PXIe-782XR digital output failure detectability

[ToKoSi]

Hi,

I am considering using PXIe-782XR cards for a safety critical system. The FPGA has a large number of digital IO: most would be used as inputs and a few as outputs. To drastically simplify: if state of any input transitions to low level, one of dedicated outputs would also be forced low.

It would be beneficial for me to detect in the FPGA if an output stage on the card fails (the critical failure mode would be if it latches high). Mitigating the failure is not required (that is done elsewhere in the system), but detecting the error in the FPGA and reporting it to the host would be very nice. Of course, cable integrity etc is also out of scope here, I am purely interested in the output buffer failure detectability.

My understanding is that the 128 DIOs on the card are bidirectional buffers (see https://www.xilinx.com/support/documentation/user_guides/ug471_7Series_SelectIO.pdf page 39). I would like to understand what happens if I am writing to and reading from the same digital output in a SCTL (see example image). Would the data just be read from an intermediate internal register (what does LabVIEW do here?), or would I actually read the logic level from the wire connected to the card's front panel, despite also writing something to it in the same cycle?

Cheers

[VA.KI · Accepted solution]

Hello ToKoSi,

You should use Set Output Data and Set Output Enable methods.

Take in mind that in SCTL (Single-Cycle Timed Loop) the entire contents of the loop will executes within one tick of the FPGA clock you have selected. So try to perform read and write functions not in the same tick.

This two articles will give answers to your questions (links are below) more detailed.

https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z0000019NgvSAE&l=en-US

http://zone.ni.com/reference/en-XX/help/371508V-01/target2devicehelp/digital_data_and_digital_enable...

[VA.KI · Accepted solution]

Hello ToKoSi,

In addition to my last comment:

The logic in the screenshot example will work, just keep in mind that you should ignore first three ticks in this case.