Modbus Error: ... read of 8 bytes without response from unit before it closed connection

[rdorsch]

Hello,

I have a strange observation here:

I do a periodic read_holding_registers() operation. If I read every 10 secs, everything works super reliable, if I read every 60 secs, I get a connection error. The server is an Alfen Eve NG910 wallbox:

Example code:

rd@evcc:~$ cat alfen-test-minimal.py
import pymodbus.payload
import pymodbus.client
import time
import datetime
import random
    
# establish wb connection
client = pymodbus.client.ModbusTcpClient('192.168.2.231', port=502, timeout=10, auto_open=True, auto_close=False)
client.connect()

while True:
    print(datetime.datetime.now())
    try: 
        rr = client.read_holding_registers(344,count=2,slave=1)
        decoder = pymodbus.payload.BinaryPayloadDecoder.fromRegisters(rr.registers, byteorder=pymodbus.constants.Endian.Big, wordorder=pymodbus.constants.Endian.Big)
        energy_measurements = {
            'real_power': decoder.decode_32bit_float(),
        }
        r=energy_measurements["real_power"]
        print("wb_real_power: ", r)
    except Exception as e:
        print('wb_real_power_sum_exception: ')
        print(e)

    time.sleep(60)

                    
rd@evcc:~$ 

With the 60 seconds sleep interval, I get as output:

rd@evcc:~$ python3 alfen-test-minimal.py 
2023-03-26 12:07:03.655449
wb_real_power:  0.0
2023-03-26 12:08:03.784190
wb_real_power_sum_exception: 
Modbus Error: [Connection] ModbusTcpClient(192.168.2.231:502): Connection unexpectedly closed 0.00012636184692382812 seconds into read of 8 bytes without response from unit before it closed connection

with a 10 seconds sleep interval everything works as expected.

The issue goes away, if I add a client.connect() before the client.read_holding_registers() and a client.close() thereafter.

I assume that the wallbox closes the connection after a certain time of inactivity.

Isn't the auto_open=True supposed to automatically open the connection again?

Thanks
Rainer

[janiversen]

Where did you see auto_open it is not in our documentation.

But yes your problem is that wallbox (like many other devices) close the connection after 30 seconds.

[rdorsch]

Hi Jan, many thanks for the quick response. Am Sonntag, 26. März 2023, 18:13:08 CEST schrieb jan iversen:

Where did you see auto_open it is not in our documentation.

It seems I used the wrong documentation without noticing it: https://pymodbustcp.readthedocs.io/en/latest/quickstart/index.html I am surprised that python does not complain about the undefined arguments.

But yes your problem is that wallbox (like many other devices) close the connection after 30 seconds.

Would you open for each operation a connect() and close() if there is a risk, that the timeout becomes bigger than 30 seconds? client.connect() rr = client.read_holding_registers(344,count=2,slave=1) client.close() Thanks Rainer

…

-- Rainer Dorsch http://bokomoko.de/

[janiversen]

That is clearly a legal way of doing it, On my production system I make a call every 10 seconds reading a sensor not very important but it keeps the connection open.