Identify the Modbus data table as well as the address when defining a requested field.
Name the access table
Modbus defines coils and discrete inputs as bit tables, and holding and input registers as 16-bit word tables, with different access roles. Primary reference: data model, section 4.3.
The table name and applicable function belong to the requested field. An offset alone does not identify its content. Protocol table roles do not prove the physical application or that a particular device implements every possible operation.
A hypothetical repeated offset
Imagine a fictional device map has offset 7 in a holding-register table and offset 7 in a discrete-input table. The repeated number does not mean the two entries are the same variable. One request concerns a word representation; the other concerns a bit under its stated map.
Write desired variable, table, offset, read function, representation and documented access on the worksheet. Keep returned bit states distinct from an integer packed with application-defined flags. A nonzero register value should not casually become one named switch state without the corresponding field definition.
Use the selected device map
Ask the supplier which variables occupy each implemented table and how the consumer should interpret them. Access labels explain a protocol role; operating authorization and actual machine behavior require their own specification. Avoid inferring that every holding register is a safe editable setting.
This guide contains no live write or read instruction and claims no HM Modbus capability. It separates the namespace/access-table question from numbering conventions and multi-register decoding. The final interface record should identify one unambiguous application field rather than an address number that could refer to several different requests.
Customer Questions
Can the same offset occur in different tables?
The table distinguishes the request.
Are all tables word-sized?
Coils/discrete inputs are bit tables.
Does holding-register access authorize an operating change?
Actual application authorization is separate.
Primary References
These references support the technical principles discussed in this guide. The worked examples and review questions are educational.
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