Knowledge base
Modbus map: old and new address notation
The same Modbus point can be documented as 40001, Holding Register 1, FC03 address 0 or 0x0000. Translating that correctly prevents wrong values and Illegal Data Address errors.
Old notation
Old maps use prefixes such as 0xxxx, 1xxxx, 3xxxx and 4xxxx to describe the data area. This is convenient for a manual, but the prefix is not sent as the protocol address.
New notation
In the protocol, the function code selects the area, and the PDU carries a zero-based address. For example, old 40070 often becomes FC03 with address 69.
| Old | Area | Function | PDU address |
|---|---|---|---|
00001 | Coils | 01 | 0 |
30001 | Input Registers | 04 | 0 |
40001 | Holding Registers | 03 | 0 |
A modern map should also include data type, scale, word order and access mode.
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