The D8255 is a programmable I/O device which is designed for use with all Intel and most other microprocessors. It provides 24 I/O pins which may be individually programmed in 2 groups of 12 and used in 3 major modes of operation:

● Mode 0 - Basic Input/Output. This functional configuration provides simple input and output operations for each of the three ports. No „handshaking'' is required, data is simply written to or read from a specified port. Mode 0 Basic Functional Definitions:
  ○Two 8-bit ports and two 4-bit ports.
  ○Any port can be input or output.
  ○16 different Input/Output configurations are possible in this Mode

● MODE 1 - Strobed Input/Output. This functional configuration provides a means for transferring I/O data to or from a specified port in conjunction with strobes or „handshaking'' signals. In mode 1, Port A and Port B use the lines on Port C to generate or accept these „handshaking'' signals. Mode 1 Basic functional Defini
tions:
   ○Two Groups (Group A and Group B).
   ○Each group contains one 8-bit data port and one 4-bit control/data port.
   ○The 8-bit data port can be either input or output Both inputs and outputs are latched.
   ○The 4-bit port is used for control and status of the 8-bit data port.

● MODE 2 - Strobed Bidirectional Bus I/O. This functional configuration provides a means for communicating with a peripheral device or structure on a single 8-bit bus for both transmitting and receiving data (bidirectional bus I/O). „Handshaking'' signals are provided to maintain proper bus flow discipline in a similar manner to MODE 1. Interrupt generation and enable/disable functions are also
available. MODE 2 Basic Functional Definitions:
  ○Used in Group A only.
  ○One 8-bit, bi-directional bus port (Port A) and a 5-bit control port (Port C).
  ○The 5-bit control port (Port C) is used for control and status for the 8-bit, bi-directional bus port (Port A)

KEYFEATURES
●Compatible with industry standard 8255
●24 I/O lines individually programmed in 2 groups of 12:
   ○Group A - Port A and upper half of Port C
   ○Group B – Port B and lower half of Port C
●3 major modes of operation
   ○Mode 0 – Basic input/output
   ○Mode 1 – Strobed Input/output
  ○Mode 2 – Bi-directional Bus
●Control Word Read-Back Capability
●Direct Bit Set/Reset Capability
●Interrupt control functions
●No internal three states busses
●Fully synthesizable technology independent source code.