The Mastercard II is a versatile dual-function interface card designed for Apple II+ and //e computers. It provides an asynchronous serial port that complies with RS232 standards, along with an 8-bit parallel "user port".

The serial port is intended for use with communications software like Data Highway and enables two-way communication with remote devices such as terminals, printers, and other computers. These devices can be connected directly or via a modem.

An example application of the user port is controlling auto-dialling modems, such as the Pace Nightingale, enabling automated connection features.