Hello, my name is Finn and I’m a control system engineer at Australian Control Engineering. So today we’re talking about MODBUS communication.
So, what is MODBUS? MODBUS is a communication protocol that allows multiple devices to communicate with each other on a central network. Basically, you have a master and a slave, so you can have multiple slave devices, each with their own slave address. This basically tells the master where to send the data.
There’s two main MODBUS protocols. There’s MODBUS ASCII and there’s MODBUS RTU. Modbus RTU is mainly a binary system to basically send a message across the network. ASCII uses characters, basically and basic ASCII table that you’ve seen in the past. This uses characters as the main medium to send data.
There’s mainly three cables as well that is worth mentioning. There’s MODBUS 232,485 and 422. The main reason you’d choose one over the other is length. Modbus 232 is mainly used for short communication. This is maybe under 15 meters or so. 485 is used for longer wire lengths.
So, if you need to send data crossword sites beyond, say, 15 meters, it’s a wiser choice to use this type of cable. We also have MODBUS TCP/IP. This is mainly using through an Ethernet cable. Previously we’ve used sending data over serial, which is generally a more reliable method, but the ease of using Ethernets shouldn’t be ignored either. And there is also its own sort of infrastructure for that as well.
Modbus is actually a really versatile and common communication protocol. Basically, all RTUs or PLCs generally have this as at least a base protocol. So, while you can have communication between, say, a PLC and a RTU or RTU to RTU, we generally use the master and slave relationship between SCADA as the master and a slave device as the RTU.
MODBUS is incredibly easy to maintain. It’s quite simple. Significantly simpler than other protocols like DNP3 or where you have a lot of data and a lot of other data that comes with it. There’s seven PDUs within Modbus or more or less registers. We have the coils, input registers, holding registers and discrete inputs.
Each one has their own application. Commonly used is coils, which we generally use for motor control or general outputs. So, this is similar to a Ladder Logic coil as well, and most common, only we use holding registers. So, this allows read and write to a single MODBUS address. And this can go basically both ways what we most commonly use within our industry.
If you’d like to know more about MODBUS or any other communications protocol, feel free to reach out to us, and we’re more than happy to help. So, thanks for watching and until next time.
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