National Instruments NI USB-621x - Manual (Page 124)

Chapter 10 PFI
NI USB-621x User Manual 10-6 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 specifications document for your device.
NI-DAQmx 8.3 and later supports programmable power-up states for PFI
and DIO lines. Software can program any value at power up 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 8.x Help for more
information about setting power-up states in NI-DAQmx or MAX.