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Check the HMI RS232/485/422 port COM with a loopback test on RX and TX pins

What is a loopback test

A loopback test involves sending and receiving data from a single COM port. A signal is sent by a device and returns to it after traversing part or all of a network.

This is a primary method to troubleshoot serial communication issues and ensure that a COM port is functioning properly.

The loopback test provides information about:

  • problems present on a network of serial devices
  • the existing means to determine if a peripheral is functioning correctly


How to Perform a Loopback Test

  • RS-232: Connecting the transmit pin 3 (TX) to the receive pin 2 (RX)
  • RS-422 and RS-485: Connect TX+ pin to RX+ and TX- to RX-

Wirring of the HMI serial port

HMI D-sub 9 pins
1 2 3 4 5
6 7 8 9
View from outside

RS232, RS485 and RS422 pins


HMI-Tool test project

Download the project for testing

Wirring issue

If you don’t see this indicator light blinking on your HMI, it’s probably a wiring cable problem.