Chapter 6 Digital I/O
NI USB-621x User Manual 6-6 ni.com
Programmable Power-Up States on USB-6212/6216 Devices
At system startup and reset, the hardware sets all PFI and DIO lines to
high-impedance inputs by default. The DAQ device does not drive the
signal high or low. Each line has a weak pull-down resistor connected to it,
as described in the NI USB-621x Specifications.
NI-DAQmx supports programmable power-up states for PFI and DIO lines.
Software can program any value at startup to the P0, P1, or P2 lines. The
PFI and DIO lines can be set as:
• A high-impedance input with a weak pull-down resistor (default)
• An output driving a 0
• An output driving a 1
Refer to the NI-DAQmx Help or the LabVIEW Help in version 8.0 or later
for more information about setting power-up states in NI-DAQmx or MAX.
Increasing Current Drive on USB-6212/6216 Devices
The total internal current limit for digital outputs and power drawn from the
+5 V terminals is 50 mA. You can increase this internal current limit by
supplying an external +5 V supply. Refer to the +5 V Power as an Input
section of Chapter 3, Connector and LED Information.
Connecting Digital I/O Signals on USB-6212/6216 Devices
The DIO signals, P0.<0..15>, P1.<0..7>, and P2.<0..7> are referenced to
D GND. You can individually program each line as an input or output.
Figure 6-4 shows P0.<0..3> configured for digital input and P1.<0..3>
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. Digital output applications include sending
TTL signals and driving external devices, such as the LED shown in the
figure.
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