industrial//fieldbus//Modbus//Modbus RTU

Modbus over a serial line, normally RS-485. One master and several devices share the bus in a chain, each with a Modbus address; the master polls them in turn.

Modbus over a serial line, normally RS-485. One master and several devices share the bus in a chain, each with a Modbus address; the master polls them in turn.

RS-485 defines the electrical link (the differential A and B pair, the multidrop wiring); Modbus RTU defines how the messages are structured on it, with a compact binary frame and a CRC. Two standards, two layers (protocol stack).

It is the form Modbus takes on the oldest and cheapest equipment in a plant, and the reason a "digital" 2003 meter has no IP address (fieldbus).