The I/O for the controls of the pinball machine is done using an OnTrak
data acquisition device via the serial port.
ADR2200 writes it's command to the SerialIO object, which uses
a SerialPort object to communicate on the serial port.
SerialIO writes and reads to and from queues in the SerialPort.
SerialPort controls the serial port. It polls the serial port
for data to read in via SerialIO.
When data is available to be written, data is written to the serial port