The pymodbus connection sometimes doesn't respond?

Question

I'm use "pymodbus" lib to connect PLC devices. The device is used Modbus RTU over TCP that devices will return the temperature and humidity of the environment.

map address list

0001: temperature
0002: humidity

I performed once to get value and it can succeed. But I'm using while loop sometimes get error. I don't know why.

code:

from time import sleep
from pymodbus.client.sync import ModbusTcpClient
from pymodbus.framer.rtu_framer import ModbusRtuFramer

from pymodbus.register_read_message import ReadHoldingRegistersResponse

client = ModbusTcpClient(host='192.168.1.1', port=5000, framer=ModbusRtuFramer)    
client.connect()
while True:
    rr = client.read_holding_registers(0, 2, unit=1)

    if isinstance(rr, ReadHoldingRegistersResponse):
        temp = rr.registers
        print(temp)
    else:
        print('error')
    sleep(1)
client.close()

output:

> ...
> [189, 444]
> [189, 443]
> [189]
> error
> error
> ...

We can see that sometimes the result is obtained normally, sometimes the result is incomplete, and sometimes the result is not available.

What should I do to solve this problem, I want to monitor this device. Thank you.

I have never seen this. The only real difference I have is not using framer=ModbusRtuFramer so perhaps the answer lies in what that does. Certainly for me, it's either a full list or no list (with an Exception thrown), but not a partial read. — roganjosh
Hi, thank you. If I don't use framer = ModbusRtuFramer will get an error. — frank
Error msg: AttributeError: 'ModbusIOException' object has no attribute 'registers' — frank
Sometimes, the slave device would not be able to process the requests (due to limitations on the read/write buffers) when read very fast (like in this case every 1 sec) and that could result in ModbusIOException. Enable debug logs and see what exactly is happening over the socket . I would use a higher interval between polls in any case. — Sanju

Sergio Flores Sergio Flores · Accepted Answer · 2019-03-28T23:20:39

Yes, I see this all the time in my pymodbus code. I suspect there's something wrong with the implementation when doing succesive reads. What I do, is quite simply, to retry the failed read after a slight delay. And that usually gets it working again. Alternatively, try closing and re-connecting the client and re-attempt the reading. Also try increasing the sleep time. Let me know how it goes!

The pymodbus connection sometimes doesn't respond?

2 Answers