Chapter 7 PFINI USB-621x User Manual 7-6 ni.comI/O ProtectionEach 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 orPFI output signals. Do not exceed the specified current output limits ofthe DAQ device. NI has several signal conditioning solutions fordigital applications requiring high current drive.• Do not drive a DI or PFI input line with voltages outside of its normaloperating range. The PFI or DI lines have a smaller operating rangethan the AI signals.• Treat the DAQ device as you would treat any static sensitive device.Always properly ground yourself and the equipment when handling theDAQ device or connecting to it.Programmable Power-Up StatesAt system startup and reset, the hardware sets all output PFI and DO linesto high-impedance by default. The DAQ device does not drive the signalhigh or low. Each line has a weak pull-down resistor connected to it, asdescribed in the NI USB-621x Specifications.NI-DAQmx supports programmable power-up states for PFI and DIO lines.Software can program any value at power up to the P1 lines. The output PFIand 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 1Refer to the NI-DAQmx Help or the LabVIEW Help in version 8.0 or laterfor more information about setting power-up states in NI-DAQmx or MAX.
