Exception 03 (0x03): Illegal Data Value

A value in the request body is outside what the server allows. This is about the contents of the request, commonly the quantity field, and NOT about whether the value you are writing is sensible for the process.

Start free Modbus → canonical

Diagnosis

When you hit this

Requesting more than the protocol maximum: 125 holding registers or 2000 coils in one read.
Writing a value outside a register's declared range, such as a scaled setpoint sent unscaled.
A byte count that disagrees with the register count in a write-multiple request, usually a client library bug or a hand-built frame.

Resolution

How to fix it

Cap the quantity
Keep reads at or below 125 registers and 2000 coils. Some devices cap lower; the manual states it.
Check scaling before writing
A register documented in tenths wants 235 for 23.5. Sending 23.5 scaled wrong is a legitimate 03.
Verify the byte count field
For FC16, byte count must equal register count times two. Log the raw frame if you are unsure.

Normalization

What Forge does about it

This is a configuration or addressing mistake on the wire, upstream of anything Forge sees. Where Forge helps is the step after you fix it: once the point returns data, its vendor tag name maps to a canonical field automatically, so you do not also have to hand-maintain a name table for every point you just re-addressed.