Modbus

Modbus is one of the oldest and most widely deployed industrial communication protocols, created by Modicon in 1979 to connect programmable logic controllers (PLCs) and field devices. It defines a simple, openly published request/response model in which a client (master) reads and writes registers and coils on servers (slaves), and its longevity comes from being easy to implement and royalty-free.

• Simple data model - Everything is expressed as coils, discrete inputs, holding registers, and input registers.
• Multiple transports - Runs as Modbus RTU and ASCII over serial lines and as Modbus TCP over Ethernet and IP networks.
• Client/server exchange - A master device polls one or more slave devices with compact function-code messages.
• Vendor-neutral and open - A published specification any manufacturer can implement without licensing fees.

In real operations Modbus is the workhorse that gets sensor readings and actuator commands off the factory floor, and modernization efforts increasingly wrap it behind REST or streaming APIs so operational data can flow into cloud analytics and dashboards. That gateway pattern — Modbus at the edge, an OpenAPI-described interface above it — is how legacy industrial systems are made observable, governable, and, increasingly, reachable by AI agents. Note that classic Modbus carries no authentication or encryption, so any bridge to the wider network has to add the security the protocol itself never defined.