Modbus is an open communications protocol originally developed in 1978 by Modicon Inc.^[1] for networking industrial PLCs (programmable logic controllers). Since its introduction, it has gained popularity with a number of control device vendors to transfer discrete/analog I/O and register data between control devices.

Apart from PLCs, Modbus-capable devices now include many with both industrial and commercial applications, such as electric-demand meters and lighting controllers, among many others. In addition, Modicon has introduced another variant of the Modbus protocol, Modbus TCP, again as an open protocol. Modbus TCP is becoming increasingly popular because it supports TCP/IP/Ethernet connectivity.

Modicon also developed a related protocol, Modbus Plus®, which is proprietary. Compared to Modbus and Modbus TCP, the Modbus Plus protocol is not widely-used. Modbus Plus is currently not supported by any NiagaraAX driver.