Chapter 4
Signal Connections
©
National Instruments Corporation
4-7
DAQCard-DIO-24 User Manual
Digital I/O Signal Connections
Pins 1 through 24 of the I/O connector are digital I/O signal pins. Pin 27
is a digital ground pin.
The following specifications and ratings apply to the digital I/O lines:
•
Absolute maximum voltage rating:
–0.5 to +5.5 V with respect to GND
•
Digital input specifications (referenced to GND):
–
Input logic high
2.2 V minimum
5.3 V maximum
voltage
–
Input logic low
–0.3 V minimum
0.8 V maximum
voltage
–
Maximum input
–1 µA minimum
1 µA maximum
current
(0 < V
in
< 5 V)
•
Digital output specifications (referenced to GND):
–
Output logic high
3.7 V minimum
—
voltage at
I
out
= –2.5 mA
–
Output logic low
—
0.4 V maximum at
voltage
I
out
= 2.5 mA
Figure 4-3 shows an example of connections to the digital input and
output ports. In this figure, port A of the 82C55A is configured for
digital output, and port B is configured for digital input. Digital input
applications include receiving TTL signals and sensing external device
states such as the switch shown in Figure 4-3. Digital output
applications include sending TTL signals and driving external devices
such as the LED shown in Figure 4-3.