The standard includes MODBUS application layer protocols and service specifications used in two communication procedures:
● MODBUS on serial link. MODBUS serial links depend on TIA/EIA standards: 232-F and 485-A.
●MODBUS over TCP/IP. MODBUS TCP/IP depends on IETF standards: RFC793 and RFC791.
MODBUS over serial links and TCP/IP are two communication procedures specified according to the corresponding ISO layer models. The following figure highlights the main parts of the standard. Green boxes indicate specifications. Gray boxes represent existing international standards (TIA/EIA and IETF standards).
The MODBUS standard is divided into three parts
The first part (“Modbus Protocol Specification”) describes MODBUS transactions.
The second part (“Implementation Guide for MODBUS Message Transmission on TCP/IP”) provides reference information to help developers implement the MODBUS application layer on TCP/IP.
Part 3 (“Implementation Guide for MODBUS Message Transmission on Serial Links”) provides reference information to help developers implement the MODBUS application layer on serial links.
