Addressing & raw data
PDU offsets, 4xxxx notation, words, bytes, and scaling.
Addressing & raw data
Most Modbus confusion happens before any decoding. Documentation notation is a label used by manuals; the wire protocol carries a zero-based PDU offset. Busloom stores the PDU address and may display a selected documentation notation alongside it.
Translate documentation addresses deliberately
| Common documentation label | Space | Typical PDU offset | Wire operation |
|---|---|---|---|
| 00001 | Coils | 0 | FC01 / FC05 / FC15 |
| 10001 | Discrete inputs | 0 | FC02 |
| 30001 | Input registers | 0 | FC04 |
| 40001 | Holding registers | 0 | FC03 / FC06 / FC16 |
From words to engineering value
Read raw words
Use Register Inspector and keep hexadecimal visible. For a 32-bit value, record both consecutive 16-bit words.
Choose the data type
Select signed/unsigned integer, float, ASCII, hexadecimal, binary, or Boolean according to the manual—not according to a plausible-looking result.
Set word order
For multi-register values, decide which 16-bit word is most significant. Vendors often call this word swap.
Set byte order
If required by the device, swap bytes inside each 16-bit word. Treat byte and word order as independent choices.
Apply engineering conversion
Busloom presents the decoded number after scale and offset. Record the unit and precision only after the numeric interpretation is proven.
Test a second operating point
Change the real process or compare another known value. One plausible sample is not enough to prove endianness and scale.