There should be a label on the card with text that starts with 184021. If the character following that number is F the card will work in either a 3.3V or 5V slot. If the character following the number is A, B, C, D, or E the card will only work in a 5V slot. The currently shipping version of the PMC-GPIB will work in either slot and all future versions of the card should also work in either slot.