Timers and counters

Timers and counters have been in existence for as long as relays and provide an important component in the development of logic. Timers were constructed in the past as an add-on device to relays slowing down the transition of the plunger from immediately opening or closing. The time delay was accomplished with a pneumatic bladder that allowed the air to escape either quickly or slowly depending on the setting of the timer. Quick was usually less than a second and slow was usually between 30 and 60 seconds. Setting this kind of timer was an inexact science and today’s traffic lights are an example of the fickle nature of timers that seldom respond in exactly the same from day to day and year to year. For the first time, function blocks are introduced in the rung output position or coil position to provide timer and counter functions. Function blocks allow inputs from the left and pass power through to the right when the function is done or when various conditions are met. Either the timer has timed out or the counter has counted to the preset. Function block usage differs from manufacturer to manufacturer. Function blocks rely on a standard format to enter information about the counter or timer. All variables in the function block must be entered correctly before the device will operate. Some timers are referred to as retentive. Retentive refers to the device’s ability to remember its exact status such that when the circuit is again activated, the timer continues from the previous point. Non-retentive timers reset to zero and start from zero each time the timer function block is energized. Retentive is similar to blowing up a balloon. One does not blow a balloon up with one blast of air. It takes quite a few. The retentive balloon has a finger along the neck of the balloon holding the air already blown in captive. When more air is blown in, the new air is added to the air already present. Many processes in the factory rely on logic needing this kind of physical property to control a machine. Other terms used in the timer and counter blocks are “preset” and “accumulated”. These words refer to the preset or target amount and the “accumulated” amount that the timer or counter has built to get to a preset. Times are really counts stored as integer numbers. Thus, counters and timers are very similar. Timers increment a number regularly each time period (usually in increments of 1 msec.).


Timers are used to provide logic when a circuit turns on or off. Traditional pneumatic timers were provided as either on-delay timers or off-delay timers. Contacts were provided both normally open and normally closed for each type of timer. The timer head was chosen as either the on-delay type or off-delay type. PLCs allow for a quick change from one type to the other with a few keystrokes on the programming panel.

Allen-Bradley SLC Timers and Counters

Timer On Delay TON is the non-retentive instruction for on-delay timers. It is used to provide signals that change state a time delay after the TON block is energized. TOF is the non-retentive instruction for off-delay timers. RTO is the retentive timer instruction. It does not reset to an initial value but rather stays at an accumulated value each time the input to the function block is energized. It ‘retains’ the count previously accumulated and continues on from that value. It must be reset with a RES (Reset) command. Reset commands are useful for not only the retentive timer instruction but also any timer or counter with a retentive nature.