12.7. Example serial code in Python

The example Python code below uses the pySerial library to communicate with the Tic via serial. It demonstrates how to set the target position of the Tic and how to read variables from it.

To use this code, you will need to connect the Tic’s TX, RX, and GND pins to a serial port on your computer or a USB-to-serial adapter as described in Section 4.5.

The Tic’s control mode should be set to “Serial / I²C / USB”.

You will need to change the line that sets port_name in order to specify the correct serial port. The correct serial port name to use depends on your operating system and what type of serial port or USB-to-serial adapter you are using between the Tic and your computer. The baud rate you select in the code should match the baud rate specified in the serial settings in the Tic Control Center.

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.

You might notice that the Tic only performs the desired movement for about a second before it stops moving and the red LED turns on, indicating an error. This is because of the Tic’s command timeout feature: by default, the Tic’s “Command timeout” error will happen if it does not receive certain commands periodically (see Section 5.4 for details), causing the motor to stop. You can send a “Reset command timeout” command every second to get around this, or you can disable the command timeout feature using the Tic Control Center: uncheck the “Enable command timeout” checkbox in the “Serial” box.

# Uses the pySerial library to send and receive data from a Tic.
#
# NOTE: The Tic's control mode must be "Serial / I2C / USB".
# NOTE: You will need to change the "port_name =" line below to specify the
#   right serial port.

import serial

class TicSerial(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.
  def exit_safe_start(self):
    self.send_command(0x83)

  # Sets the target position.
  #
  # For more information about what this command does, see the
  # "Set target position" command in the "Command reference" section of the
  # Tic user's guide.
  def set_target_position(self, target):
    self.send_command(0xE0,
     ((target >>  7) & 1) | ((target >> 14) & 2) |
     ((target >> 21) & 4) | ((target >> 28) & 8),
     target >> 0 & 0x7F,
     target >> 8 & 0x7F,
     target >> 16 & 0x7F,
     target >> 24 & 0x7F)

  # Gets one or more variables from the Tic.
  def get_variables(self, offset, length):
    self.send_command(0xA1, offset, length)
    result = self.port.read(length)
    if len(result) != length:
      raise RuntimeError("Expected to read {} bytes, got {}."
        .format(length, len(result)))
    return bytearray(result)

  # Gets the "Current position" variable from the Tic.
  def get_current_position(self):
    b = self.get_variables(0x22, 4)
    position = b[0] + (b[1] << 8) + (b[2] << 16) + (b[3] << 24)
    if position >= (1 << 31):
      position -= (1 << 32)
    return position

# Choose the serial port name.
port_name = "/dev/ttyACM0"

# Choose the baud rate (bits per second).  This must match the baud rate in
# the Tic's serial settings.
baud_rate = 9600

# Change this to a number between 0 and 127 that matches the device number of
# your Tic 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)

tic = TicSerial(port, device_number)

position = tic.get_current_position()
print("Current position is {}.".format(position))

new_target = -200 if position > 0 else 200
print("Setting target position to {}.".format(new_target));
tic.exit_safe_start()
tic.set_target_position(new_target)