Being the industry standard since 1979, millions of devices use MODBUS for reliable industrial communication. MODBUS is an application level protocol positioned at level 7 of the Open Systems Interconnection (OSI) model. It supports client/server communication between devices connected on RS485/RS422 or TCP/IP networks.
MODBUS is a request/reply protocol where different functions provide a set of standardized services and, if required, can be extended by application-specific services. The standardized services include a pre-defined data model suitable for the exchange of analog and digital information.
The four primary elements of the data model are:
- 1-bit read only discrete inputs (e.g. the state of a digital input)
- 1-bit read/write coils (e.g. an digital output)
- 16-bit read only input registers (e.g. the value of an analog input)
- 16-bit read/write holding registers (e.g. the analog output value of an actuator)
In addition, the standard includes a large set of supporting diagnostic services and additional data types (File records, FIFOs, etc.).