Minimalmodbus - RS485 shield for Raspberry Pi

Question

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)

error: minimalmodbus.NoResponseError: No communication with the instrument (no answer)

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

Pyhton 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,

Answer 1

The write_bit function is reading the response after the message is sent. And Seeed RS-485 Shield needs to toggle the GPIO18 pin to switch between reading and writing. As is shown in the sample code in documentation https://wiki.seeedstudio.com/RS-485_Shield_for_Raspberry_Pi/#communication-test-code.

By checking the code of write_bit function I don't think that there is an easy way to toggle that pin as between serial.write and serial.read these no place to register own hook and toggle the PIN there (https://github.com/pyhys/minimalmodbus/blob/c08208523b729d178a9f662725a69195924f7c34/minimalmodbus.py#L1379).