Digital inputs allow connection of the AC signal. However, due to the fact that these inputs are fast and allow counting pulses at 1 kHz frequency, it is not possible to use them to monitor the presence of a variable frequency signal like 50Hz because, depending on the moment at which the input state is read with the Modbus protocol, different results can be obtained despite the fact that AC voltage is still present. To be able to monitor the AC signals, the possibility of filtering the digital inputs has been introduced. Filtration works in a way that the high state occurs if the voltage is maintained for at least 40 ms. The low state appears when there is no voltage for at least 80 ms.
The filter can be turned on for each input of the device independently using the 40018 register.