Installing Slower 386s on Faster Boards +Other 386 Questions

[hardwarecop]

Hi all,

I have a 386 motherboard without any jumper documentation (nothing can be found on the internet) - however, it came with an AMD Am386DX/DXL-40. So my question is: Is the motherboard fixed to clock at 40 MHz or can it also run lower clocked 386s without overclocking them? I'd like to put different CPUs like Intel i386 DX-33, CHIPS 386 etc. on that board without having the fear of damaging them by overclocking them (unconsciously).

One oscillator says AC14.318 - do you need photos to identify better?
The board has OPTi 82C391/2 chipset and on the back it says "307R5.1"...

Some other questions maybe:

- Is it also possible to run an FPU that is either higher or lower clocked than the CPU?
- Is every 386 motherboard compatible with Cyrix Cx486DLC/Drx2, Intel RapidCad and other rarities?
- Is it possible to combine any random 386 CPU with any random 386 FPU? Like Am386 with CHIPS 387 FasMath, Cyrix 486 DLC with ULSI or Intel i386 with IIT 4C87?
- Last question: Can I run any Multi I/O ISA controller on any 386 motherboard or does each 386 board has its own compatible Multi I/O controller?

Any ideas? Thanks
(I'm really interested in 386s)

Add: Another question: Any information about this motherboard shown in the picture? It has 486, 487, 386 and 387 socket?! This isn't my 386 board described above - but I'm just curious. That seem to be a cool thing if I want to test CPUs. I can test them all together on one motherboard ? (Might move my question to the information requests section? )

[iulianv]

This is from my own experience with 386 boards (that is, don't take it as absolute truth, there might be situations that I haven't encountered yet):

- CPU frequency on 386 boards is given by an on-board oscillator - in your case it seems to be the chip on the corner of the board, next to the CPU (the one secured with a plastic tie); in order to "match" the CPU in the photo it should be a 66MHz oscillator - if you replace it with, say, an 80MHz one, then the CPU will run at 40MHz

- on some boards the Turbo button has the effect of lowering the CPU frequency (to half, for example), making it possible to safely test other CPUs

- FPU frequency on 386 boards is either the same as the CPU's, or (on some boards) is set as "async to the CPU" by a jumper and given by a second on-board oscillator - you can combine random CPUs and FPUs, but some will run over/underclocked (or may not run at all, if they don't tolerate the frequency boost)

- make sure you combine SX CPUs with SX FPUs and DX CPUs with DX FPUs (I'm not sure what happens in case of a mismatch, I haven't tested it )

- any I/O ISA controller should work on any 386 board

- I believe the second socket on your board can accomodate either a 387 or a Weitek FPU

- not every 386 board is compatible with those rarities you mentioned - some work, some may work but don't use the CPU's on-die cache, some may not work at all

- when looking for info on such an old board I usually find a TH99 repository online - such as http://stason.org/TULARC/pc/motherboards/index.html -, go through the list of boards and try to match a layout (location of slots, RAM, cache, etc) to the board in question

[aberco]

All the infos for configuring the jumpers is on the motherboard silkscreen.
Unfortunately the picture is not good enough to read it.
That should answer all your configuration questions for speed and more.
The battery seems to be a bit tired.

[hardwarecop]

Great, thank you very much for taking time answering my questions!

Now I feel much safer to use 386 CPUs/motherboards. By the way, I've also found a good PDF that explains all the jumpers on my motherboard.

That repository is just awesome. I wanna merry the one who created all this Thanks alot!

[misace]

[hardwarecop]

[misace]

[andy2000]

You'll want to attend to that leaking Ni-Cd CMOS battery near the keyboard connector. Hopefully it hasn't damaged the board beyond repair.