Modbus addressing — 40001 vs. offset 0

The four tables

The function code selects the table, not the address. Function 03 at address 0 always reads the first holding register; function 04 at address 0 reads the first input register. These are different areas of the device (although some vendors map them onto the same data).

40001 is a documentation convention

The specification distinguishes:

• the data model — elements of each table are numbered 1 to n;
• the PDU (message on the wire) — an element is addressed 0 to 65535.

Element number X is sent with address X − 1. The Modicon notation puts the table in the first digit and the one-based element number in the remaining digits.

Five-digit notation covers element numbers 1…9999; six-digit notation (400001…465536) covers the full range.

The number 40001 is never transmitted. "Address 40001" means function 03, starting address 0.

Why off-by-one errors happen

Vendors document registers in different ways:

1. 4xxxx notation — 40101 means address 100.
2. One-based register number — "register 101" also means address 100.
3. Zero-based PDU address — "register 100" or 0x0064 means address 100.
4. Hex addresses — usually zero-based (0x2000), but not always.

SCADA and PLC drivers differ too: some expect 40101, others the offset 100, some have a "zero-based addressing" option. If a value looks like a neighbouring parameter or half of another number, check for an off-by-one first.

Practical trick: find a parameter with a known value in the register map (rated voltage, serial number, firmware version), read a few registers around it and see where the value matches.

Bits inside registers

Many devices pack flags into a register (status word, alarm word). Bits are usually numbered from the least significant: bit 0 = 0x0001, bit 15 = 0x8000. Decode such words with the Bit / Byte Tool.

Example

Meter manual: "Phase A voltage — 30001, FLOAT32, 2 registers". This is function 04, starting address 0, quantity 2. The two registers form one 32-bit value; the word order must be taken from the manual (see Byte and word order).