THDIO-G Modules Digital Inputs

The inputs are optically coupled. The optical coupler (also called an opto-coupler or opto-isolator) provides the transference of the input state while providing complete electrical isolation. Electrical isolation is of paramount importance in applications such as greenhouse control.

You must supply a 5-24V AC or DC signal to the inputs. The “Active Low” box determines if the input is considered logically on (active) when a signal is applied or not. If the “Active Low” box is unchecked, then the input reads “ON” or active when an input signal is applied. If the box is checked, then the input reads “ON” or active when the input signal is removed.

The following diagram gives a simplified schematic.

As seen from the above circuit, the input is rectified by a diode bridge. A 1.2K (kilo-ohm) resistor and a 1 mF (micro-Farad) capacitor provide the current limiting and filtering functions. The input current should not exceed 20 mA (milli-Amperes).

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