MOS Technology 6526

The 6526/8521 Complex Interface Adapter (CIA) was an integrated circuit made by MOS Technology. It served as an I/O port controller for the 6502 family of microprocessors, providing for parallel and serial I/O capabilities as well as timers and a Time-of-Day (TOD) clock. The device's most prominent use was in the Commodore 64 and Commodore 128(D), each of which included two CIA chips. The Commodore 1570 and Commodore 1571 floppy disk drives contained one CIA each. The Amiga home computers employed two and Commodore 1581 floppy disk drive one 8520 chip, which is functionally equivalent to 6526/8521 except the simplified TOD circuitry.

The CIA had two 8-bit bidirectional parallel I/O ports. Each port had a corresponding Data Direction Register, which allowed each data line to be individually set to input or output mode. A read of these ports always returned the status of the individual lines, regardless of the data direction that had been set.

An internal bidirectional 8-bit shift register enabled the CIA to handle serial I/O. The chip could accept serial input clocked from an external source, and could send serial output clocked with one of the built-in programmable timers. An interrupt was generated whenever an 8-bit serial transfer had completed. It was possible to implement a simple "network" by connecting the shift register and clock outputs of several computers together. The maximum bitrate is 500 kbit/s for the 2 MHz version.

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