The PC port can be used with the MCUpdate software to flash or update the motor control (MC) firmware on your CPC scope. It can also be used with Nexremote to control your scope. You need the programming cable from Celestron (part number 93922).
If you have hand control firmware version 4.13 or higher, the MC firmware can be upgraded via the port on the bottom of the hand control. For this you need the NexStar RS-232 cable (part number 93920).
If your computer lacks a serial port, you’ll also need a USB to RS-232 converter cable for either method (Celestron part number 18775).
For the wiring diagram for the cable, please see the NexStar Resource Site's webpage: