ERROR: request ask for id= but got id=

[seud0nym]

Hello.

I have a couple of questions about this error message.

I have a user of my project who is seeing these messages very frequently. In my environment, I have only ever seen these errors when there is a connection failure or communication issue. The application still seems to work as expected for them, but they are obviously concerned by the large number of errors they are seeing.

I am certain that the requested device must be the correct id, as no-one else has reported this error. In this users environment, device ids 247, 1 and 2 would be used.

This is an example log:

>>>>> recv: 0x13 0x64 0x0 0x0 0x0 0x7 0xf7 0x4 0x4 0xff 0xff 0xff 0xff extra data: 
>>>>> send: 0x13 0x65 0x0 0x0 0x0 0x6 0x2 0x4 0x7d 0xd 0x0 0x1
>>>>> recv: 0x13 0x65 0x0 0x0 0x0 0x5 0x2 0x4 0x2 0x0 0x0 extra data: 
>>>>> send: 0x13 0x66 0x0 0x0 0x0 0x6 0x1 0x4 0x79 0x3a 0x0 0x1
>>>>> recv: 0x13 0x65 0x0 0x0 0x0 0x5 0x2 0x4 0x2 0x0 0x0 extra data: 
2025-09-02 06:45:50,463 WARNING transaction:235 ERROR: received pdu without a corresponding request, IGNORING
2025-09-02 06:45:50,753 ERROR base:78 ERROR: request ask for id=1 but got id=2, Skipping.
>>>>> recv: 0x13 0x66 0x0 0x0 0x0 0x5 0x1 0x4 0x2 0x0 0x0 extra data: 
>>>>> send: 0x13 0x67 0x0 0x0 0x0 0x6 0x1 0x4 0x79 0x36 0x0 0x1
>>>>> recv: 0x13 0x67 0x0 0x0 0x0 0x5 0x1 0x4 0x2 0x0 0x27 extra data: 
>>>>> send: 0x13 0x68 0x0 0x0 0x0 0x6 0x2 0x4 0x7d 0xe 0x0 0x1
>>>>> recv: 0x13 0x68 0x0 0x0 0x0 0x5 0x2 0x4 0x2 0x0 0x0 extra data: 
>>>>> Repeating....
>>>>> send: 0x13 0x69 0x0 0x0 0x0 0x6 0x2 0x4 0x7d 0x3 0x0 0x2
>>>>> recv: 0x13 0x69 0x0 0x0 0x0 0x7 0x2 0x4 0x4 0x0 0x0 0x0 0x0 extra data: 
>>>>> send: 0x13 0x6a 0x0 0x0 0x0 0x6 0x1 0x4 0x77 0x7b 0x0 0x2
>>>>> recv: 0x13 0x69 0x0 0x0 0x0 0x7 0x2 0x4 0x4 0x0 0x0 0x0 0x0 extra data: 
2025-09-02 06:45:55,938 ERROR base:78 ERROR: request ask for id=247 but got id=2, Skipping.
>>>>> send: 0x13 0x75 0x0 0x0 0x0 0x6 0xf7 0x4 0x75 0x6a 0x0 0x2
>>>>> recv: 0x13 0x75 0x0 0x0 0x0 0x7 0xf7 0x4 0x4 0xff 0xff 0xfe 0x34 extra data: 
>>>>> send: 0x13 0x76 0x0 0x0 0x0 0x6 0xf7 0x4 0x75 0x9e 0x0 0x2
>>>>> recv: 0x13 0x76 0x0 0x0 0x0 0x7 0xf7 0x4 0x4 0x0 0x0 0x0 0x0 extra data: 
>>>>> send: 0x13 0x77 0x0 0x0 0x0 0x6 0xf7 0x4 0x75 0xb0 0x0 0x2
>>>>> recv: 0x13 0x77 0x0 0x0 0x0 0x7 0xf7 0x4 0x4 0x0 0x0 0x0 0x0 extra data: 
>>>>> send: 0x13 0x78 0x0 0x0 0x0 0x6 0xf7 0x4 0x75 0xa6 0x0 0x2
>>>>> recv: 0x13 0x78 0x0 0x0 0x0 0x7 0xf7 0x4 0x4 0x0 0x0 0x0 0x0 extra data: 
>>>>> send: 0x13 0x79 0x0 0x0 0x0 0x6 0xf7 0x4 0x75 0xb8 0x0 0x2
>>>>> recv: 0x13 0x79 0x0 0x0 0x0 0x7 0xf7 0x4 0x4 0x0 0x0 0x0 0x0 extra data: 
>>>>> send: 0x13 0x7a 0x0 0x0 0x0 0x6 0xf7 0x4 0x75 0xec 0x0 0x2
>>>>> recv: 0x13 0x7a 0x0 0x0 0x0 0x7 0xf7 0x4 0x4 0x0 0x0 0x0 0x0 extra data: 
>>>>> send: 0x13 0x7b 0x0 0x0 0x0 0x6 0xf7 0x4 0x75 0xe6 0x0 0x2
>>>>> recv: 0x13 0x7b 0x0 0x0 0x0 0x7 0xf7 0x4 0x4 0x0 0x0 0x0 0x0 extra data: 
>>>>> send: 0x13 0x7c 0x0 0x0 0x0 0x6 0xf7 0x4 0x75 0x3f 0x0 0x2
>>>>> recv: 0x13 0x7c 0x0 0x0 0x0 0x7 0xf7 0x4 0x4 0x0 0x0 0x0 0xd2 extra data: 
>>>>> send: 0x13 0x7d 0x0 0x0 0x0 0x6 0x2 0x4 0x7d 0x3 0x0 0x2
>>>>> recv: 0x13 0x7d 0x0 0x0 0x0 0x7 0x2 0x4 0x4 0x0 0x0 0x0 0x0 extra data: 
>>>>> send: 0x13 0x7e 0x0 0x0 0x0 0x6 0xf7 0x4 0x75 0x63 0x0 0x1
>>>>> recv: 0x13 0x7d 0x0 0x0 0x0 0x7 0x2 0x4 0x4 0x0 0x0 0x0 0x0 extra data: 
2025-09-02 06:45:56,583 ERROR base:78 ERROR: request ask for id=247 but got id=2, Skipping.
>>>>> send: 0x13 0x84 0x0 0x0 0x0 0x6 0xf7 0x3 0x9c 0x6c 0x0 0x2
>>>>> recv: 0x13 0x84 0x0 0x0 0x0 0x7 0xf7 0x3 0x4 0xff 0xff 0xff 0xff extra data:

Any ideas?

A related question is whether or not this message should be logged as an ERROR. It seems to be more like a WARNING. If it was a "real" error, I would expect to be able to catch it and examine the actual related request from my code perspective.

Thanks in advance.

[janiversen]

As you can see in the log, you have a tcp/ip connection to the device.

The send contains id=1, but the device responds with id=2, that is wrong, but I cannot tell you why the device does that.

The message is discarded, and the next message is a correct response from id=1, so the app get a proper response.

Since the message is discarded this is clearly an ERROR and not a warning, in your case the normal response comes in the next message so discarding it do not have any consequences.

Pymodbus do provide tracepoints where the app can examine the incoming/outgoing message at byte stream level or at PDU level, so you are able to catch it.

[seud0nym]

Thanks for your quick response and explanation.

[janiversen]

Since you state that valid id´s are 1, 2 and 247, then I would be concerned !!! because the user might get false data, in general this ERROR should not be taken lightly.

[seud0nym]

My theory is that the user has a network issue, and that the device is retrying to send the response. You can see that with the message with id 0x13 0x65. There is one send and then two recv messages, which I assume is the device retrying because of a network glitch. Message ids 0x13 0x69 and 0x13 0x7d are the same.

Would it be possible to track the message ids and ignore any recv messages with a used id?

[seud0nym]

Where are you seeing that the responses are from two different devices? (Sorry if that is a silly question, but this is the first time I have looked at the low level messages.)

What I think I am seeing is that the second recv message with the same id is identical to the first recv message with that same id (using 0x13 0x65, 0x13 0x69 and 0x13 0x7d as examples). The send message in between those two recv messages with the next id and different device id has its own recv message. The fact that the recv messages are identical leads me to assume they are retransmitted by the server, and I thought network issues the likely culprit, since the only time I have ever seen anything remotely similar was during a connection issue.

It seems that there could be an assumption in the code somewhere that the next recv message must be for the last send message, and that the message identifier is therefore not checked. The device id obviously is checked, which throws the error message. Is that possible?

[janiversen]

Please read the modbus standard!! I marked earlier the id in the messagees!

And how can you say the receive messagees are identical ? did you bother to compare ? byte 7 is different (as I showed in the image I attached earlier.

[seud0nym]

how can you say the receive messagees are identical ?

If you number the first 8 lines from my original message thus:

1 >>>>> recv: 0x13 0x64 0x0 0x0 0x0 0x7 0xf7 0x4 0x4 0xff 0xff 0xff 0xff extra data: 
2 >>>>> send: 0x13 0x65 0x0 0x0 0x0 0x6 0x2 0x4 0x7d 0xd 0x0 0x1
3 >>>>> recv: 0x13 0x65 0x0 0x0 0x0 0x5 0x2 0x4 0x2 0x0 0x0 extra data: 
4 >>>>> send: 0x13 0x66 0x0 0x0 0x0 0x6 0x1 0x4 0x79 0x3a 0x0 0x1
5 >>>>> recv: 0x13 0x65 0x0 0x0 0x0 0x5 0x2 0x4 0x2 0x0 0x0 extra data: 
6 2025-09-02 06:45:50,463 WARNING transaction:235 ERROR: received pdu without a corresponding request, IGNORING
7 2025-09-02 06:45:50,753 ERROR base:78 ERROR: request ask for id=1 but got id=2, Skipping.
8 >>>>> recv: 0x13 0x66 0x0 0x0 0x0 0x5 0x1 0x4 0x2 0x0 0x0 extra data: 

And then you compare the 2 recv messages with id 0x13 0x65 (lines 3 and 5):

3 >>>>> recv: 0x13 0x65 0x0 0x0 0x0 0x5 0x2 0x4 0x2 0x0 0x0 extra data: 
5 >>>>> recv: 0x13 0x65 0x0 0x0 0x0 0x5 0x2 0x4 0x2 0x0 0x0 extra data: 

They are identical.

Please read the modbus standard!!

I hope this is the correct version:

https://modbus.org/docs/Modbus_Messaging_Implementation_Guide_V1_0b.pdf

Page 24:

According to the sequence of the log messages, transaction identifer 0x13 0x66 is the current pending transaction (line 4), so the second recv message for transaction identifier 0x13 0x65 (line 5) should have been discarded, and then line 8 should have been processed. Now, that is effectively what has happened, but the discarding of line 5 happened because of the device id mismatch with the pending transaction, not the fact that it was the wrong transaction identifier. The problem could occur that a resent message with the same device id but a different transaction identifier could be incorrectly accepted just because the device id matches.

[janiversen]

I have already told you the client SENDs a request to device 0x01 (byte 7) in line 4, and receives 2 responses l First one in line 5 have the device id 0x13 (byte 7), and that is WRONG and the transaction ID is from an earlier request.
In line 8 the correct response from device 0x01 with the correct transaction id is received and handled.
So the correct transaction are being processed !!!

But it is still correct to log an error for line 5 since it is received without having been requested, and the server do NOT do retries (the standard specifies that the client is responsible for retries).

[janiversen]

But this is not a pymodbus issue, so can we please stop.

[seud0nym]

My apologies. It was my first time looking at the actual messages being exchanged, and I was confused by what I was seeing.

Thank you for your explanantions.

[seud0nym]

What is the correct way to stop a message being processed when using a trace packet handler?

    def _trace_packet_handler(self, is_send: bool, packet: bytes) -> bytes:
        if len(packet) < 2:
            self._logger.warning(f"[TRACE] invalid packet: {packet.hex()}")
            return packet
        tid = int.from_bytes(packet[0:2], byteorder="big")
        if is_send:
            self._tid_tracker.add_expected(tid)
            return packet
        if self._tid_tracker.is_valid(tid):
            self._tid_tracker.mark_processed(tid)
            return packet
        return b"" # is this correct?

[janiversen]

The are no correct or not correct way, just a way that works :-)

Your code do not look bad, but you have so much going on there, so I cannot tell you if it will work.