The serial port (often called the COM port) is perfect for connecting various electronic circuits to the PC. The complexity of the connected circuit varies from a few I/O lines to control something and read the state of switches, to a complex communications system between several computers. This circuit is in the first category and uses the serial port to perform a few simple I/O tasks.The COM Port Tester is a simple circuit that shows how the signals on a serial port can be used to control another electronic circuit. We will have to disappoint those of you who expect to use the serial port to control motors and lights etc. This circuit is limited to driving three LEDs and reading the state of four switches, nothing more and nothing less.