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What Pins Are Needed for 2- and 4- Wire Transmission with RS-485 Serial Communication?

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

What pins are needed for 2- and 4- wire transmission with RS-485 serial communication?

For 2- wire and 4- wire transmission, TXD+, TXD-, RXD+, and RXD- are used. The + and - represent two separate wires which allows a differential communication.

In 4-wire transmission (full duplex), 4 wires run from TXD or RXD on the master to the opposite (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-

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 slaves. 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 is shown below:

Related Links:
KnowledgeBase 41OICGAC: How do I 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++


Report Date: 09/27/2004
Last Updated: 08/26/2014
Document ID: 3DQBQ0OY

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