17
GPI Port Circuitry Details
The gBOX GPI input and output circuits are detailed in the
following section.
The inputs of the gBOX GPI ports are CMOS inputs. The input
circuitry has a 4700 ohm pullup resistor to +5 volts as referenced
to pin 1 of the GPI Input Port.
Note: Because the inputs are CMOS, the input voltage MUST be
limited to voltage levels between 0 and 5 volts. If this is not possible,
consider using the gBOX I/O.
The internal pullup resistor insures that the input pin is set to a
known state. The default state of the GPI Inputs is +5 volts or a
logic state of ‘1’ in the GPI message bitmap. The internal pullup
resistor also allows a simple switch or ‘dry contact’ to be
connected between a GPI Input pin and ground as shown in the
example below.
74HC244
5 Volts
Detail of GPI Input
4700
Ω
4700
Ω
74HC244
5 Volts
GPI Input Example with Pushbutton Switch
1
2