National Instruments VXI-USB - Manual (Page 47)

Appendix D Common Questions
VXI-USB User Manual D-4 ni.com
When all LEDs except the SYSFAIL LED are on, the hardware detected
that the FPGA PLL lost lock, and the board may be in an unstable state.
Power cycle the board to repair it.
In an empty chassis, when both the SYSFAIL and FAIL LEDs are solid on,
the firmware image is corrupted. Recover the firmware to repair the board.
What kind of signal is CLK10 and what kind of signal do I need for an
external CLK10?
CLK10 is a differential ECL signal on the VXIbus backplane. However, the
oscillator for the VXI-USB and the EXT CLK input from the front panel
use TTL. Therefore, supply a TTL-level signal for EXT CLK, and onboard
voltage converters automatically convert the signal to differential ECL.
What is the accuracy of the CLK10 signal?
The CLK10 generated by the VXI-USB is ±100 ppm accurate. If you need
a more accurate CLK10 signal, you can use the EXT CLK input at the front
of the VXI-USB.