MOS Technology SPI
The6529 Single Port Interface (SPI aka PIO)was anintegrated circuitmade byMOS Technology.It served as an I/O controller for the6502family of microprocessors, providing a single8-bitdigitalbidirectionalparallelI/O port.Unlike the more sophisticated6522 VIAand6526 CIA,it did not allow the data direction for each I/O line to be separately specified, nor did it supportserialI/O or contain anytimercapabilities. Because of this, it did not achieve widespread use.
6529 ICs were available in 1MHz,2 MHz, and 3 MHz versions. Theform factorwas aJEDEC-standard 20-pinceramicorplasticDIP.
The 6529 differs from a 74(LS)639 bidirectional three-state/open-Collector-busdriver in that the 6529 has passive output pullups and power-on reset circuitry.