• There are some devices, and concepts that are Temporal. This means that they keep track of events over time, as opposed to conditional logic that decides based on instantaneous conditions.



• Controls that have states, or time dependence will require temporal controls (also known as sequential)


• Some devices that are temporal are


Flip-Flops - these can be latched on or off.

Latches - Will stay on until reset (Similar to flip-flops)

Counters - Keeps a count of events

Timers - Allows inputs and outputs to be delayed, or prolonged be a known amount.



1.2.1 Latches




• Assuming that the PLC is much faster than the fastest event (i.e. no aliasing), then we can approximate the timing diagram above with a simpler form.



• As an example consider the ladder logic for a SLC-150,



• In most PLCs these will keep their last state even when the PLC is turned off, and back on.


• Write a short ladder logic program that will,




1.2.2 Flip-Flops


• There are many type, but one of the best known is the SR (Set-Reset) Flip-Flop.




1.2.3 Timers


• We use timers to do some or all of,

- Delay turning on

- Delay turning off

- Accumulate time passed







• When using timers (especially retentive) we must reset values when done. The (RES) instruction does this.


• The timers also have timer timing bits that indicate when the accumulator value is changing.


• Consider the example below, Write out the values for the accumulator, enable, timer timing and the done bits.



• Repeat the last example for the timer below.




• Consider the example of the SLC timers given below,




• As an example,



• When more than one event should happen in a sequence, timers can be cascaded. This means that the end of the first timer starts the next.


• As an example,




1.2.4 Counters


• Count up/count down counters will track input events.



• Count down counters are similar but can be used for convenience.


• Consider the example below for a SLC-150,



• Write a simple program that will,