The Modbus is a serial communication protocol that was originally published by Modicon (now Schneider Electric) in 1979 for use with its programmable logic controllers (PLCs). It has since become a de facto standard communication protocol in the industrial electronics field. Its popularity stems from the fact that it is an open protocol (its specification is publicly available and royalty-free), it is relatively simple to implement, and it is robust.
Modbus is designed as a master-slave (or more modernly, client-server) protocol. A client device sends a request to a server device, which then processes the request and returns a response. The protocol defines a simple data model and a set of function codes for accessing that data. For example, a client can use specific function codes to read or write values in the server's memory registers, which could correspond to a power supply's voltage setting or a temperature sensor's current reading. The protocol itself does not define the physical communication layer.
The original and most common implementation is Modbus RTU, which is used over serial lines like RS-485 or RS-232. Another popular variant is Modbus TCP, which encapsulates the Modbus data packets within a TCP/IP frame, allowing it to be used over standard Ethernet networks. Because of its simplicity and widespread adoption, many industrial devices, including programmable power supplies, offer a Modbus interface for easy integration into existing industrial control systems.