In microcomputer systems, Input/Output (I/O) operations are crucial for the CPU to interact with external devices, known as peripherals. Parallel I/O involves transmitting multiple bits of data simultaneously over separate lines. This contrasts with serial I/O, where data is sent bit by bit over a single line. Parallel I/O offers higher data transfer rates over shorter distances and is commonly used for interfacing with devices like printers, keyboards, displays, and sensors where multiple data lines are available.

The 8255 is a versatile, programmable peripheral interface device designed to interface microprocessors with parallel I/O devices. It provides 24 programmable I/O pins, which can be configured by software in various modes to suit different application requirements.

The 8255 PPI consists of the following functional blocks:
● Data Bus Buffer: This is a tristate 8-bit bidirectional buffer that interfaces the 8255 with the system data bus (D0-D7 of 8085). It allows the CPU to read data from or write data to the 8255's internal registers (Port A, Port B, Port C, or Control Word Register).
● Read/Write Control Logic: This block manages the internal read and write operations. It accepts control signals from the microprocessor (RD, WR, A0, A1, CS) and generates appropriate internal control signals for the 8255's various functional units.
● Group A Control: This block controls the functionality of Port A and the upper 4 bits of Port C (PC4-PC7). It handles the configuration of these ports based on the control word written by the CPU.
● Group B Control: This block controls the functionality of Port B and the lower 4 bits of Port C (PC0-PC3). It configures these ports based on the control word.
● Port A (PA0-PA7): An 8-bit I/O port, programmable as either an 8-bit input or an 8-bit output. It can operate in Mode 0, Mode 1, or Mode 2.
● Port B (PB0-PB7): An 8-bit I/O port, programmable as either an 8-bit input or an 8-bit output. It can operate in Mode 0 or Mode 1.
● Port C (PC0-PC7): An 8-bit I/O port, which can be divided into two 4-bit nibbles.

Key Pins for 8085 Interfacing:
● D0-D7 (Data Bus): 8-bit bidirectional data lines for communication with the microprocessor.
● A0, A1 (Register Select): These are input pins connected to the microprocessor's lower address lines (A0, A1).
● CS (Chip Select): An active-low input. When CS is low, the 8255 is enabled for communication with the CPU.
● RD (Read): An active-low input. When CS is low and RD is low, the 8085 reads data from the selected port or internal register.
● WR (Write): An active-low input. When CS is low and WR is low, the 8085 writes data to the selected port or internal register.
● RESET: An active-high input. When high, it clears all internal registers, sets all ports to input mode, and clears the control word.
● Vcc: +5V power supply.
● GND: Ground reference.
● PA0-PA7: 8-bit I/O lines for Port A.
● PB0-PB7: 8-bit I/O lines for Port B.
● PC0-PC7: 8-bit I/O lines for Port C.

The 8255 can operate in two primary modes:
1. Bit Set/Reset (BSR) Mode: Only applicable to Port C. Allows individual bits of Port C to be set (high) or reset (low) without affecting other bits.
2. I/O Mode: Configures Ports A, B, and C as input or output ports. This mode has three sub-modes:
○ Mode 0 (Basic I/O): All ports (A, B, C) can be configured as simple latched outputs or buffered inputs. No handshaking signals are used.
○ Mode 1 (Strobed I/O): Used for data transfer with handshaking signals.
○ Mode 2 (Bidirectional I/O): Only Port A can be configured in this mode. It allows Port A to be used for both transmitting and receiving data simultaneously, with handshaking.

The functionality of the 8255 is configured by writing a specific 8-bit Control Word to its Control Word Register (CWR). The format of this word depends on whether you are setting up I/O modes or using the Bit Set/Reset feature.
A. I/O Mode Set Control Word (D7 = 1):
B. Bit Set/Reset (BSR) Mode Control Word (D7 = 0):

Connecting the 8255 to the 8085 involves connecting data lines, control lines, and address lines, along with address decoding logic.
● Data Lines: The D0-D7 pins of 8255 are connected directly to the AD0-AD7 (multiplexed data lines) of 8085.
● Control Lines:
○ 8255.RD is connected to 8085.RD.
○ 8255.WR is connected to 8085.WR.
○ 8255.RESET is connected to 8085.RESET OUT.
● Address Lines and Chip Select (CS):
○ 8255.A0 is connected to 8085.A0 (from the de-multiplexed address bus).
○ 8255.A1 is connected to 8085.A1 (from the de-multiplexed address bus).
○ The CS (Chip Select) pin of the 8255 needs to be activated (brought low) when the 8085 wants to communicate with the 8255. This is achieved using address decoding logic.