Trying to read motor values using Raspberry pi 3

Question

Currently I am doing project where I want to use a Raspberry pi 3 as a monitoring device for motors. The Raspberry pi itself communicates with Roboteq controller using USB connection. Below is the simple code I'm trying to send via USB to the motor controller. I want to get motor Amps and show values in the terminal:

import time
import serial

def init_serial():
  global ser
  ser = serial.Serial(
  port = '/dev/ttyACM0',
  baudrate = 9600,
  parity = serial.PARITY_NONE,
  stopbits = serial.STOPBITS_ONE,
  bytesize = serial.EIGHTBITS,
  timeout = 1
  )

  if ser.isOpen():
    print("Connected to: " + ser.portstr)

def motor_ampers():

  motorChannel = 1
  command = '?A '+str(motorChannel)+' \r'
  ser.write(command.encode()) 
  data = ser.readline().decode().strip() 
  print(data)

  value = 0
  if data.startswith('A='):
     value = int(data.split('=')[1])
  print(value)


init_serial()
motor_ampers()

A little explanation to my code. By sending ?A 1 command to roboteq controller I can get first motor ampers. As an output I'm getting ?A 1 ▫A=-2 where -2 is motor ampers. After I'm trying to extract motor ampers from string and store it as integer. However I'm always getting value = 0even if motors are running. I can't figure out mistake by myself so any help is greatly appreciated.

Answer 1

Your problem is the data you are getting back on the port starts with a non-ASCII character so this line:

if data.startswith('A='):

will never be tested as true.

Try replacing it with:

if 'A=' in data: