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RS-485 Pins for 2- and 4-Wire Transmission

Updated May 27, 2022

Reported In

Hardware

  • RS-422/RS-485
  • Serial Cable
  • PXI-8433/4
  • PXI-8433/2
  • PCI-8433/4

Issue Details

  • I am using RS-485 serial communication, what pins do I need to connect for 2- and 4- wire transmission?
  • What are the digital output pins on the RS-485 port to communicate with another device?

Solution

Full Duplex (4-Wire)

For 4-wire transmission, you will need to wire the following signals together from the master to slave device: 
 RS-485 Signal (Master) Third Party Device Signal (Slave)
 TXD+ RXD+
 TXD- RXD-
 RXD+ TXD+
 RXD- TXD-

The corresponding pins will depend on your device’s user manual and the RS-485 cable connector type. For NI serial hardware connector pinout diagrams, refer to the Serial Quick Reference Guide. 


Half Duplex (2-Wire)

For 2-wire transmission, you will need to short the transmit (TXD) and receive (RXD) signals together on the RS-485 port. Refer to Set Up 2-Wire (Half-Duplex) Communication with RS-485 Port for the pinouts and more details. 


 

Additional Information

NI-Serial directly supports 4-wire full duplex operation on peer-to-peer RS-485 networks. Multidrop RS-485 networks require additional software development. For cabling solutions, refer to NI Serial Cable product page.

To prevent reflections for both 2- and 4-wire transmission, you must properly terminate the transmission lines. For more information, refer to RS485 Termination. 

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