mesa GPIO voltages

Just wondering... I know that the mesa GPIO ports are 5v tolerant..... and I noticed that the 7i76E is actually 3.3v output rated.

I was wondering if the 7i92 is 5v output or 3.3v?   (I suspect 5V from the manual as there is no mention of 3.3v like the 7i76E)

I was also wondering what the input voltage tolerance was?

If I use a 5.1V zener diode (1N4733A // BZT52C5V1 // ZMM5V1-M)   or  as a voltage clamp.... would that be ok or would it possibly kill the FPGA?

The range on a 5.1V zener could be anywhere from 4.8 to 5.4V due to manuf tolerance (and I suspect operating temp too).

should I use a lower voltage zener?

The GPIO on the 7I92 or 7I76E is 3.3.V but has a 4/7K pullup to 5V
so will swing to 5V if there is no load (and the card is set for 5V tolerance mode)

The bus switches on the GPIO pins will take positive voltages up to 7V
however they will only take about -0.5V due to the input clamp diodes.

Negative voltages without a current limit are the most likely way to damage
GPIO pins. How do you get a negative input voltage with unlimited current?
Its very easy, simple connect a GPIO pin to frame ground when you have a step drive
or VFD running and you have a volt or so of MHz frequency ground bumping, (at RF
frequencies generated by power electronics switching edge rates, "ground" is just a
tapped inductor..) The easiest way to protect against this is to add a small series
resistor (say 220 Ohm) on the GPIO pins