APeripheral Interface Adapter (PIA) is aperipheralintegrated circuit providing parallel I/O interfacing formicroprocessor systems.
┌─────⊔︀─────┐Vss ┤ 1 40├ CA1PA0 ┤ 2 39├ CA2PA1 ┤ 3 38├ !IRQAPA2 ┤ 4 37├ !IRQBPA3 ┤ 5 36├ RS0PA4 ┤ 6 35├ RS1PA5 ┤ 7 34├ !RESPA6 ┤ 8 33├ D0PA7 ┤ 9 32├ D1PB0 ┤10 31├ D2PB1 ┤11 30├ D3PB2 ┤12 29├ D4PB3 ┤13 28├ D5PB4 ┤14 27├ D6PB5 ┤15 26├ D7PB6 ┤16 25├ EPB7 ┤17 24├ CS1CB1 ┤18 23├ !CS2CB2 ┤19 22├ CS0Vcc ┤20 21├ R/!W └───────────┘Common PIAs include theMotorola MC6820 and MC6821, and theMOS Technology MCS6520, all of which are functionally identical but have slightly different electrical characteristics. The PIA is most commonly packaged in a 40 pinDIP package.
The PIA is designed for glueless connection to theMotorola 6800 stylebus, and provides 20 I/O lines, which are organised into two 8-bit bidirectional ports (or 16 general-purpose I/O lines) and 4 control lines (forhandshaking andinterrupt 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.[1]
The MC6820 was used in theApple I to interface theASCII keyboard and the display.[2]It was also deployed in the 6800-powered first generation ofBally electronic pinball machines (1977-1985), such asFlash Gordon[3]andKiss.[4]The MCS6520 was used in theAtari 400 and 800[5] andCommodore PET[6] family of computers (for example, to provide four joystick ports to the machine).TheTandy Color Computer uses two MC6821s to provide I/O access to the video, audio and peripherals.[7]
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