Error in connecting 2 modbus slave pymodbus

Question

I am working on pymodbus. Trying to read the data from multiple units (multiple slaves) it gives me data but when the 1 of the slaves did not respond or is off the other slaves also shows the error after some time. please help. Thank you

from pymodbus.client.sync import ModbusSerialClient as ModbusClient
from pymodbus.constants import Endian
from pymodbus.payload import BinaryPayloadDecoder
from pymodbus.exceptions import ModbusIOException
from pymodbus.exceptions import ConnectionException

def length(x):
    client = ModbusClient(method='rtu', port='/dev/ttyUSB0', timeout=2, stopbits = 1, bytesize = 8,  parity='N', baudrate = 9600)
    client.connect()
    rr = client.read_holding_registers(address=1, count=4, unit=x);
    decoder = BinaryPayloadDecoder.fromRegisters(rr.registers, Endian.Big, wordorder=Endian.Big);
    a=str(decoder.decode_32bit_float());
    c = int (float(a))
    client.close()
    print("length",c)
    client = ModbusClient(method='rtu', port='/dev/ttyUSB0', timeout=2, stopbits = 1, bytesize = 8,  parity='N', baudrate = 9600)
    client.connect()
    rr = client.read_holding_registers(address=7, count=4, unit=x);
    decoder = BinaryPayloadDecoder.fromRegisters(rr.registers, Endian.Big, wordorder=Endian.Big);
    b=str(decoder.decode_32bit_float());
    d = int (float(b))
    client.close()
    print("speed",d)
    print(x)
    return c,d

after running some time the code shows error 'ModbusIOException' object has no attribute 'registers' if one of the connected slave is off — Eng, Oct 24 '18 at 03:45
I posted an [answer](https://stackoverflow.com/a/52990404/3702377), I hope it will be helpful. — Benyamin Jafari, Oct 27 '18 at 08:50
`count=4` reading and `decode_32bit_float()` are contradicted together, because you read 4regs, and each register in Modbus is a *Word Type*, each word is *2Byte*, Thus you read *8Byte*, So you read *64bit* that you need decode it with `decode_64bit_float()`. — Benyamin Jafari, Oct 28 '18 at 05:47
I had solve the above issue. The actual problem was because of the thread. — Eng, Nov 11 '18 at 09:19

Benyamin Jafari · Answer 1 · 2019-09-14T11:46:01.103

I've improved your code with the .isError() method in pymodbus as the following:

from pymodbus.client.sync import ModbusSerialClient as ModbusClient
from pymodbus.constants import Endian
from pymodbus.payload import BinaryPayloadDecoder

def error_checker(instance):
    if not instance.isError():
        '''.isError() implemented in pymodbus 1.4.0 and above.'''
        decoder = BinaryPayloadDecoder.fromRegisters(
            instance.registers, byteorder=Endian.Big, wordorder=Endian.Big
        )
        return decoder.decode_32bit_float()
    else:
        # Error handling.
        return None

def length(x):
    client = ModbusClient(
        method='rtu',
        port='/dev/ttyUSB0',
        timeout=2, stopbits=1, bytesize=8, parity='N', baudrate=9600
    )
    try:
        client.connect()
        rr = client.read_holding_registers(address=1, count=4, unit=x)
        c = error_checker(rr)
        print("length: ", c)
        rr = client.read_holding_registers(address=7, count=4, unit=x)
        d = error_checker(rr)
        print("speed: ", d)
        return c, d
    except Exception as exc:
        print(exc)
    finally:
        client.close()

Error in connecting 2 modbus slave pymodbus

1 Answers1