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What is Modbus?

BACnet FAQ Optigo Networks Modbus

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BACnet gets compared to a lot of other protocols on the market, including LonWorks, Modbus, and KNX. Over the next few articles, we’re explaining how each of these protocols are different, and how they stack up against each other. In this post, find out about Modbus.

Modbus is a communications protocol that was developed by then-Modicon (now Schneider Electric), and is currently maintained by the Modbus Organization. It’s basically the protocol in the industrial world. It’s also been adopted across other systems, according to Facilities Net, including building automation, transport, and energy. National Instruments describes it as a “request-response” protocol, meaning that it follows a master/slave format: the master devices have control over the flow of information, as they request information that the slave devices then provide. This diagram from National Instruments illustrates the process well.

Send request and read response

The master devices send requests for information, and read responses from slave devices.  

Facilities Net notes that Modbus has a lot of strengths that have contributed to this growth:

  • It’s completely open, with no charges or fees required.

  • It’s also quite simple to understand, and not much hardware is needed to use the protocol.

  • It supports both traditional serial and Ethernet protocols.

  • And its use of TCP/IP means Modbus can easily be used over the Internet.  

As with LonWorks, there is a Modbus certification to ensure installations really do adhere to the standard. Facilities Net notes though that because the protocol is so widespread, there are likely many installations that have not gone through the certification process.

There are some drawbacks to the protocol which National Instruments details, including that “In many cases, a user has no choice but to use it because of hardware constraints such as existing sensors or other devices that must talk over Modbus.”

For more technical information, National Instruments has a few whitepapers on Modbus that are quite detailed and helpful.

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