When a device understands your request but will not answer it, it returns an exception. That is good news: the device is alive, the framing is right, and the problem is in what you asked for. A timeout tells you much less.
How an exception looks on the wire
The device echoes your function code with the high bit set, then one byte of exception code. A failed FC03 comes back as 0x83 followed by the code.
Request and exception response
request 01 03 00 64 00 02 read 2 holding registers from address 100
response 01 83 02 exception 02: illegal data addressThe codes you will actually see
| Code | Name | What it usually means |
|---|---|---|
| 01 | Illegal function | The device does not implement this function code. Reading input registers from a device that only has holding registers gives this. |
| 02 | Illegal data address | The address, or the address plus the quantity, is outside what the device maps. The most common exception by far. |
| 03 | Illegal data value | The quantity is out of range — asking for 0 registers, or more than 125 in one read. |
| 04 | Server device failure | The device hit an internal error trying to serve the request. |
| 05 | Acknowledge | The request was accepted but will take a while. Rare in reads. |
| 06 | Server device busy | The device is occupied. Retry after a pause. |
| 0B | Gateway target device failed to respond | A Modbus gateway could not reach the downstream device — the unit ID is wrong, or the serial device is off. |
Exception 02 is usually one of four things
- Off-by-one from the 4xxxx convention — you asked for 40100 instead of protocol address 99.
- The wrong table — the value is in input registers and you used FC03.
- A read that starts inside a valid block but runs past its end, because the quantity is too large.
- A gap in the device's map. Many devices map scattered blocks, not a continuous range.
A block read that fails with exception 02 does not mean every register in it is missing. Splitting the block and re-reading the halves finds the boundary, which is how an adaptive scan narrows down a real map.
Exception versus timeout
An exception is an answer. A timeout is silence, and the causes are different: wrong IP or port, wrong unit ID on a serial line, wrong baud rate or parity, a cable, or a device that is simply off. If some unit IDs answer and others time out on the same line, the line is fine and the unit ID is wrong.
Never treat a timeout as a zero. A value that could not be read is not a value; passing it downstream as 0 has caused more than one spurious alarm.
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