Peripheral Interface Adapter
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.
In 1976 Motorola switched the MC6800 family to a depletion-mode technology to improve the manufacturing yield and to operate at a faster speed. The Peripheral Interface Adapter had a slight change in the electrical characteristics of the I/O pins so the MC6820 became the MC6821. 
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.
- Advanced Information: 1.5 and 2.0 MHz Components for the M6800 Microcomputer System. Austin, Texas: Motorola Semiconductor Products. April 1977. pp. 4–6. ADI-429. The MC6820 became the MC6821 because the electrical characteristic of PA0–7 and PB0–7 pins changed slightly. The typical Input High Current went from -250 μAdc to -400 μAdc and the Input Low Current went from 1.0 mAdc to 1.3 mAdc.
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