PLC COUNTER FUNCTIONS

A counter is a simple device intended to do one simple thing - count. Using them, however, can sometimes be a challenge because every manufacturer (for whatever reason) seems to use them a different way. Rest assured that the following information will let you simply and easily program anybody's counters.

What kinds of counters are there?

Well, there are up-counters (they only count up 1,2,3...). These are called CTU,(count up) CNT,C, or CTR. There are down counters (they only count down 9,8,7,...). These are typically called CTD (count down) when they are a separate instruction. There are also up-down counters (they count up and/or down 1,2,3,4,3,2,3,4,5,...) These are typically called UDC(up-down counter) when they are separate instructions.

Many manufacturers have only one or two types of counters but they can be used to count up, down or both. Confused yet? Can you say "no standardization"? Don't worry, the theory is all the same regardless of what the manufacturers call them. A counter is a counter is a counter...

To further confuse the issue, most manufacturers also include a limited number of high-speed counters. These are commonly called HSC (high-speed counter), CTH (CounTer High-speed?) or whatever. 

Typically a high-speed counter is a "hardware" device. The normal counters listed above are typically "software" counters. In other words they don't physically exist in the plc but rather they are simulated in software. Hardware counters do exist in the plc and they are not dependent on scan time.
A good rule of thumb is simply to always use the normal (software) counters unless the pulses you are counting will arrive faster than 2X the scan time. (i.e. if the scan time is 2ms and pulses will be arriving for counting every 4ms or longer then use a software counter. If they arrive faster than every 4ms (3ms for example) then use the hardware (high-speed) counters. (2xscan time = 2x2ms= 4ms)

To use them we must know 3 things:

1. Where the pulses that we want to count are coming from. Typically this is from one of the inputs.(a sensor connected to input 0000 for example)
2. How many pulses we want to count before we react. Let's count 5 widgets before we box them, for example.
3. When/how we will reset the counter so it can count again. After we count 5 widgets lets reset the counter, for example.