industrial//industrial communication//fieldbus//Modbus//register map

A register map is the vendor document that says what each Modbus address means. The client must know two things: which address to read and what data type it holds.

A register map is the vendor document that says what each Modbus address means. The client must know two things: which address to read and what data type it holds.

Modbus sends numbers and the register map says what they are.

The device reports 217 at 40001, 64 at 40002 and 1 at 40003. The register map says that 40001 is temperature in tenths of a degree (21.7 C), 40002 is pressure in tenths of a bar (6.4 bar), 40003 is the pump running flag.

Modbus contributes practically no semantics. Without the map you receive numbers and cannot tell what they are; scaling factors, signedness, byte order and 32-bit values split across two registers are all conventions of the manufacturer.

Knowing the downstream device's register manual is therefore the first step of any integration: the PLC or gateway logic is programmed against those addresses, and every device of another brand has a different map (system integrator).

The register map is the smallest possible data model, and the contrast that explains why OPC UA exists: an address and a raw number against a named node with value, unit, quality and timestamp.