SCXI-1162HV Input Readings Flickering When They Should Be Digital Low

Updated Nov 6, 2019

Reported In

Hardware

  • SCXI-1162HV

Issue Details

I am using an SCXI-1162HV module with an SCXI-1326 terminal block to sense both DC and AC signals. I am connected to a PC-DIO-96. Everything reads correctly with DC signals, but with the AC signals, the digital values intermittently flicker on and off.

Solution

If flickering occurs in the energized state (i.e. when the AC signal is on, and the module should return a logic high), it is possible that the signal is not clean.
  • For example, if a power line is not clean, it may miss cycles, which will look to the SCXI-1162 module like the source is off momentarily. This can happen if the AC signal stays around 0 V (0-2 V) for 1.5 ms or longer. This is not incorrect behavior for the module.
     
If flickering occurs in the de-energized state (i.e. the AC signal is off), then noise could be the culprit.
  • Check connections and grounding.
  • Make sure that connections are across a load and not across a relay switching the load.
  • If using shielded wires, be careful to ground the one end of the shield to the same ground that the signal uses, not some other ground. This means you need to have different shields for each bank of signals on the SCXI-1326. Note that shielding is not required. Incorrect shielding is worse that no shielding.