80386 mainboards with socket and soldered cpu - Page 1
Forum section: electronics / general

author: 386SX
author_rank: Newbie
timestamp: July 03, 2020, 05:09:55 pm
content: Hello,I've got an 80386 mainboard built in 1992 called MG 38606 I'm using to build an "high-end" 80386 computer; I found the same pcb layout was equal to other mainboards like the QD-U386DX, Magitronics A-B341H, 3DIUD-1.2 and the TK 82C390A, based on the UMC 481/482 chipset. The mainboards came with various configuration of cpu/fpu/socket; some had the soldered Am386DX in the plastic QFP package and almost always the NPU/FPU socket installed but the PGA-132 cpu socket removed; other times with the FPU socket installed, the PGA-132 cpu socket installed instead of the soldered cpu removed and other times with both the cpu solderedandthe PGA-132 cpu socket too (and the FPU socket too) and this last is like my mainboard.If I understood correctly to use the PGA-132 socket, if originally intended to be used, I read discussions that the soldered one should not be there obviously or should be disabled theorically with the FLT# pin usage (even if I suppose both would still be powered). Considering how rare these mainboards are nowdays I didn't try to use a socket cpu cause I don't know what may happen with two cpu powered at the same time probably on the same lines etc not to mention desoldering the cpu itself with the risk that even after that the PGA socket may still not be usable, but I'm studying into the possibility for an upgrade.Casually following with a multimeter the FLT# pin trace of the soldered Am386DX the trace end up to a factory-removed jumper (that in most other equal mainboards is installed called W6) while the other pin of the removed jumper instead end up into the FPU socket to the CKM pin that if I understood correctly is related to the sync/async clock with the cpu.Does this mean the jumper is only related simply to the FPU async clock? But then why seems to be connected to the FLT# pin of the main cpu? The fact that the jumper was removed might suggest the board coming with a DX40 version would have not needed a different cpu/fpu clock (there're anyway two oscillator on the board) but still what would be the point of the installed PGA-132 socket?I was thinking if that jumper might actually disable the soldered cpu leaving space for a faster Cyrix 486DLC or if instead if that is not its purpose, why has been connected to the FLT# pin to an FPU socket that probably most didn't have installed.Also are there any possibilities the mainboard might be configured to actually "detect" if a PGA cpu is installed? Near both cpu/fpu sockets there's an IC called CD74ACT244E and I was thinking if that would be involved into this possibility.Thanks for any opinions.Bye

author: 386SX
author_rank: Newbie
timestamp: July 16, 2020, 01:23:44 pm
content: I add a photo of the angle of the mainboard and the cpu situation. I wish to eventually be able to use the ceramic PGA-132 socket one instead of the plastic soldered one. The W6 jumper is clearly seen on the left of the main cpu.

author: Halcyon
author_rank: Unknown
timestamp: July 18, 2020, 12:44:17 pm
content: Thanks for sharing! Certainly an oddity, I've never seen one like that before. Curiously those boards have a jumper to enable/disable the math co-processor (which they refer to as the Numeric Processing Unit or NPU) but nothing to do with the CPU socket.Asimilar board with an ALI chipset has a number of factory jumper settings which are not further described. I wonder if they have anything to do with the on-board or external CPU or those pins you traced?

author: dexters_lab
author_rank: Unknown
timestamp: July 19, 2020, 09:08:12 am
content: i have seen boards like this before but never actually used one...can i ask though why don't you want to use the on-board AM386DX40? it's the fastest 386 available.

author: 386SX
author_rank: Newbie
timestamp: July 22, 2020, 05:36:57 pm
content: Thanks for both answers. I suppose there were some different chipset based boards having similar situations but as much as I looked for specific answers I didn't find sure facts without having a real manual of this board. I asked people that did the cpu switch modding the QFPA cpu or someone even removing it. But I don't like the idea of such mods on such old rare board also considering there's no idea if the board would actually boot from the PGA-132 socket at all and at that point it'd be useless. No way.Maybe the only way is to look for the others equivalent boards having the same layout and that I had found listed in the first post (and it wasn't easy) but back then mainboard manuals probably were not as common as nowdays or got simply lost.That ALi based board seems at least to have some "factory configured jumpers" to have some hope for this, mine has a W4 jumper that is related to the NPU enable socket, then a W7 jumper that in my case is already factory shorted and I discovered ending up to the PGA-132 ERROR# pin related to some NPU error situation and than that W6 jumper as above said. Than there're some jumpers, one for the external battery I am not sure which kind of battery it'd need anyway, and other two jumpers close to the AMI keyboard chip probably related to the color/monochrome monitor choice.A very interesting read I found in this linkhttp://www.os2museum.com/wp/the-nearly-ultimate-386-board/where the person testing its board had the same situation with a different newer chipset but still interesting to see a similar removed jumper.If I had to choose I'd have preferred to have only the PGA-132 one even if I'd have probably used the same DX40 again but that in the PGA-132 socket version is a larger more expensive and better looking ceramic package, not these plastic one that are just equally fast but make the board look and feel cheap and I suppose was the idea at first of the many 386 on this package back than. The idea was also to try sometimes the Cyrix 486DLC 132-pin upgrade solution maybe just for some tests, indeed a faster cpu. Also I can't imagine why to make a board with a socket not usable and it'd be interesting to understand it.