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8.8. Example serial code in Python
<p>The example Python code below uses the <a href="http://pyserial.readthedocs.io">pySerial</a> 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.</p>
<p>For this example to work, the Simple Motor Controller G2’s input mode must be <strong>Serial/USB</strong>, the serial mode must be <strong>Binary</strong>, 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.</p>
<p>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 <a href="http://pyserial.readthedocs.io">pySerial documentation</a> to install it.</p>
<pre name="code" class="python">
# 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)</pre>
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