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

Updated Oct 9, 2018

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?

Solution

For 2- wire and 4- wire transmission, you need to use TXD+, TXD-, RXD+, and RXD-. The + and - represent two separate wires which enable differential communication across the cable.

The pintout for a DB-9 connector is shown below:
 

Additional Information

Full-Duplex
In 4-wire transmission (full duplex), 4 wires run from Transmit Data (TXD) or Receive Data (RXD) on the master to the corresponding TXD or RXD on the slave(s). This allows simultaneous bi-directional communication.

1. Master to Slave(s):
  • TXD+ ↔ RXD+
  • TXD- ↔ RXD-
2. Slave(s) to Master
  • RXD+ ↔ TXD+
  • RXD- ↔ TXD-

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, see the related links section.

Half-Duplex
In 2-wire transmission (half-duplex), Transmit Data (TXD+) and Receive Data (RXD+) on the master are wired together to TXD+ and RXD+ on the slave(s). TXD- and RXD- on the master are wired together to TXD- and RXD- on the slave(s). Only one pair of cables connects the master and the slave(s). Since the RS-485 protocol requires communication to flow in one direction at a time, 2-wire half-duplex mode results in non-simultaneous bi-directional communication.

Terminating Resistors
In all cases terminating resistors must be placed at the extreme end or ends of a cable. As signals travel through cabling it creates reflections, terminating resistors aim to reduce and minimise these reflections. In the case of circuits with only 1 or 2 slaves terminating resistors must still be used.

 
 

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