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Tearing my hair out over ISA card dimensions

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First post, by HwAoRrDk

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Does anyone have, or can point me to, any kind of clear, definitive reference for dimensions of ISA cards?

In particular, the dimension between card edge connector and bracket outside face (i.e. the face that contacts the chassis). I am going mad trying to correlate several different sources that are sometimes slightly ambiguous, sometimes contradictory, and sometimes not in agreement with reality. 😠

My first port of call was the IBM PC 5150 Technical Reference, page 1-35, which provides the following diagram:

The attachment pc-5150-card-dimensions.png is no longer available

But that doesn't include the mounting bracket at all. 🙁 It also doesn't agree with any ISA cards I have in my possession, because it specifies a distance of 98.933mm from back of card edge connector to front edge of board, whereas nearly all my cards measure approx. 102.5mm.

My next call was the EISA Specification 3.1, figure 97, page 254, which has the following diagram for an 8-bit ISA card:

The attachment eisa-card-dimensions.png is no longer available

This appears to have the dimension I am looking for, but the scan (and/or original document) is too crappy and I can't tell whether the 106.17mm dimension is to the inner face of the bracket, outer face, or centre line. On my selection of physical cards, most are all about 1mm short of that on the outside face; only one matches. So maybe the inner face? But the drawing doesn't quite look like the line is aligned that way...

Next, I remembered seeing some drawings for ISA cards in the PCI specification. And, lo, PCI Bus Specification Revision 2.2, figure 5-8, page 160, has a drawing:

The attachment pci-card-dimensions.png is no longer available

Although, it and its same-page companion drawing are for some mythical extended and keyed 5V and 3.3V variations of an ISA card that I'm not sure ever existed. Regardless, it actually specifies that its dimension is to the outer face of the bracket, but the other end of the dimension is in the centre of the key notch, which isn't quite the back edge of the regular 8-bit edge connector, so is of little use. 🙁

Finally, I also found this:

The attachment andavno-card-dimensions.png is no longer available

Some guy's project to make his own POST card, for which he compiled specifications from various sources into one drawing. While the back-of-edge-connector-to-bracket-outer-face dimension of 105.156mm roughly corresponds to all my cards measured, it doesn't correspond for front-of-edge-connector-to-board-edge to the IBM drawing, which he himself mentions as reference. 😕

I think generally what also isn't helping the problem is that card manufacturers all use slightly different thicknesses of metal bracket. The only official source to specify a bracket thickness is the PCI standard, which says in a note on figure 5-12 "Material: .034 thk (20 ga)".

Help me before I go crazy!

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA 🫨

(Edit: I've no idea why the first picture isn't showing up inline.)

Reply 1 of 2, by rmay635703

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Your not the only one that had trouble with this, nearly every beige box pc I had , I had to leave motherboard hardware loose because the cards couldn’t seat properly or fully down, lots of instability because of this nonsense

Reply 2 of 2, by HwAoRrDk

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I think I might be on to something. I was re-reading the aforementioned blog post, and it mentions the Intel ATX specification. So I checked out the Version 2.2 document I had to hand, and there is indeed a drawing of example motherboard layout - Figure 3, page 12. It contains dimensions for distance from centre line of ISA slot connector pin 1 to top-left mounting hole (which is the [0,0] datum), 0.6", and from mounting hole to board edge, 0.4". But this doesn't yet help with finding out what I want.

So I thought that there must be somewhere that the ATX specification mandates the distance to the chassis, rather than rely on a third-party source as per the blog post. Two pages further on there is Figure 4, showing back panel I/O aperture requirements. This actually gives a distance between the zero datum and the chassis back panel of 0.483". The chassis ISA slots and I/O aperture, as far as I'm aware (at least, as far as I can see on a chassis I have here), are planar with each other.

The attachment atx-dimensions.png is no longer available

This makes the required distance between chassis and pin 1 of ISA slots as 0.6+0.483=1.083 inches.

The distance between pin 1 centre and the back edge of edge connector can be calculated from the EISA specification's fig. 98 (p255). Distance between pin 1 and 31 is 3.000". Width of edge connector tab is 3.190". So distance from back of edge connector to pin 1 is 3.190-((3.190-3.000)/2)=3.095 inches.

Add the two together and we get 1.083+3.095=4.178 inches.

While this is all from the ATX specification, I think it's safe to rely on this calculated value for two reasons:

  • It correlates with the EISA specification's dimension - only 0.002" less, which would seem to be within the realm of rounding error.
  • Intel no doubt made everything in this respect the same as prior AT/XT chassis and motherboard convention for backwards compatibility.

So I guess the EISA spec's figure is to the bracket's outside face after all! 😀

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