Minimalmodbus - RS485 shield for Raspberry Pi

Products & Technology Raspberry Pi
#1

I am trying to interface Seeed Studio’s RS485 shield for Raspberry Pi along with the Python Minimalmodbus library on Raspberry Pi 2B - NOT WORKING!!!

shield : https://www.seeedstudio.com/RS-485-Shield-for-Raspberry-Pi.html

I tested following on Raspberry Pi:

  • It worked fine if I send data using Python serial library. It turns on a relay on a slave device and responds correctly.

ser.write(’\xff\x05\x00\x00\xff\x00\x99\xe4’)

  • It is not working if I use Minimalmodbus library for Python

instrument.write_bit(0,1,5)
minimalmodbus.NoResponseError: No communication with the instrument (no answer)

note: I used the same Minimalmodbus python code using another USB-to-RS485 converter on same RPi and it worked fine.

Python Code:

import minimalmodbus

import time

import serial 

instrument = minimalmodbus.Instrument('/dev/ttyAMA0', 255)


instrument.serial.baudrate = 9600      
instrument.serial.bytesize = 8
instrument.serial.stopbits = 1
instrument.serial.timeout  = 1          
instrument.mode = minimalmodbus.MODE_RTU  
instrument.clear_buffers_before_each_transaction = True
instrument.debug = True

while 1:
    
    instrument.write_bit(0,1,5)

    time.sleep(5)
...
**error:**  minimalmodbus.NoResponseError: No communication with the instrument (no answer)

Can anybody help if I can use this specific rs485-shield with Minimalmodbus library? Thank you in advance

Regards,
#2

We haven’t tried Minimalmodbus, but we’ve tested Pyserial and it works.

#3

Yes it worked with Pyserial Library in Python. But we need to use Minimalmodbus or any other similar library in our application. Can you propose any solution? Because we are planning to use this board in some numbers. Thanks

#4

i have the same problem. can anyone provide a sample code for Modbus or Pyserial?

#5

Hi @Sebastian_von_Bentze
Here is a Modbus library we have once used, but this is not our library.

1 Like
#6

can you send me your working sample of Pyserial?

#7

Hi @Sebastian_von_Bentze
Here you go.

#8

Hello, thank you but this is not a Modbus library - right?

#9

Hi @Sebastian_von_Bentze
Yes, you are right.