Before we talk about the key concepts that new users need to know about Modbus, it’s important to understand what we’re talking about. Modbus was invented when factories had no way for their Programmable Logic Computers to communicate with each other. So what does Modbus do? It acts as an information highway between your devices. Keep reading for a brief overview of Modbus for beginners.
Even though Modbus may seem technical and complicated, it has only two data types, known as registers and coils. A coil is a single bit—a zero or a one. Zero means “off,” while one means “on.” Coils represent inputs and outputs with a simple descriptor (for instance, is an LED on or off? A conveyor belt moving or not moving?).
Registers are 16-bit unsigned register data—we just saw your eyes glaze over. That’s okay; it will make more sense when we discuss the two types of registers. Input registers are a digital representation of analog signals, so they could tell you the temperature of a machine, for example. Simply put, holding registers store data.
RTU vs. TCP
Modbus devices come with a host of acronyms, and it’s crucial to understand what they mean. Modbus remote terminal unit (RTU) is a transmission protocol, and it uses the traditional “master-slave” technique for communication. The master is a supervisor for the slave devices.
Modbus transmission control protocol (TCP) allows you to connect your Modbus devices via Ethernet. Without it, Modbus devices can only communicate when they are within a few hundred feet of each other. Ethernet allows you to connect Modbus data acquisition devices that are miles apart!
Most modern Modbus devices use the RTU protocol to encode their messages. What does that mean? With the 16-bit registers we talked about earlier, the RTU protocol delivers the message with the Most Significant Byte first. This allows the protocol to cut to the chase, giving you access to critical information before anything else.
Now that you have a brief overview of Modbus for beginners, get back to your Modbus manual—it should make a little more sense now!