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:

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.

[symygy]

Yes, I tried to read i.e. register 100 and it was ok, I got proper value. But when I am trying to read register 1000 (which does not exists) I got Slave Device Failure instead of Invalid Data Address.

result = client.read_holding_registers(address=100, count=5, slave=1) - OK
result = client.read_holding_registers(address=1000, count=5, slave=1) - I am expecting Illegal Data Address, I have Slave Device Failure instead

My library version is 3.9.2

[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.