Counters are used to detect pieces numbers and events. Controllers frequently need to operate with counters in practice. For example: a counter in circuit is required if exactly 20 identical components are to be conveyed to a converyor belt via a sorting device.
There are two basic counter types a) Count Up b) Count down
When the input to count up counter goes true the accumulator value will increase by 1 ( no matter how long the input is true). If the accumulator value reaches the preset value the counter bit will be set. A count down counter will decrease the accumulator value until the preset vaule is reached. Symbols are shown in Figure 1.8