Modbus RTU master - python script with minimalmodbus

Question

I would like to control an actuator with a python script in MODBUS RTU master. I tried to use the library minimalmodbus to communicate (write bit, write & read registers) with my slave.

When I start my code, I have some errors. So, someone can I help me to find a solution?

My code:


    import minimalmodbus
    import os
    import struct
    import sys
    import serial
    import time

    instrument = minimalmodbus.Instrument('/dev/ttyRS485', 1)
    instrument.serial.port
    instrument.serial.baudrate = 9600
    instrument.serial.parity = serial.PARITY_NONE
    instrument.serial.bytesize = 8
    instrument.serial.stopbits = 1
    instrument.mode = minimalmodbus.MODE_RTU
    instrument.serial.timeout = 0.05


    modbus = instrument.write_bit(0x0427, 1)
    print (modbus)

    alarme = instrument.write_bit(0x0404, 1)
    print (alarme)

    alarme = instrument.write_bit(0x0404, 0)
    print (alarme)

    on = instrument.write_bit(0x0403, 1)
    print (on)

    home = instrument.write_bit(0x040B, 1)
    print (home)

    position = instrument.write_register(0x9900, 0, number_of_decimals=2,functioncode=16, signed=False)
    print (position)

    posi = instrument.write_register(0x9901, 6000, number_of_decimals=2,functioncode=16, signed=False)
    print (posi)

Errors:

    ========================= RESTART: /home/pi/test.py =========================
    None
    None
    None
    None
    None
    None
    Traceback (most recent call last):
    File "/home/pi/.local/lib/python3.5/site-packages/minimalmodbus.py", line 2448, in _pack
    result = struct.pack(formatstring, value)
    struct.error: 'H' format requires 0 <= number <= 65535

    During handling of the above exception, another exception occurred:

    Traceback (most recent call last):
    File "/home/pi/test.py", line 36, in <module>
    posi = instrument.write_register(0x9901, 6000, number_of_decimals=2, functioncode=16, signed=False)
    File "/home/pi/.local/lib/python3.5/site-packages/minimalmodbus.py",line 518, in write_register 
    payloadformat=_PAYLOADFORMAT_REGISTER,
    File "/home/pi/.local/lib/python3.5/site-packages/minimalmodbus.py",line 1166, in _generic_command
    payloadformat,
    File "/home/pi/.local/lib/python3.5/site-packages/minimalmodbus.py",line 1514, in _create_payload
    value, number_of_decimals, signed=signed
    File "/home/pi/.local/lib/python3.5/site-packages/minimalmodbus.py", line 1991, in 
    _num_to_twobyte_string outstring = _pack(formatcode, integer)
    File "/home/pi/.local/lib/python3.5/site-packages/minimalmodbus.py", line 2454, in _pack
    raise ValueError(errortext.format(value, formatstring))
    ValueError: The value to send is probably out of range, as the num-to-bytestring conversion failed. 
    Value: 600000 Struct format code is: >H

what I do suspect (though I am no expert in modbus) is that the port `dev/ttyRS485` itself may be wrong. Can you verify if the variable `instrument` actually points at the device you're looking for? — learner, Feb 27 '20 at 15:18
The port is working because when I wrote bits to activate the actuator, it's working but when I wrote registers to move the actuator I have these errors — Théo LE BORGNE, Feb 27 '20 at 15:25
Yes, I think because they correspond to the activation of the bits — Théo LE BORGNE, Feb 27 '20 at 15:54
Why not try pymodbus? If you do I can help you with that - I've never tried minimalmodbus, so don't have experience with your particular errors. — Hayden Eastwood, Feb 27 '20 at 23:09
Yes with pleasure. If you have experience and examples with this library, I could use some help. Do you have a framework for initiating communication with the slave? — Théo LE BORGNE, Feb 28 '20 at 10:18

Hayden Eastwood · Answer 1 · 2023-02-20T18:39:40.457

4

In response to your request in the comments for an alternative library, here is what I use to read modbus with the pymodbus library:

import pymodbus
from pymodbus.pdu import ModbusRequest
from pymodbus.client.sync import ModbusSerialClient as ModbusClient
from pymodbus.transaction import ModbusRtuFramer

client = ModbusClient(
  method = 'rtu'
  ,port='/dev/tty.usbserial-AQ00BYCR'
  ,baudrate=38400
  ,parity = 'O'
  ,timeout=1
  )
connection = client.connect()
registers  = client.read_holding_registers(0,100,unit=1)# start_address, count, slave_id
print (registers.registers)

Note that in the above, the reading begins from address 0 and continues to address 100, for slave_id 1.

To write registers, do the following:

write  = client.write_register(1,425,unit=1)# address = 1, value to set = 425, slave ID = 1

edited Feb 20 '23 at 18:39

answered Feb 28 '20 at 13:30

Hayden Eastwood

928
2
10
20

Thank you so much ! I will try that on monday! Do you have an example of write register and multiple registers with two values? – Théo LE BORGNE Feb 28 '20 at 15:31
I have updated the answer above to include a write example. I'm not sure what you mean by "multiple registers with two values". If you elaborate on that I can possibly help. If this solved your problem please tag it as "correct". Thanks! – Hayden Eastwood Feb 28 '20 at 19:44
Correct! Thank you – Théo LE BORGNE Mar 02 '20 at 16:08

score 1 · Answer 2 · answered Mar 02 '23 at 14:45

1

Pymodbus in RTU, not only allows to read 13 registers, the maximum reading length is 16 registers, including headers and crc. In this case the best option, if you are obviously using RTU is minimalmodbus.

answered Mar 02 '23 at 14:45

joelk klein

11
1

Modbus RTU master - python script with minimalmodbus

2 Answers2