Frame delivery problem while updating slave data

Posted by / 18-Feb-2019 16:10

Conformance class summary When defining a new protocol from scratch, it is possible to enforce consistency of numbering and interpretation.

MODBUS by its nature is implemented already in many places, and disruption to existing implementations must be avoided.

For function codes where the request and response are of fixed length, the function code alone is sufficient.

For function codes carrying a variable amount of data in the request or response, the data portion will be preceded by a byte count.

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.

This means that when a numerical quantity larger than a single byte is transmitted, the MOST significant byte is sent first.

So a MODBUS message requesting the read of a register at offset 0 would return the value known to the application programmer as found in register 001 (memory type 4 = output register, reference 00001)1.4 Implied length philosophy All MODBUS requests and responses are designed in such a way that the recipient can verify that a message is complete.

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.

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.

For historical reasons, user reference numbers were expressed as decimal numbers with a starting offset of 1.

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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.

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