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From this paragraph, I understand that the reason for putting a notch on a 32 bit PCI card is simply to prevent people that doesn't read the specifications from frying their cards.

In other words, what I understand is, 32 bit PCI cards (and their sockets) could very well be made without any notches whatsoever. But in that case, people would need to read the specification of their motherboard and of the PCI card, in order not to connect a card that has a different voltage requirement than of the motherboard.

I.e, putting notches on PCI cards is not a technical requirement, but a user experience requirement.

For 32 bit double voltage PCI cards, I understand that having two notches is to be able to insert them on both 3.3V and 5V type sockets.

My question is, why do single voltage versions of 64 bit PCI cards have two notches? Is it to create some kind of backward compatibility with 32 bit PCI card slots?

What is the reason for an extra notch in 64 bit PCI?

Edit

I am talking about the rightmost notches in the card and slot of 64 bit PCI. Both images are from Conventional PCI Wikipedia page.

enter image description here

enter image description here

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    \$\begingroup\$ You really need to illustrate this. The 64-bit PCI cards does not have a notch in the extended part, and the other notches are obviously designed so that they can fit in the sockets - the 64-bit sockets must have them because they can accept 32-bit cards. So it's unclear to me which notches and socket combinations you mean. \$\endgroup\$
    – pipe
    Commented Jul 9, 2016 at 18:20
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    \$\begingroup\$ I have a strong suspicion that the slot you refer to is wide enough to allow a 64-bit card to be installed in a 32-bit slot, whereupon it would use 32-bit mode. Compare with open-ended 4-channel (say) PCIe slots - the open end of the socket allows a card capable of communicating on more channels to be installed. \$\endgroup\$
    – Andrew Morton
    Commented Jul 9, 2016 at 18:34
  • \$\begingroup\$ @AndrewMorton So, the rightmost notches on 64 bit cards are to be able to insert them to 32 bit slots and use them as if they were 32 bit cards? \$\endgroup\$
    – Utku
    Commented Jul 9, 2016 at 18:36
  • \$\begingroup\$ @Utku Yes - you could measure the thickness of the end-wall of a 32-bit PCI socket to check it is compatible with the width of the slot in question. \$\endgroup\$
    – Andrew Morton
    Commented Jul 9, 2016 at 18:39

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Compatibility with 32-bit slots

There are no electrical reasons for the notches, but having at least one notch makes everyone's lives much easier. With that out of the way, the reason for the notches in the 64-bit card is for compatibility with 32-bit cards.

Semi-inserted PCI-X card in a 32-bit PCI slot Semi-inserted PCI-X card in a 32-bit PCI slot

32-bit cards in a 64-bit slot

A 64-bit slot can accept 32-bit cards, so the slots must be designed with the same properties. Since the question is not about 32-bit cards, we can simply accept that the 32-bit slot has notches.

A 64-bit card designed to fit in a 64-bit slot must have notches in the same place as the 32-bit cards, for this reason.

64-bit cards in a 32-bit slot

As for the rightmost notch. It could technically have been left out, and the connector could just continue on. The "problem" is, again, compatibility with the 32-bit slot.

A 64-bit card will often (always?) work well in a 32-bit slot, you will just not get the maximum bandwidth. The notch in the edge connector is necessary for the 64-bit card to physically fit in the 32-bit slot.

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    \$\begingroup\$ I think the last point is the most important here. To allow a 64bit card to be installed in a 32bit slot. \$\endgroup\$
    – Tom Carpenter
    Commented Jul 9, 2016 at 19:08
  • \$\begingroup\$ @TomCarpenter Thanks, I added some formatting to make it easier to see the different points. \$\endgroup\$
    – pipe
    Commented Jul 9, 2016 at 19:16
  • \$\begingroup\$ Ensuring a snug fit for 32-bit cards in 64-bit slots is also relevant, I suspect. \$\endgroup\$
    – Jules
    Commented Jul 10, 2016 at 12:23

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