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



Hardware: Serial>>RS-485>>PCI-485/2

Problem:
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.

Full-Duplex:
In 4-wire transmission (full duplex), 4 wires run from TXD or 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.

Half-Duplex
In 2-wire transmission (half-duplex), TXD+ and 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.

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



Related Links:
KnowledgeBase 41OICGAC: How to Connect and Configure a 2-Wire Device With RS-485
KnowledgeBase 17M6L0TI: What Are the Different Transceiver Modes on the NI RS-485 Serial Cards?
Product Manuals: Serial Quick Reference Guide (July 2013)
Example Program: How to Do Serial I/O and Change Transceiver Modes Programmatically using Visual C++
White Paper: RS-232, RS-422, RS-485 Serial Communication General Concepts

Attachments:





Report Date: 09/27/2004
Last Updated: 11/13/2017
Document ID: 3DQBQ0OY

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