Knowledge Base

HD8: External inputs and outputs

The HD8 contains 4 digital I/O points which can be used as inputs or outputs. We call these “bidirectional”. In effect they consist of an input and an output brought out to a single pin. If the output is OFF, an externally connected device (say an NPN sensor or a contact) can control the on/off state of the pin. If the output is turned on, it connects the I/O pin to 0V via a transistor switch and it registers an ON input. Hence to use a pin as in input you simply make sure its output circuit is turned off.