Modbus Register Addressing: Reconcile Numbering and Zero-Based Offsets

Write the address convention

The Modbus application specification distinguishes data-model numbering from zero-based PDU addressing. Device mapping remains application-specific. Primary reference: addressing model, section 4.4.

A human-facing register label is therefore incomplete without its convention. Some tools display conventional prefixed labels; others expect an offset or another documented notation. Identify what each field means before comparing two integration files.

A hypothetical one-based mapping

Suppose an invented device document explicitly maps its first holding-register label 40001 to protocol offset zero. Under that same documented convention, label 40017 corresponds to offset 16. Entering the printed label as if it were the offset would ask a different addressing question.

This example does not claim every tool accepts 40001-style labels or needs the same manual conversion. Write the device map, data table, displayed label, protocol offset and client-tool convention in different worksheet columns. Preserve the mapping evidence rather than repeatedly adding or subtracting one until a number looks familiar.

Confirm the requested field

Ask the integration owner to relate the desired application variable to its exact mapping and read function. Keep an expected representative value and its meaning as separate acceptance evidence; plausible content alone does not prove the correct address was selected.

This guide gives no live read or write procedure and claims no Modbus interface on HM equipment. It addresses the translation between documented numbering conventions, leaving table selection, data representation, scale and access permissions as their own review questions.

Customer Questions

Primary References

These references support the technical principles discussed in this guide. The worked examples and review questions are educational.

• Primary reference: addressing model, section 4.4