What a Register Map Is
A register map is a vendor-provided document listing all available Modbus registers for a device. It defines the addresses, data types, scaling factors, access modes, and descriptions for every point the device exposes.
Register maps are the single most important document for a Modbus integration. Without an accurate register map, configuration is guesswork.
Typical Register Map Contents
| Column | Description |
|---|---|
| Address | Register number or offset |
| Name | Human-readable point name |
| Data type | UINT16, INT16, FLOAT32, etc. |
| R/W | Read-only, Read/Write, or Write-only |
| Scale | Multiplier or divisor for engineering units |
| Units | °F, kW, PSI, etc. |
Common Problems
[!WARNING] Register maps often don’t state the addressing convention — whether addresses are 1-based references or 0-based offsets. This is the #1 source of integration errors. See Modbus Addressing & Register Reference.
Other issues:
- Outdated revision — register map doesn’t match firmware version
- Missing data types — vendor omits byte order or multi-register layout
- Ambiguous R/W — vendor marks registers as “R/W” when some are actually read-only in practice
Best Practice
- Always request the register map revision matching the installed firmware
- Confirm addressing convention (reference vs offset)
- Confirm byte order and word order for multi-register values
- Validate at least one known-good engineering value after mapping