Glossary · Industrial communication
Modbus RTU
Also known as: Modbus serial, Modbus RTU over RS-485
German: Modbus RTU
In industrial communication, Modbus RTU is the binary serial transmission mode of Modbus, typically used over RS-485, in which one master polls up to 247 addressed slave devices and frames are protected by a CRC checksum.
- Industrial communication
- Standards
In one sentence
Modbus RTU is the binary serial form of Modbus, usually over RS-485, with one master polling addressed slaves.
Example
A heating controller polls three frequency inverters on an RS-485 line with Modbus RTU at 19,200 baud, 8 data bits, even parity and one stop bit.
How it applies
- Engineering: RS-485 segments need termination at both ends, correct biasing and a common reference. Only one master is allowed per line; slaves answer only when addressed.
- Commissioning: All devices must use the same Baud rate, parity and stop bits, and each slave needs a unique Node address. Response timeouts and inter-frame silent times must fit the slowest device.
- Maintenance: Replacement devices often come with default address and serial settings that differ from the installed ones.
- Documentation: The documentation team states the supported serial settings and defaults, the address range, wiring and termination, and the register map.
Modbus RTU vs. Modbus TCP
Modbus RTU runs over serial lines with a master-slave model and CRC. Modbus TCP runs over Ethernet with a client-server model, uses TCP for error detection and adds the MBAP header. Gateways convert between the two.