They can  count up; count down or count up and down. Counters can start at zero and count up or be loaded with a pre-determined value and count down to zero. Since they are in memory their counting speed is limited an so manufacturers usually include “high speed” counters that are hardware not software based.

The Counter needs certain information such as its “number” and preset count value.

In the above example it is Counter 1 and its preset count value is 100
When I 0.1 goes to status “1” then the counter will count up, when I 0.1 goes to status “1” Then the counter will count down.

Other inputs will load the counter with its pre-determined valve or reset it to zero. The counter will only count, up or down, when it sees the change in state from “0” to “1”. It will not count on the change in state from “1” to “0”.