Use of counters on logic inputs

The SysGetCounter FB manages the physical hardware counter that is available on the DI00 of CPU module and on some I/O extension modules (please refer to its documentation). The Relay I/O card hasn’t the counter it has only the quadrature encoder counter and for this the FB returns an error.

Now the question is how faster is the clock signal you have to count, because by reading the logic input module on Fast task you can sample frequencies up to 400 Hz.

The range of frequency can be read surely in a Fast task program but you have to make some considerations.

First consideration, the program at every 1 mS acquires the clock signal status, so other than the frequency it’s important to consider the signal duty cycle. It’s important to know how much time the signal remain active and not active, these times must be longer than the execution time of the program (1 mS).

Second consideration, in the Fast task program you cannot use the logic inputs mapped in the memory (I.e. %IX0.0) because these inputs are not acquired immediately but they represent the process image. To acquire the logic inputs immediately you have to use the SysGetPhrDI, get peripheral digital input FB.

Third consideration, the inputs on CPU module can be read fast they are directly attached to the CPU. The peripheral modules are connected by using a I2C bus and to read a module it’s required about 200 uS. For these reasons it’s better to use the inputs on CPU module for the signals with higher frequency and other inputs for the lower frequency. Moreover a single read acquires all the 16 inputs on the module so if you have many clock signals it’s better to connect them to the same module.