Classification of Ports

"Programmable Port" 

This port is also an integrated circuit in which function of a
port can be changed with the help of a program. One port can be used
as an input port at one time. The same port can be used as an output
port at other time by giving some command to this port. Also each port
can be utilized in different modes with the help of software. Each
programmable port must have a CONTROL REGISTER. The function of
control register is to hold the necessary command word for the
operation of the port .


The programmable ports also have STATUS
REGISTER to read the status of different ports in the chip. For
example IC8255 is having three 8-bit programmable ports.
"Non - Programmable Port "
This port operates as input port or as output port for the whole
life. The function of this port cannot be changed with the help of a
program. For example a tristate Buffer always works as an input port
and a latch always works as an output port. Intel IC 8212 is a non -
programmable input - output port. That IC can be connected to work as
an input port or as an output port. But ones it is connected it the
circuit us function cannot be changed by user.

USE OF PORTS

"Ports" 

The input-output devices are not directly connected to the CPU. Because he the IO devices are directly connected, it will reduce the number of IO devices that can be connected to CPU.
The interface between IO devices and CPU is called Port. Ports are the electronic circuits used to transfer the data to an output device and used to accept data from an input device. Each port in the computer system is identified with the help of an address called PORT ADDRESS.
Now a days the whole circuit of the port can be packed in a single integrated circuit (IC). There are so many ways to classify a port.

1. Input Port - This ports accepts data from an input device and then
supply this data to the CPU. The CPU enables this port by providing
appropriate port address and necessary control signals to the
supporting logic circuit of the port.

2. Output Port - This port accepts data from CPU and supplies this
data to output device. CPU enables this port by providing appropriate
port address and necessary control signals.