8255s

The Intel 8255 Programmable Peripheral Interface (PPI) is an integrated circuit designed to provide a versatile interface between a microprocessor and peripheral devices. It was commonly used in older microcomputer systems, particularly those based on the Intel 8085 and 8086 microprocessors. The 8255 offers 24 parallel I/O pins that can be programmed in various configurations to suit different input and output needs.

The 8255 features three 8-bit ports, Port A, Port B, and Port C. Port C can be further divided into two 4-bit nibbles, allowing for more flexible control. The device supports two main modes of operation: Mode 0 and Mode 1. In Mode 0, the ports function as simple, independent input or output ports. Mode 1 introduces strobed communication, where Port C provides handshake signals for Port A and Port B, enabling more controlled data transfer. A more advanced Mode 2 operation is also available, offering bidirectional data transfer on Port A, utilizing Port C for handshake signals.

Configuration of the 8255 is achieved through a control word written by the microprocessor. This control word dictates the mode of operation for each port and whether they are set as input or output. The 8255 was instrumental in connecting a wide range of peripherals, such as keyboards, printers, displays, and analog-to-digital converters, to early microprocessor systems before the widespread adoption of more integrated peripheral controllers.