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Connect and Configure a 2-Wire Device with an RS-485 Serial Port

Updated Jan 11, 2019

Reported In

Hardware

  • RS-485
  • RS-232
  • Serial Cable

Issue Details

I have a 2-wire serial device (RS-232) that I want to connect to an RS-485 serial port. Since RS-485 communication is usually 4-wire, how do I connect it and how do I configure Windows to communicate with the device?

Solution

RS-485 is, by default, set up for four-wire communication. Two pins send data (TXD+ and TXD-) and two pins receive data (RXD+ and RXD-) as differential signals as shown below.

To connect a 2-wire device, you must connect the 2-wire device's send pin (TXD) to both TXD- and RXD- and the device's receive pin (RXD) to both TXD+ and RXD+. In the image above, jumper wires would be placed between pins 4 and 8, and between pins 5 and 9.

Because the send and receive pins are shorted together on the RS-485 COM port, the COM port must be configured to communicate in 2-wire half-duplex mode. To do this, follow the steps below:

  1. Go to Windows Control Panel

  2. Click on System

  3. In the system menu choose the Hardware tab.

  4. Click on Device Manager

  5. Expand the Ports menu and right-click on the RS-485 serial port in use and choose Properties.

  6. Choose the Port Settings tab and choose Advanced.

  7. Choose the 2-Wire Auto setting from the Tranceiver Mode drop-down menu.

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