Intel 8279 is a specially designed Input Output port chip which performs two major functions. It scans a matrix keyboard, as well as it displays a multiplexed display such that the processor gets relieved from the extreme tasks and let it performs systematically. We had given a brief description about the working of 8279 as we have indicated in the following paragraphs.
We continuously scan an 8279 matrix keyboard. During the scanning process, if a key gets pressed on the keyboard a scanning is performed again about 10 μs, resulting bouncing of the key to die down. If it is found that the key is still pressed, we interpret it as a valid key depression. After that the scan code of the key gets pressed and is stored in the FIFO RAM. The location of the key is indicated by the scan code including the status of control and shift key. FIFO RAM is present within 8279 consisting of size length of eight words, ranging each of width of 8 bits. Then IRQ which is the interrupt request output, gets activated by the 8279. The IRQ output gets connected to a pin of interrupt request of the microprocessor. The FIFO RAM is read by the processor on a first-in first-out basis just for receiving information of 8279 resulting the scan code to get pressed. For refreshing multiplexed display 8279 is used. The interesting fact is that only due to the principle of persistence vision the total 16 LED displays the characters continuously and in a simultaneous manner. The display of 16 LEDs takes about 10μs.