS to RI
  • Connect DTR to DCD to DSR

For RS-422, RS-485 serial devices:

  • Connect TX+ to RX+ and TX- to RX-
  • Connect RTS+ to CTS+ and RTS- to CTS-

For RS-530 serial devices:

  • Connect TX+ to RX+ and TX- to RX-
  • Connect RTS+ to CTS+ and RTS- to CTS-
  • Connect DTR+ to DCD+ to DSR+ and DTR- to DCD- to DSR-