Reply, exception or timeout

Three outcomes

After a request the client waits. A normal reply echoes the function code and carries the data. An exception reply carries the function code plus 0x80, so 03 comes back as 83, followed by an exception code. Or nothing valid arrives before the response timeout. The first two are answers, and the third is an absence.

The exception is a reply

The codes include 01 illegal function, 02 illegal data address, 03 illegal data value and 04 a failure inside the device. An exception exists only if the server received a valid frame: the right address, a correct check value, and a request it understood well enough to refuse. So it is good news about wiring, baud rate and framing, even though the request failed.

The timeout has many causes

A timeout is no valid reply. A wrong address gives one. So do a baud rate or parity that does not match, no power, a cut cable, a driver released late, and a client timeout shorter than the server needs. A damaged request does too: a frame with a wrong check value is discarded without a reply, so it looks the same as a server that is not there.

Swapped A and B inverts every bit, so no valid frame arrives. It is harmless to the hardware, and swapping back fixes it. Missing ends or bias on a long run produce errors and stray bytes, not by themselves a total silence.

Finding the cause

Because a timeout does not say why, change one thing at a time: the address, then baud rate and parity at both ends, then polarity, then ends and bias, using a request you know is good.

That is the last lesson of the course. The RS-485 book covers the board and its wiring, and RS-485 and Modbus on the ESP32 puts it into code.

Common mistakes

• Reading a timeout as a dead server. It is the absence of a valid reply. The server may be fine and the request may never have arrived intact.
• Treating an exception as a link fault. The link worked, or there would be no exception. Read the code: 02 points at an address and 03 at the request.
• Changing several settings at once. Two changes can hide each other. Change one, test, then the next.