📖 8. Programmable Peripheral Interface (PPI) 8255A

📖 From An Introduction To Microprocessor 8085 • 163 questions available

About 8. Programmable Peripheral Interface (PPI) 8255A

The 8085 is the "brain," but it needs a way to connect to the outside world, like LEDs, motors, or switches. The 8255A is a helper chip that acts like a "personal assistant" for the CPU. It provides three 8-bit ports (Port A, Port B, and Port C) that the CPU can program to be either Input ports (to read data from external devices) or Output ports (to send data to external devices). It makes the complex job of interfacing with I/O devices much simpler.

How It Works: You program the 8255A by writing a "Control Word" to its internal "Control Register." This Control Word tells each port whether it is input or output. For example, you can set Port A as output to turn on a set of LEDs, and Port B as input to read the state of a bank of switches. The 8255A has three operating modes (Mode 0, 1, and 2). Mode 0 is the simplest (basic I/O), while Mode 1 and 2 are "handshaking" modes, where the peripheral device and the CPU use extra signals (like "Data Ready" or "Acknowledge") to ensure data is transferred correctly.

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🔄 Last updated: 2026-07-30