Chapter 7 PFI
NI USB-621x User Manual 7-6 ni.com
I/O Protection
Each DI, DO, and PFI signal is protected against overvoltage,
undervoltage, and overcurrent conditions as well as ESD events. However,
you should avoid these fault conditions by following these guidelines:
•Do not connect a DO or PFI output lines to any external signal source,
ground signal, or power supply.
• Understand the current requirements of the load connected to DO or
PFI output signals. Do not exceed the specified current output limits of
the DAQ device. NI has several signal conditioning solutions for
digital applications requiring high current drive.
•Do not drive a DI or PFI input line with voltages outside of its normal
operating range. The PFI or DI lines have a smaller operating range
than the AI signals.
• Treat the DAQ device as you would treat any static sensitive device.
Always properly ground yourself and the equipment when handling the
DAQ device or connecting to it.
Programmable Power-Up States
At system startup and reset, the hardware sets all output PFI and DO lines
to high-impedance 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 P1 lines. The output PFI
and DO 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.
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