First of all, let’s talk about the most important thing: What is Modbus? Modbus is a serial communication protocol, i.e. it communicates bit by bit.
To get around, it uses two well-known serial communication interfaces, RS-485 and RS-232.
RS-232 uses 3 wires, so we have a cable that transmits, one that receives and a common, this facilitates the duplex communication to send and receive bits of information in a point to point communication. In this way we can only have one device on one side and one device on the other.
In contrast, RS-485 uses only 2 wires and relies on the potential difference between the two lines to determine which bits are sent in which direction. The receiving side cannot communicate back until the initial communication bit has been received. One of the most important parts of this interface is the ability to communicate with a wide variety of Modbus RTU speaking devices.
How does it do this? Well, Modbus RTU is a differential and cascaded data line so the delivery is done by organizing the devices into master and slave. You can have only one master who initiates the communication and then upon receiving a message the slave can respond with information. No slave can talk unless a message is delivered to them. Since the channel can only support one message at a time, you can only have one master, it would be impossible to have two.
We all always want to know, how do I talk to my devices on Modbus RTU, what should I read? Most of the time we have 16-bit unsigned registers arranged in packets, so we would only have to select the right space to get information. It doesn’t really matter what kind of layout the manufacturer of the device we are communicating with has decided, as long as we know the data structure; after that it is easy to call the space we need to read.
If you want to learn more technical topics about Modbus RTU and many other communication protocols, check out our youtube channel.
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