Chapter 4 Digital I/O
© National Instruments Corporation 4-9 E Series User Manual
Power-On States of the PFI and DIO Lines
At system startup and reset, the hardware sets both the PFI and digital lines
to high-impedance. This setting means that the device circuitry is not
actively driving the output either high or low. However, these lines might
have pull-up or pull-down resistors connected to them, as shown in the
I/O Terminal Summary table in the specifications of each device. These
resistors weakly pull the output to either a logic high or logic low state.
For example, P0.0 is in the high-impedance state after startup, and the
I/O Terminal Summary table shows that there is a 50 kΩ pull-up resistor.
This pull-up resistor sets the P0.0 pin to a logic high when the output is in
a high-impedance state.
Caution If you enable a PFI line for output, do not connect any external signal source to
it. Doing so could damage the device, the computer, and the connected equipment.
Connecting Digital I/O Signals
All devices have DIO signals P0.<0..7> and D GND. P0.<0..7> are the
eight digital lines making up the DIO port, and D GND is the
ground-reference signal for the DIO port. You can individually program all
lines as inputs or outputs. Figure 4-6 shows P0.<0..3> configured for
digital input and P0.<4..7> configured for digital output. Digital input
applications include receiving TTL signals and sensing external device
states such as the state of the switch shown in the Figure 4-6. Digital output
applications include sending TTL signals and driving external devices,
such as the LED shown in Figure 4-6.
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