8.8. Example serial code in Python

The example Python code below uses the pySerial library to communicate with the Simple Motor Controller G2 via serial. It demonstrates how to read variables from the controller and set its target speed.

For this example to work, the Simple Motor Controller G2’s input mode must be Serial/USB, the serial mode must be Binary, and the CRC must be disabled. These are the default settings that the controller is shipped with. If you are using TTL serial, you should set the controller to use a fixed baud rate of 9600, or change the baud rate in the code below to match whatever fixed baud rate you choose.

If you run the code and get the error “ImportError: No module named serial” or “ModuleNotFoundError: No module named ‘serial’”, it means that the pySerial library is not installed, and you should follow the instructions in the pySerial documentation to install it.

# Uses the pySerial library to send and receive data from a
# Simple Motor Controller G2.
#
# NOTE: The Simple Motor Controller's input mode must be "Serial/USB".
# NOTE: You might need to change the "port_name =" line below to specify the
#   right serial port.

import serial

class SmcG2Serial(object):
  def __init__(self, port, device_number=None):
    self.port = port
    self.device_number = device_number

  def send_command(self, cmd, *data_bytes):
    if self.device_number == None:
      header = [cmd]  # Compact protocol
    else:
      header = [0xAA, device_number, cmd & 0x7F]  # Pololu protocol
    self.port.write(bytes(header + list(data_bytes)))

  # Sends the Exit Safe Start command, which is required to drive the motor.
  def exit_safe_start(self):
    self.send_command(0x83)

  # Sets the SMC's target speed (-3200 to 3200).
  def set_target_speed(self, speed):
    cmd = 0x85  # Motor forward
    if speed < 0:
      cmd = 0x86  # Motor reverse
      speed = -speed
    self.send_command(cmd, speed & 0x1F, speed >> 5 & 0x7F)

  # Gets the specified variable as an unsigned value.
  def get_variable(self, id):
    self.send_command(0xA1, id)
    result = self.port.read(2)
    if len(result) != 2:
      raise RuntimeError("Expected to read 2 bytes, got {}."
        .format(len(result)))
    b = bytearray(result)
    return b[0] + 256 * b[1]

  # Gets the specified variable as a signed value.
  def get_variable_signed(self, id):
    value = self.get_variable(id)
    if value >= 0x8000:
      value -= 0x10000
    return value

  # Gets the target speed (-3200 to 3200).
  def get_target_speed(self):
    return self.get_variable_signed(20)

  # Gets a number where each bit represents a different error, and the
  # bit is 1 if the error is currently active.
  # See the user's guide for definitions of the different error bits.
  def get_error_status(self):
    return self.get_variable(0)

# Choose the serial port name.
# Linux USB example:  "/dev/ttyACM0"  (see also: /dev/serial/by-id)
# macOS USB example:  "/dev/cu.usbmodem001234562"
# Windows example:    "COM6"
port_name = "/dev/ttyACM0"

# Choose the baud rate (bits per second).  This does not matter if you are
# connecting to the SMC over USB.  If you are connecting via the TX and RX
# lines, this should match the baud rate in the SMC's serial settings.
baud_rate = 9600

# Change this to a number between 0 and 127 that matches the device number of
# your SMC if there are multiple serial devices on the line and you want to
# use the Pololu Protocol.
device_number = None

port = serial.Serial(port_name, baud_rate, timeout=0.1, write_timeout=0.1)

smc = SmcG2Serial(port, device_number)

smc.exit_safe_start()

error_status = smc.get_error_status()
print("Error status: 0x{:04X}".format(error_status))

target_speed = smc.get_target_speed()
print("Target speed is {}.".format(target_speed))

new_speed = 3200 if target_speed <= 0 else -3200
print("Setting target speed to {}.\n".format(new_speed));
smc.set_target_speed(new_speed)