This content is not available in your preferred language.

The content is shown in another available language. Your browser may include features that can help translate the text.

Loop Back Test with a RS-485 Port in Two Wire Mode

Updated Jan 8, 2019

Reported In


  • RS-485

Issue Details

How do I do a loop back test with a RS-485 port in two wire mode (Auto or non-Auto mode)?


Note: This procedure does not apply to the RS-485 ports in a Touch Panel Computer. To perform a Loopback Test in the RS-485 port from a TPC-2206,TPC-2212, TPC-2215 or TPC-2230, refer to Performing a Serial Loopback Test on the COM2 Port of a Touch Panel Computer.

Two-wire Auto Mode:

In RS-485 two wire mode you cannot do a traditional loopback test. A loop back test allows for simple testing of a port by sending and receiving data from the same serial port. To put a 485 port in two wire auto mode, connect pin four with pin eight and pin nine with pin five on the same port. You can connect the send and receive line on the port (4/8 with 5/9) and see the echo, but this will not allow you to send and receive data to the same port as with other loop back tests. To achieve a test setup similar to traditional loopback test you need to have two 485 ports in two-wire auto mode. Connect:
  • pin 1 (port 1) to pin 1 (port 2)
  • pin 4/8 (port1) to pin 4/8 (port 2)
  • pin 5/9 (port 1) to pin 5/9 (port 2)
You can now send data to one port in two-wire mode and receive it on the other port in two-wire auto mode.

Two-wire Echo (non-Auto) Mode:

In RS-485 two wire mode, if you are using auto mode, you cannot see your serial port's reflection by running a basic loopback test. In this case, use echo mode to send and receive data on the same port. Echo mode can be implemented with a serial property node—the Wire Mode property. Property nodes can be obtained by right-clicking on the block diagram and navigating to Programming » Application Control » Property Node. Once you have wired in the VISA resource name, click on Property and navigate to Serial Settings » Wire Mode. Right-click on Wire Mode and select Change All to Write. Right-click on the input terminal and Create » Constant and choose RS485/Wire2-EchoDTR. See the image below for what this function should look like.

In order for this to work properly, you must have the serial port wired in the manner displayed below.



Not Helpful