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Multidrop Networks for RS232, RS422, and RS485

Hardware: Serial>>RS-485, Serial>>RS-232, Instrument Connectivity Other>>USB>>USB-485/4, Serial>>RS-485>>USB-485/4

Problem:
Is it possible to set up a multidrop network using RS232, RS422, or RS485?

Solution:
Multidrop networks are supported by RS422 and RS485. Since the RS232 protocol only allows one transmitter and one receiver, it does not support multidrop.

The 422 standard will support 10 instruments on a multidrop. The 485 standard will support up to 32 instruments in a multidrop system. All of the devices in a multidrop system must use the same protocol. Furthermore, when using modbus communication, all devices on the multidrop network must use either the RTU protocol or the ASCII protocol. You cannot mix modbus RTU and modbus ASCII on the same multidrop network.

Many times an instrument manufacturer will either not program the device with a multidrop protocol or will not release its protocol and insist that its own proprietary software be used. Please refer to the manual of your instrument to see if it is compatabile in a multidrop configuration.

Related Links:
KnowledgeBase 3DRAGUOY: A Quick Comparison of RS-232, RS-422, and RS-485 Serial Communication Interfaces
Developer Zone Tutorial: Serial Communication General Concepts
KnowledgeBase 1M9D8H6Q: Serial Communication Starting Point

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Report Date: 02/28/1995
Last Updated: 06/18/2013
Document ID: 06R9GIRV

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