Peripheral Interface Adapter

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Motorola MC6820 and MC6821 Peripheral Interface Adapters

The Peripheral Interface Adapter (PIA) is a peripheral integrated circuit providing parallel I/O interfacing capability for microprocessor systems.


Common PIAs include the Motorola MC6820 and MC6821, and the MOS Technology MCS6520, all of which are functionally identical but have slightly different electrical characteristics. The PIA is most commonly packaged in a 40 pin DIP package.

The PIA is designed for glueless connection to the Motorola 6800 style bus, and provides 20 I/O lines, which are organised into 2 8-bit bidirectional ports (or 16 general-purpose I/O lines) and 4 control lines (for handshaking and interrupt generation). The directions for all 16 general lines (PA0-7, PB0-7) can be programmed independently. The control lines can be programmed to generate interrupts, automatically generate handshaking signals for devices on the I/O ports, or output a plain high or low signal.

It was used in Atari 800 family of computers (to provide four joystick ports to the machine). It was also used in the Apple I to interface the ASCII keyboard and the display.

The chip was also famously deployed in first generation of Bally electronic pinball machines (1977-1985), such as Flash Gordon and Kiss.


  • Leventhal, Lance A. (1986). 6502 Assembly Language Programming 2nd Edition. Osborne/McGraw-Hill. ISBN 0-07-881216-X.