Set Up 2-Wire (Half-Duplex) Communication with RS-485 Port

Updated Apr 14, 2021

Environment

Hardware

  • RS-485

Operating System

  • Windows

How do I wire my 2-wire device to a RS-485 port? 

RS-485 is set up for four-wire communication by default. To connect a 2-wire device, you will need to short the transmit and receive signals together on the RS-485 port. Note that this is necessary to connect RJ45 (8-pin modular jack) ports to RJ50 (10-pin modular jack) ports. Follow the instructions below.

 
  1. On the RS-485 connector, place jumper wires between TXD+ and RXD+, and between TXD- and RXD-. Refer to the pins below based on your connector type. 
     Connector   Pins Diagram
     DB-9 Pin 8 and Pin 4 (TXD+ and RXD+)   
     Pin 9 and Pin 5 (TXD- and RXD-)
     
     DB-25 Pin 5 and Pin 20 (TXD+ and RXD+)   
     Pin 22 and Pin 7 (TXD-and RXD-)
     
     RJ-50 Pin 3 and Pin 7 (TXD+ and RXD+)   
     Pin 2 and Pin 6 (TXD- and RXD-)
     
 
  1. Wire the 2-wire device's send pin (TXD) to both TXD- and RXD-. Wire the device’s receive pin (RXD) to both TXD+ and RXD+. Consult your device’s user manual for reference.
 
  1. Configure the COM port to communicate in 2-wire Auto mode by referring to Setting the Transceiver Wire Mode of Ports on National Instruments Serial Boards.
 
  1. Test communication with VISA Interactive Control. Refer to your device’s user manual for a supported command.