what is modbus

Modbus is a client/server data communications protocol in the application layer of the OSI model. It was originally published by Modicon in 1979 for use with its programmable logic controllers (PLCs) . In simple terms, it is a method used for transmitting information over serial lines between electronic devices. The device requesting the information is called the Modbus Client, and the devices supplying information are Modbus Servers. In a standard Modbus network, there is one Client and up to 247 Servers, each with a unique Server Address from 1 to 247. The Client can also write information to the Servers. Modbus is an open protocol, meaning that its free for manufacturers to build into their equipment without having to pay royalties. It has become a standard communications protocol in the industry and is widely used to connect industrial electronic devices. It is typically used to transmit signals from instrumentation and control devices back to a main controller or data gathering system, for example, in supervisory control and data acquisition (SCADA) systems. Modbus is a request-response protocol implemented using a master-slave relationship, where communication always occurs in pairs, with one device initiating a request and the other responding. It is commonly used in process automation communication and is defined as a master/slave protocol, meaning a device operating as a master will poll one or more devices operating as a slave.