Don't forget that most "store bought" 486 and Pentium systems used LPX form factor like the video you linked. They use a riser card (allowing a desktop case to be shorter) and allow ports at the back of the motherboard, but both features mean having to have a custom case.

If you had your system built at a local computer shop with off the shelf components, you got AT form factor. And white-box brands at Sams Club, mail order catalogs, etc. would be like that as well.

Since AT cases had only a hole for a keyboard port, if you want to add other onboard ports you have to have those little ribbon cables to connect them to brackets in the back of the case. The Biostar MB-8443UUD-A loved on here is a major exception. It seems like most PCI boards do at least have the pins to connect a PS/2 mouse port. I don't know why they didn't catch on earlier.

ATX solved the problem of the proprietary cases by allowing an arbitrary selection of ports but in exchange requires the backplate.

Laptop and compact systems found a PS/2 port went neatly in line with the D connectors.

However, it all just kinda happened, there wasn't any master plan until Intel's PC design guides PC-97 released late in early 98 and PC-98 in December ... and that only sorta got followed. If there was any following of any of the industry leaders grand master plans, there never would have been AT 386es, microchannel architecture would have been licensed as the 32 bit bus of choice and gone on for some years, super socket 7 would never have happened, ISA slots would have been killed completely in 1998, etc etc etc.

Industrial MB aside, what were factors deciding an MB to use a DIN-5 AT keyboard (and nothing else) or a pair of PS/2 connectors?

dormcat wrote on 2021-08-07, 16:13:

Industrial MB aside, what were factors deciding an MB to use a DIN-5 AT keyboard (and nothing else) or a pair of PS/2 connectors?

I think you might be mixing up AT and ATX cases.
Here's an example of a "true" AT case someone posted elsewhere on the forum: https://i.imgur.com/iy7Ibar.jpg
As you can see, it isn't a matter of choosing what to put instead of an AT keyboard but the fact that only an AT keyboard port would fit. To use a pair of keyboard connectors the would-be AT case would need two PS/2 holes instead.

Here's someone's LPX case: https://i.imgur.com/EXq1pDtm.jpg
The LPX case allows the motherboard to have whatever ports it wants, but the cases were pretty proprietary. You couldn't just buy an "LPX case" and stuff whatever motherboard you wanted into it. I don't know if LPX motherboards were even sold at retail.

Adapters are readily available to over come issues. A lot of generic AT cases had headers (labelled PS/2) on the mobo close to keyboard output dimm for PS/2 mice.

I'm not really sure what the OP is on about quite honestly.

jakethompson1 wrote on 2021-08-07, 22:19:

I think you might be mixing up AT and ATX cases. Here's an example of a "true" AT case someone posted elsewhere on the forum: ht […]
Show full quote

I think you might be mixing up AT and ATX cases.
Here's an example of a "true" AT case someone posted elsewhere on the forum: https://i.imgur.com/iy7Ibar.jpg
As you can see, it isn't a matter of choosing what to put instead of an AT keyboard but the fact that only an AT keyboard port would fit. To use a pair of keyboard connectors the would-be AT case would need two PS/2 holes instead.

Here's someone's LPX case: https://i.imgur.com/EXq1pDtm.jpg
The LPX case allows the motherboard to have whatever ports it wants, but the cases were pretty proprietary. You couldn't just buy an "LPX case" and stuff whatever motherboard you wanted into it. I don't know if LPX motherboards were even sold at retail.

Thanks for the photos; I know their differences quite well. Maybe I shouldn't use that video as an example: I mentioned it only because it was an early example of non-IBM machine with PS/2 ports, not to discuss its LPX form factor.

As I said, I've got a Dell Dimension XPS in 1995, with a Dell-customized MB based on Intel Advanced/EV in a fairly standard mini-tower case.

(Sorry for the awful cable management)

Two PS/2 ports can be seen at the upper right corner. Those brand name computer manufacturers had models in LPX form factor so they might want to keep their keyboard/mouse with just one connector format, as demonstrated in those two examples: the MB with a Dell sticker has two PS/2 while the other MB with no OEM sticker has one AT keyboard port.

Caluser2000 wrote on 2021-08-07, 22:25:

Adapters are readily available to over come issues. A lot of generic AT cases had headers (labelled PS/2) on the mobo close to keyboard output dimm for PS/2 mice.

I'm not really sure what the OP is on about quite honestly.

I was just curious what's the benefit of producing a P3 (a rare Slot 1 / Socket 370 hybrid) MB like M748LMRT without an ATX-compliant back panel I/O arrangement, when many Socket 7 MB like TXP4-X followed the standard more than a year earlier.

Sure, there are headers and adaptors, but why would PCChips force its potential customers to continue using AT keyboards in 1999?

That Dell motherboard is arguably not AT because it wouldn't fit in an AT case due to the nonstandard keyboard port, and it also doesn't use AT power either. Perhaps it's some kind of "draft-ATX" board?

As to why PCChips kept the AT keyboard port... because anyone who still wanted an AT board in 1999 was because they wanted to fit it in an AT case.

jakethompson1 wrote on 2021-08-08, 03:33:

That Dell motherboard is arguably not AT because it wouldn't fit in an AT case due to the nonstandard keyboard port, and it also doesn't use AT power either. Perhaps it's some kind of "draft-ATX" board?

Dell was notorious for creating a proprietary power supply that looked like ATX power but with different pinout and worked like AT power (with a "hard" instead of "soft" switch).
A Permanent Solution to the Dell 'Fake ATX' Power Supply Problem?
Converting ATX Power Supply to DELL w/ Adapter (Safety Check!)
https://community.spiceworks.com/topic/908128 … 2-0-spec-pinout
https://www.dell.com/community/PowerEdge-Hard … ut/td-p/4582086

jakethompson1 wrote on 2021-08-08, 03:33:

As to why PCChips kept the AT keyboard port... because anyone who still wanted an AT board in 1999 was because they wanted to fit it in an AT case.

Well that's a reasonable answer; just like lots of folks on VOGONS use XT-IDE: products are supplied when demands reach a certain level.

We still have these distinctions between store-bought and home-built or small computer shop-built systems today. Consider which never have a 'reset' button and which sometimes still do.

dormcat wrote on 2021-08-08, 15:42:

Well that's a reasonable answer; just like lots of folks on VOGONS use XT-IDE: products are supplied when demands reach a certain level.

Sometime we have no choice. My Redstone XT Turbo 10MHz system's original MFM hdd was toast. I wasn't going to get another one so a XT-IDE card with BigFoot drive (plenty of spare spinning rust drives) made total sense to me. Fitting high density FDDs and nic were no brainers as well.

These systems were made with expansion/upgrading in mind. Not set to be dormant in the same configuration until the end of time. How boring is that...😉