Exception 01 (0x01) — Illegal Function

The server received a function code it does not implement — or does not implement for the addressed unit. It is a statement about the function, not about the address or the value.

Start free Modbus → canonical

Diagnosis

When you hit this

Reading holding registers with FC03 from a device that only exposes input registers (FC04), or vice versa. This is the single most common cause.
Using FC23 (read/write multiple) or FC43 (device identification) against a low-cost slave that implements only the basic four.
Talking through a gateway that forwards to a serial unit which does not support the function, even though the gateway itself does.

Resolution

How to fix it

Confirm the function the device implements
Check the register map in the device manual: coils are FC01, discrete inputs FC02, holding registers FC03, input registers FC04. A register listed as read-only is almost always FC04, not FC03.
Match the function to the register table
Change the client's function code rather than the address. An address that works under FC04 and fails under FC03 is exception 01, not exception 02.
Check the unit ID
Over TCP the unit ID is often ignored, but a gateway uses it to select a serial slave. The wrong unit ID can surface as 01 rather than a timeout.

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.