The parallel input-output port chip 8255 is also called a programmable peripheral input-output port. Intel‟s 8255 is designed for use with Intel‟s 8-bit, 16-bit, and higher capability microprocessors. It has 24 input/output lines which may be individually programmed in two groups of twelve lines each, or three groups of eight lines.
The two groups of I/O pins are named Group A and Group B. Each of these two groups contains a subgroup of eight I/O lines called an 8-bit port and another subgroup of four lines or a 4-bit port. Thus Group A contains an 8-bit port along with a 4-bit port. C upper. The port A lines are identified by symbols PA0-PA7 while the port C lines are identified as PC4-PC7.
Similarly, Group B contains an 8-bit port B, containing lines PB0-PB7 and a 4-bit port C with lower bits PC0- PC3. The port C upper and port C lower can be used in combination with an 8-bit port C. Both ports C are assigned the same address. Thus one may have either three 8-bit I/O ports or two 8-bit and two 4-bit ports from 8255.
All of these ports can function independently either as input or as output ports. This can be achieved by programming the bits of an internal register of 8255 called a control word register ( CWR ). This buffer receives or transmits data upon the execution of input or output instructions by the microprocessor. The control words or status information is also transferred through the buffer.
