Difference between 2-key lockout and n-key rollover of 8279 processor?

Answer 1

Key depressions in 8279 can be processed as a two key lockout or an n-key rollover.

In scanned keyboard mode with two key lockout, when a key is pressed debounce logic comes into operation. During the next two scans, the other keys are checked for closures and no other key is pressed, the first key is taken into consideration. The key code of the key pressed along with the SHIFT and CNTL status is entered into the FIFO if it is not full. If the FIFO is full, then the data will not be entered and an error flag is set.

In scanned keyboard with N-key rollover, each key depression is treated independently. When a key is pressed, the debounce circuit waits for two keyboard scans and then checks whether the key is still depressed. If it is still depressed, the code is entered in FIFO. Any number of keys can be pressed simultaneously and recognized in the order, the keyboard scan recorded them. All the codes of such keys are entered into the FIFO.