The input and output of the timer cards is addressed and switched by software. If a card receives a signal over a timer input, the preprogrammed processes can be executed. If signals only have to be counted, the on-board counter can count them. The cards have a wide range of usability because of the inputs and outputs as well as their interrupt ability.
Timer cards are often used in PC measuring technique. With the Timer cards, several impulses can be counted and the duration can be calculated. The card can also be programmed to create an interrupt after a number of impulses. With this interrupt, an alarm clock could be switched on, for example. Rotation speeds can be calculated which can be used for the supervision of a machanical device. Another field of use is the creation of frequencies.
The 8253 and 8254 are often used counters. Counter modules only can count distinct events.
The 8253 is built by several factories. It is a counter module and supplies 3 16-bit timers.