For historical reasons, user reference numbers were expressed as decimal numbers with a starting offset of 1. One potential source of confusion is the relationship between the reference numbers used in MODBUS functions, and the ‘register numbers’ used in Modicon PLC’s. Today there are no PLC devices which take advantage of that. The ‘read and write general reference’ function codes are defined to carry a 32 bit reference number, and could be used to allow direct access to data items within a VERY large space. There is no assumption that the data items represent a true contiguous array of data, although that is the interpretation used by most simple PLC’s It is perfectly acceptable, and very common, to regard all four tables as overlaying one another, if this is the most natural interpretation on the target machine in question.įor each of the primary tables, the protocol allows individual selection of 65536 data items, and the operations of read or write of those items are designed to span multiple consecutive data items up to a data size limit which is dependent on the transaction function code. The distinction between inputs and outputs, and between bit-addressable and word-addressable data items, do not imply any application behavior. 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 MODBUS bases its data model on a series of tables which have distinguishing characteristics. 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. 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. Since MODBUS is already widely known, there should be little information in this document which could not be obtained elsewhere. The MODBUS TCP/IP protocol is being published as a (‘de-facto’) automation standard. 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. Specifically, it covers the use of MODBUS messaging in an ‘Intranet’ or ‘Internet’ environment using the TCP/IP protocols. MODBUS TCP/IP is a variant of the MODBUS family of simple, vendor-neutral communication protocols intended for supervision and control of automation equipment. An Introduction to MODBUS TCP/IP Modbus TCP/IP Unplugged – An introduction to Modbus TCP/IP Addressing, Function Codes and Modbus TCP/IP Networking
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