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What is Modbus?


Modbus is one of the most widely used standard protocols in the automation industry. The protocol is open, does not require licensing and is easy and low cost to implement for device manufacturers.

There are three main versions;

  • Modbus ASCII - this is the least common
  • Modbus RTU - this is the most common, and sometimes also called Modbus 485 or RS485
  • Modbus TCP - also common, but uses Ethernet hardware instead of serial port

The ezeio system supports Modbus RTU.

Please note that 'RS485' by itself is not the same as Modbus. If your device says it has a RS-485 port, but does not mention Modbus specifically, it probably doesn't support Modbus at all.

The protocol is very versatile, but the downside is that the standard only dictates HOW to communicate, and not WHAT to communicate. Thus, when a device communicates a value, the recipient just has to know what that value means.

Every Modbus device publishes a list of 'registers', where each register has an address, a type and is documented with the meaning, unit and range. This register list cannot be discovered automatically, so to communicate between two devices, they both have to have the same register list programmed into them.

The Modbus standard also does not specify exactly how to connect two devices. The type of connection may be screw terminals, modular jack, DB9 connector or something else. The communication speed is recommended, but may be changed, and there are other parameters that are left to the manufacturer to interpret the way they want. There is even some confusion around the polarity of the data bus (usually called 'A'/'B' or 'D0'/'D1' or '-'/'+') so you may need to do some trial-and-error troubleshooting to get things going.

So even if all your hardware supports Modbus, you have to make sure they are all set to the same parameters and are connected together in the correct way. Every device looks different, and have different methods for configuration. Some use DIP-switches or Hex-knobs. Some have a display and menus/buttons, and some require a computer connected via USB or serial port and special software. There are also devices that are set-up via Modbus, which can create a chicken-egg problem; in order to configure the Modbus settings to work you may need to communicate via Modbus...

Please consult the documentation for your device and seek support with the manufacturer of the device.

When a device is listed as supported by the ezeio, we (eze System) has usually spent some time with the device and figured out how to configure it, connect it and get it to work with the ezeio. All supported devices are listed when you go to the Configuration tab and click Add Device.

If you device is not listed. Please read this article. Please contact eze System for more information.

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Category: General
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