Specifically, it covers the use of MODBUS messaging in an ‘Intranet’ or ‘Internet’ environment using the TCP/IP protocols.

The most common use of the protocols at this time are for Ethernet attachment of PLC’s, I/O modules, and ‘gateways’ to other simple field buses or I/O networks. The MODBUS TCP/IP protocol is being published as a (‘de-facto’) automation standard. Since MODBUS is already widely known, there should be little information in this document which could not be obtained elsewhere. However, an attempt has been made to clarify which functions within MODBUS have value for interoperability of general automation equipment, and which parts are ‘baggage’ from the alternate use of MODBUS as a programming protocol for PLC’s. This is done below by grouping supported message types into ‘conformance classes’ which differentiate between those messages which are universally implemented and those which are optional, particularly those specific to devices such as PLC’s. MODBUS bases its data model on a series of tables which have distinguishing characteristics. Single bit, alterable by an application program, read-writeġ6-bit quantity, provided by an I/O system, read-onlyġ6-bit quantity, alterable by an application program, read-write Single bit, provided by an I/O system, read-only The four primary tables are input discretes

The distinction between inputs and outputs, and between bit-addressable and word-addressable data items, do not imply any application behavior.

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