Counter

The counter is a software-independent, high-speed counter of rising edges of changing signal on the digital input. In simple words, it can count fast changing signals very precisely.

Counter inputs are suitable especially for collecting data from energy meters, water meters, gas meters and other pulse meters installed in HVAC systems. They are also useful for reading engine revolutions. For this purpose, Unipi units feature 32-bit registers. When the maximum value of the counter (4 294 967 295) is exceeded, the counter resets to 0. Counter inputs can be used at up to 10 kHz frequency. In the Mervis IDE library devices, they are labelled as CNT inputs.

The CNT input variables are read-only. If you want to change the counter you must use CNT_set variable.

The speed of Mervis programs is not enough for detecting quick changes of digital inputs traditionally (with the “Pulse counter (BD30)” FUPLA box for example). If you have a slot car track and you want to count the number of passes of slot cars, you wouldn't be able to detect it, since the length of pulses from the “pass” sensors will be in units of milliseconds. Speed of the slot car is up to 80 kmph and with the length of the car about 10 cm is the pass of the whole car above the sensor about 4.5 ms.

To demonstrate how slow is the software counting, look at this program in debugging mode:

As you can see, we are counting the rising edge events on the Patron_S167_DI_1.01 through Pulse counter (BD30) function block, and reading the HW counter of the same input via the Patron_S167_CNT_1.01 variable. Afterwards, we compare those two variables in eq function block, and if they are equal, we will turn the X2 LED on the controller ON. Once the two counters are not equal, the LED will be OFF.

The debugging started with both counter values at 0. We connected a momentary switch to the DI1.1 input and start pressing the switch very quickly. As you can see, the software counter detected only 76 presses, but the hardware counter detected 81. The difference seems small, but in more practical applications like reading speed of rotor the impulses can overwhelm the SW counter very easily.

Each digital input is equipped with counter and you don't need to set anything to use it. The counter is available through a global variable. For the input named Patron_S167_DI_1.01 we have counter variable named Patron_S167_CNT_1.01.

The counter variables are read-only, therefore you cannot set or reset them directly from the program or the variable browser in debugging mode. You can set or reset the variables via the “set” variable. For the Patron_S167_CNT_1.01 the “set” variable is called Patron_S167_CNT_1.01_set. For example, changing Patron_S167_CNT_1.01_set variable to value 23 will set the Patron_S167_CNT_1.01 counter to 23.

To reset the counter, the “set” variable need to have different value than your desired starting point, then you can change the “set” variable to your desired starting point. This works fine in debug mode, but if you want to reset the counter with fbd/st you should use different way.

To be able to work with following variables you need to set the autogen settings to include attributes. This method also uses the “set” variable, but only to copy the value. First make sure the “set” variable has the desired value. Next you can reset the counter by rising edge on ForceComm variable, in this case the variable would be Patron_S167_NCnt1.01_wr_woch_forcecomm. Shortly the Patron_S167_CNT_1.01 counter should be set to same value as Patron_S167_CNT_1.01_set variable.