SPLatco Knowledge Base

Each of the 16 I/O pins on the XBIO16 is bidirectional. That means each one can be used as an input or as an output. In actual fact, each pin is connected onboard to an input circuit and to an output circuit, in parallel. This works because when an output is OFF, it does nothing. Only when an output is turned on does it become a low resistance path to 0V. Hence, if the output circuit is left OFF, the input can respond to an external input device, say a switch or a sensor.

There is a single shared LED indicator per I/O pin. One end of the LED is connected to the + (positive) supply via a resistor, and the other end is connected to the I/O pin. Hence, when the I/O pin is connected to 0V the LED lights up. This can be due either to the output being switched on by your program, or by an external input device connecting the pin to 0V.

The input sensing circuit represents a large resistance value to 0V (some 100’s of kOhms). A very small current will flow from the LED circuit through this resistance when the I/O pin is OFF. That means that when the LED should be off it will glow dimly. This is perfectly normal and does not mean there is anything wrong.