Industry Standard For Communication Protocols in The Industrial Field: Modbus Communication Protocol

Modbus is a serial communication protocol published by Modicon (now Schneider Electric) in 1979 for communication with Programmable Logic Controllers (PLCs). Modbus has become a de facto industry standard for communication protocols in the industrial sector and is now a commonly used connection method between industrial electronic devices. The main reasons why Modbus is more widely used than other communication protocols are: • It is publicly available and has no copyright requirements. • It is easy to deploy and maintain. • For suppliers, there are not many restrictions on modifying and moving local bits or bytes. Modbus allows multiple (approximately 240) devices to be connected on the same network for communication. For example, a device that measures temperature and humidity can send the results to a computer. In Supervisory Control and Data Acquisition (SCADA) systems, Modbus is usually used to connect monitoring computers and Remote Terminal Units (RTUs). Protocol versions:Currently, there are versions of the Modbus protocol used for serial ports, Ethernet, and other networks that support Internet protocols. Most Modbus device communications are conducted through the serial EIA-485 physical layer. For serial connections, there are two variants that differ slightly in numerical data representation and protocol details. Modbus RTU is a compact way of representing data in binary, while Modbus ASCII is a human-readable and verbose representation. Both variants use serial communication. Commands/data in RTU format are followed by a checksum using cyclic redundancy check, while ASCII format uses a checksum with longitudinal redundancy check. Nodes configured as the RTU variant will not communicate with nodes set as the ASCII variant, and vice versa. For connections via TCP/IP (such as Ethernet), there are multiple Modbus/TCP variants, which do not require checksum calculation. All three communication protocols are identical in terms of data models and function calls; only the encapsulation methods are different. Modbus has an extended version called Modbus Plus (Modbus+ or MB+), but this protocol is proprietary to Modicon and different from Modbus. It requires a dedicated coprocessor to handle high-speed token rotation similar to HDLC. It uses 1 Mbit/s twisted-pair cables, and each node is equipped with a conversion isolation device that uses conversion/edge triggering instead of voltage/level triggering. Connecting Modbus Plus to a computer requires a special interface, usually a board supporting ISA (SA85), PCI, or PCMCIA buses.