In the I/O mode, the 8255 ports work as_______.

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Examoogle · Accepted Answer

Programmable I/O ports

Explanation: The Intel 8255 Programmable Peripheral Interface (PPI) is a versatile chip designed to provide I/O interface for microprocessors. Its ports (Port A, Port B, and Port C) are software-configurable, meaning they can be programmed to function as input, output, or bidirectional ports via control word register settings.

Working Principle: The 8255 operates under the control of a Control Word Register (CWR). By writing a specific bit pattern to the CWR, the user configures the direction (Input/Output) of Ports A, B, and C. In I/O mode (Mode 0, 1, or 2), the ports are treated as memory-mapped or I/O-mapped registers, allowing the CPU to read from or write to these pins as per the selected configuration.

Key Formulas: $Mode_{Selection} = Bit D_7 of CWR$; $ControlWord = 1XXXXXXX_2$ (for I/O mode selection)

Key Points: The 8255 has three 8-bit ports: Port A, Port B, and Port C.; Mode 0 is the Basic I/O mode, Mode 1 is Strobed I/O, and Mode 2 is Strobed Bi-directional I/O.; Port C can be split into two 4-bit ports (PC_upper and PC_lower) for individual bit control.; Initialization is performed by sending a command byte to the Control Register address.

Advantages: Reduces the number of chips needed for I/O interfacing.; Provides high flexibility through software control.; Supports handshaking signals for efficient data transfer.

Disadvantages: Requires complex software initialization compared to static latches.; Uses I/O address space, which might be limited in older 8-bit systems.

Applications: Interfacing keyboards and displays.; Driving relays and sensors in industrial automation.; Data acquisition systems.

Additional Info: The 8255 PPI is a 40-pin IC.. Option A and B are incorrect as 'Set/Reset' refers to the capability of bit-level manipulation (BSR mode) within Port C, not the general designation of the ports.

Answer: C is correct — The 8255 ports are highly versatile and can be configured as input, output, or bidirectional via software command, hence they are called Programmable I/O ports.

Exam Tip: Always remember that in the 8255, the control register is written to at the base address + 3, and only one mode can be active for a specific port group at any given time.

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