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7.5 TIMERS/COUNTERS/LATCHES

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

Note: As we follow this graph from left to right we are going through time. When the line moves up (on) or down (off) we can see how inputs and outputs

•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

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7.6 LATCHES

- Will stay on when set, until unlatched.

A D

L

A C

B D

U

• We can show how these latches respond with a simple diagram.

A

B

C

D

• As an example consider the ladder logic:

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

(Note: In some other PLCs latches are only used for keeping the state of the PLC when it was turned off, they don’t ‘stick’ on or off)

7.7 TIMERS

• We use timers to do some or all of the following: