Lack of IllegalAddress error while reading from not existing holding registers

[symygy]

I am trying to create simple SerialModbusServer using pymodbus library and then I want to read some not exisiting registers. I expect getting a modbus "Illegal Data Address" error but as I understand it correctly I'm getting "Slave Device Failue":
DEBUG:pymodbus.logging:Processing: 0xf1 0x80 0x4 0x40 0x30

Can anyone explain what am I doing wrong? I was also trying to use ModbusSequentialDataBlock but with similat results.
I am using that simple script to get required serial ports:
import virtualserialports virtualserialports.run(2, loopback=False, debug=True)

PS I was really trying to show my code in a proper code block using editor but I always ended up like this:

(EDIT by alexrudd2: Use three backticks ``` for code blocks. )
Server code:

def run_server():
    nreg = 200
    hr_data = {i: 17 for i in range(nreg)}

    store = ModbusSlaveContext(
        di=ModbusSparseDataBlock({i: 15 for i in range(nreg)}),
        co=ModbusSparseDataBlock({i: 16 for i in range(nreg)}),
        hr=ModbusSparseDataBlock(hr_data),  # tylko adresy 0-199
        ir=ModbusSparseDataBlock({i: 18 for i in range(nreg)})
    )

    context = ModbusServerContext(slaves=store, single=True)

    StartSerialServer(
        context=context,
        port=com_port, 
        baudrate=9600,
        bytesize=8,
        parity="N",  
        stopbits=1,
        timeout=1
    )

Client code:

def test_virtual_device():
    client = ModbusSerialClient(port="/dev/pts/3", baudrate=9600, bytesize=8, parity="N", stopbits=1)

    try:
        print("\n--- Reading register 1000 ---")
        result = client.read_holding_registers(address=1000, count=5, slave=1)

        if result.isError():
            print("error", result)

[janiversen]

Not sure I understand what you want, a server do not read data.

You do not write which version of the library you use, so I assume it is the newest.

If you refer to the client, then the reason is probably that you did not define slave 1 in the server. Did you try to to read valid data first ? Normally you would get illegal address if the slave is correct.

[janiversen]

Seems like a bug, pull requests are welcome.

[janiversen]

Just a follow up on this....Slave device failure is a generic error response, used by devices with a limited/simple firmware, it covers the more detailed errors like Illegal Data Address and are legal to use.

Your client software should be able to handle both the detailed responses as well as the generic responses, at least if you want to be able to connect to a variety of modbus devices.

[symygy]

What should I change in my code to see IllegalDataAddress error? Should I manually return ModbusResponse with specific exception code or modify the client's code?

[janiversen]

Use another server, or use the simulator datastore, which returns illegal address.

[symygy]

Ok I'll give it a try. Thanks for help :)

[alexrudd2]

See #2716 and corresponding PR #2717

[alexrudd2]

@symygy Jan came up with a better solution in #2733

If you try the very latest dev your existing client code should now see IllegalDataAddress.

[symygy]

Thank you very much for fixing that. I'm not sure if I'll be able to verify those changes, as I've switched to another Modbus library in the meantime.

PS I've just read your discussion under #2717 and #2733 PRs and that was interesting :D I wish I could be skilled enough in programming one day to debate code with such expertise.