The tradeoff is that, because it uses TCP/IP protocol for all messages, it is slow compared to other Ethernet industrial protocols – but still fast enough for monitoring applications.įigure 1: Modbus message frame Figure 1.1: Modbus function codesĪs already noted, Modbus is a simple master-slave protocol. As a result, Modbus TCP/IP is also very simple to implement. There is a bit more to it than that, but this is essentially what Modbus did. One simple way of thinking about Modbus TCP/IP is to picture it as simply encapsulating a Modbus RTU packet within a TCP/IP packet. In general, RTU is more popular, particularly in new installations. ASCII encodes the message in ASCII characters, while RTU uses bytes, thus increasing the protocol’s throughput. The only difference is in the encoding of the data. Modbus RTU is really just a small variation on the Modbus ASCII protocol.The message frame and function codes, shown in Figures 1 and 1.1, are very simple. The message frame can be up to 252 bytes in length, and up to 247 addresses are possible.

All slaves are polled on demand by the master, and there is only one master.

Modbus ASCII was the first Modbus and is a serial protocol, typically running on either the RS-232 or RS-485 physical layer.

The term “Modbus” typically refers to one of three related protocols: Modbus ASCII, Modbus RTU, or Modbus TCP/IP In comparison, Profibus’ specification document is thousands of pages long.

The specification document is fewer than 100 pages in length, which is a good indication of the protocol’s low level of complexity. As a result, there are now quite a few variations in the field. When it was published, a number of companies started using it, creating different interpretations and modifications of the original specification. However, it was later published royalty-free so that anyone could use it.įinally, Modicon made it an open protocol. When it was first introduced, it was a proprietary protocol that only Modicon could use. “Simple” is a key descriptor for Modbus – and also its biggest strength. It was originally designed in the mid-1970s by Modicon as a way to link intelligent devices with PLCs using a simple master/slave concept. Modbus is the “granddaddy” of industrial communication protocols.

Pm 5340 modbus rtu over tp