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I was reading Why is the James Webb Space Telescope's data storage space so small? and it mentioned Mercury's 440GB space-qualified RH3440 as being bleeding-edge technology for the space industry. The data sheet says it uses the 3U VPX form factor. I looked up VPX and found VPX on wikipedia, which says "[VPX] has been designed specifically with defense applications in mind".

My question is... why would the defense / space industry use VPX instead of PCI / PCIe?

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PCI can be used over VPX.

On the Wikipedia page in the specification section, you can see that PCI Express over the VPX connector is standardized as VITA 46.4.

VPX lets you use any supported protocol with the same connectors. Given how difficult it is to get hardware "space rated," that makes sense. One set of hardware that covers a large range of uses.

VPX also has a different form factor (shape) from normal PCI cards. VPX is sized to fit standardized racks. The total effect is to get more equipment in less space than if you were using PCI or PCIe cards.

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    $\begingroup$ It seems like VPX is not a superset of PCIe, but rather an electromechanical specification allowing the use of multiple high speed buses on it's connectors. PCIe is much more just electromechanical. If I'm understanding this right, by your logic you could call M.2 a superset of PCIe. $\endgroup$
    – jaskij
    Feb 3, 2021 at 14:19
  • $\begingroup$ I have to agree with Jan. The word "superset" does not work like that. If VPX really is a superset of various high speed buses then both the M.2 connector and the USB type C connector would both be valid VPX connectors because both M.2 and USB type C (when implemented as Thunderbold) are both PCI connectors. $\endgroup$
    – slebetman
    Feb 3, 2021 at 18:30
  • $\begingroup$ @JanDorniak: I agree. For a more familiar example, we could maybe look at the USB-C connector and cable specification when used in Alternate Mode. There are Alternate Modes for HDMI, DisplayPort, Thunderbolt, even Analog Audio that use the USB-C connector and cable, yet have nothing to do with USB whatsoever. Or Cat 5e cables with RJ45 connectors which can be used for anything from Gigabit Ethernet to RS-232 serial to … again 4 channels of balanced analog audio. $\endgroup$
    – Jörg W Mittag
    Feb 3, 2021 at 19:17
  • $\begingroup$ @JörgWMittag depends on what is the most familiar to who, I'm just a curious lurker with an occasional comment here on SX and used what came to mind first. While I'm loosely aware of alternate modes, especiall DP, USB-C is something I have yet to seriously dig into. $\endgroup$
    – jaskij
    Feb 3, 2021 at 19:21
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    $\begingroup$ Also, I vehemently object to using PCI and PCIe interchangeably. On the higher software levels they're probably similar, but electrically they're completely different and that is the most important in this question. $\endgroup$
    – jaskij
    Feb 3, 2021 at 19:28

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