Industrial Engineer · phase 5 of 16 · noun

Modbus RTU

Modbus protocol encoding commonly carried over RS-485 serial networks.

The Explain card

Plain English: Modbus RTU is the original Modbus, sent as compact binary frames over a serial line, usually RS-485. One master polls up to 247 slaves by address, one at a time.
Example: A PLC serial port polls three power meters and a small drive on one two wire RS-485 bus. Each device answers only when its address is called.
Why it matters: It is the lowest common denominator of industrial communication, so almost any device speaks it, and its problems are almost always physical. Wrong baud rate, wrong parity, swapped A and B wires, or a missing terminator explain most dead links.
Hook: Old, slow, and everywhere.

How the word is built, where it came from, and what it sits beside in memory.

In a sentence

During commissioning, the technician sets the VFD and PLC to matching Modbus RTU baud rate, parity, device address, and register map.

Why these words

Where it came from

Origin: English technical coinage by Modicon, with RTU formed from Remote Terminal Unit
Entered the language: 1979
What changed: Modbus broadened from a Modicon product name to a protocol-family name, while RTU shifted from an equipment initialism to the label for a transmission mode.

How it is spelled

Pattern: Modbus takes an initial capital followed by lowercase letters
Pattern: RTU keeps all capitals because it is an initialism
Pattern: The phrase is written as two unhyphenated elements
Breaks the pattern: Older specifications sometimes print MODBUS in all capitals

Spelled like

Broken into chunks

What it sits beside

Same subject

Same shape

Modbus transmission modes

industrial communication protocols

Phase 5: PLC Industrial Communications

The protocols that let controllers, drives and operator panels talk, plus the addressing and description files that make them agree on what was said.