industrial//industrial communication//fieldbus//Modbus//Modbus RTU

Modbus RTU is the variant of Modbus that runs over a serial line, normally RS-485, and it is how the protocol reaches the oldest and cheapest equipment in a plant. One master and several devices share the bus in a chain, each with a Modbus address; the master polls them in turn.

Modbus RTU is the variant of Modbus that runs over a serial line, normally RS-485, and it is how the protocol reaches the oldest and cheapest equipment in a plant. 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. Frames are separated by silence (at least three and a half character times), so the timing of the serial port matters as much as the bytes, and a USB adapter that buffers can break a bus that worked with a native port. On a two-wire bus the adapter must also release the line the moment it finishes sending; one that switches direction late talks over the device's reply. Wiring faults dominate: a missing or doubled terminator, a star topology, A and B swapped. They are two standards on two layers (protocol stack). It is also the reason a digital meter from 2003 has no IP address (fieldbus).