Sometime I received an Exception Response: IllegalFunction | IllegalAddress | GatewayNoResponse

[zarch]

Hi all,

I'm using pymodbus to read values from a bunch of devices via modbus tcp.

Looking at the log, I see that sometime the device raise some exceptions.
Bellow I reported the exceptions logs from about 10 minutes of test, please consider that I'm sending a request to the registers every 30 seconds.

SLAVE 1

time	error	register	count	slave
2024-10-16 08:22:02.156	GatewayNoResponse	20522	20	1
2024-10-16 08:22:31.098	IllegalFunction	20522	20	1
2024-10-16 08:23:01.081	IllegalFunction	20522	20	1
2024-10-16 08:23:31.100	IllegalFunction	20522	20	1
2024-10-16 08:25:01.085	IllegalFunction	20522	20	1
2024-10-16 08:25:31.100	IllegalFunction	20522	20	1
2024-10-16 08:27:02.150	GatewayNoResponse	20522	20	1

SLAVE 2

time	error	register	count	slave
2024-10-16 08:22:32.098	IllegalFunction	20480	35	2
2024-10-16 08:23:02.081	IllegalFunction	20480	35	2
2024-10-16 08:23:32.099	IllegalFunction	20480	35	2
2024-10-16 08:24:32.095	IllegalFunction	20480	35	2
2024-10-16 08:25:02.085	IllegalFunction	20480	35	2
2024-10-16 08:25:32.100	IllegalFunction	20480	35	2
2024-10-16 08:26:02.084	IllegalFunction	20480	35	2
2024-10-16 08:28:02.087	IllegalFunction	20480	35	2
2024-10-16 08:29:02.096	IllegalFunction	20480	35	2
2024-10-16 08:31:32.088	IllegalFunction	20480	35	2
2024-10-16 08:22:04.155	GatewayNoResponse	20522	20	2
2024-10-16 08:22:33.098	IllegalAddress	20522	20	2
2024-10-16 08:23:03.081	IllegalAddress	20522	20	2
2024-10-16 08:23:33.099	IllegalAddress	20522	20	2
2024-10-16 08:24:03.101	IllegalFunction	20522	20	2
2024-10-16 08:24:33.096	IllegalFunction	20522	20	2
2024-10-16 08:25:03.087	IllegalFunction	20522	20	2
2024-10-16 08:25:33.101	IllegalAddress	20522	20	2
2024-10-16 08:26:03.085	IllegalFunction	20522	20	2
2024-10-16 08:27:03.076	IllegalFunction	20522	20	2
2024-10-16 08:28:03.087	IllegalFunction	20522	20	2
2024-10-16 08:29:03.096	IllegalFunction	20522	20	2
2024-10-16 08:31:03.095	IllegalFunction	20522	20	2
2024-10-16 08:31:33.088	IllegalFunction	20522	20	2
2024-10-16 08:32:03.093	IllegalFunction	20522	20	2

and SLAVE 3

time	error	register	count	slave
2024-10-16 08:22:05.301	GatewayNoResponse	20480	35	3
2024-10-16 08:23:04.080	IllegalFunction	20480	35	3
2024-10-16 08:24:04.101	IllegalFunction	20480	35	3
2024-10-16 08:29:04.096	IllegalAddress	20480	35	3
2024-10-16 08:30:05.164	GatewayNoResponse	20480	35	3
2024-10-16 08:30:35.174	GatewayNoResponse	20480	35	3
2024-10-16 08:31:04.095	IllegalFunction	20480	35	3
2024-10-16 08:31:34.087	IllegalFunction	20480	35	3
2024-10-16 08:32:04.094	IllegalFunction	20480	35	3
2024-10-16 08:23:05.081	IllegalFunction	20522	20	3
2024-10-16 08:24:05.101	IllegalFunction	20522	20	3
2024-10-16 08:24:35.096	IllegalFunction	20522	20	3
2024-10-16 08:27:05.076	IllegalFunction	20522	20	3
2024-10-16 08:27:35.084	IllegalFunction	20522	20	3
2024-10-16 08:30:05.238	IllegalFunction	20522	20	3
2024-10-16 08:30:35.249	IllegalFunction	20522	20	3
2024-10-16 08:31:05.094	IllegalAddress	20522	20	3
2024-10-16 08:32:05.094	IllegalFunction	20522	20	3
2024-10-16 08:32:36.167	GatewayNoResponse	20522	20	3

As you can see the requests are not raised every 30s and indeed I'm acquiring some data! :-)
I can understand the GatewayNoResponse issue that might be due to some connectivity problem (even if everything is cabled). What it is hard to unsderstand to me is why the same requests can raise sometime an IllegalFunction or an IllegalAddress.

What can explain this nondeterministic device's behavior? How can I further investigate this issue?

Here a single request raising an IllegalFunction with pymodbus log level to DEBUG:

2024-10-16 08:12:01,028 DEBUG transaction:66 Adding transaction 14
2024-10-16 08:12:01,028 DEBUG transport:379 send: 0x0 0xe 0x0 0x0 0x0 0x6 0x1 0x4 0x50 0x2a 0x0 0x14
2024-10-16 08:12:01 [T:MainThread] DEBUG:pymodbus.logging: Adding transaction 14
2024-10-16 08:12:01 [T:MainThread] DEBUG:pymodbus.logging: send: 0x0 0xe 0x0 0x0 0x0 0x6 0x1 0x4 0x50 0x2a 0x0 0x14
2024-10-16 08:12:01,101 DEBUG transport:322 recv: 0x0 0xe 0x0 0x0 0x0 0x3 0x1 0x84 0x1 old_data:  addr=None
2024-10-16 08:12:01,102 DEBUG base:101 Processing: 0x0 0xe 0x0 0x0 0x0 0x3 0x1 0x84 0x1
2024-10-16 08:12:01,102 DEBUG factory:259 Factory Response[132]
2024-10-16 08:12:01,102 DEBUG base:119 Frame advanced, resetting header!!
2024-10-16 08:12:01,103 DEBUG transaction:77 Getting transaction 14
2024-10-16 08:12:01.103 | CRITICAL | Exception Response(132, 4, IllegalFunction) | client=pymodbus.client.tcp.AsyncModbusTcpClient | func_type='input_registers' | address=20522, count=20, slave=1

[janiversen]

As you can see from the debug log, it is your device that reports IllegalFunctiomRequest, so pymodbus is ok.

Looking at the previous send, it is a legal modbus message, but maybe your device do not like the address or the amount of data you requiere.