Glossary · Industrial communication
Modbus
Also known as: Modbus protocol
German: Modbus
In industrial communication, Modbus is an open request-response application protocol, originally published by Modicon in 1979, in which a client reads and writes coils and registers in a server. It runs over serial lines (Modbus RTU, Modbus ASCII) and over TCP/IP (Modbus TCP) and is maintained by the Modbus Organization.
- Industrial communication
- Standards
In one sentence
Modbus is an open request-response protocol for reading and writing coils and registers, over serial lines (RTU) or TCP/IP.
Example
A PLC reads the energy values of ten power meters by requesting their holding registers over Modbus every second.
How it applies
- Engineering: Modbus is simple and widely supported, which makes it the common denominator for meters, drives, inverters and building equipment. The protocol only knows four data tables (coils, discrete inputs, input registers, holding registers); what a register means is defined by each device.
- Commissioning: Register addresses, data types (16-bit registers, 32-bit values spread over two registers, byte and word order) and scaling must be agreed. Off-by-one errors between 0-based and 1-based addressing are common.
- Security: Classic Modbus has no authentication or encryption. Modbus devices belong behind firewalls in protected zones; Modbus/TCP Security (with TLS) exists but isn't widely deployed.
- Documentation: The documentation team provides a complete register map with address, access, data type, unit, scaling and value range. That map is the actual interface of the device.
Protocol vs. data model
Modbus illustrates the difference clearly: the protocol defines how to read register 40001, but not what the value means. The register map provides the Data model; without it, the protocol is useless.