486dx2 DTK board turbo switch and cache

[cmc]

I recently picked up two 486dx2 motherboards off Craigslist for $25. I'm trying to get at least one of them working as a computer for education. Teaching intro x86 assembly is much more straightforward on a 66mhz 486 than a 3.4ghz quad-core giant with all the new extensions to distract us, and the basics really haven't changed. In case anyone wonders about the usefulness of some old machines!

Here are some pictures of one of the boards:





It's a DTK PKM-0037S. 8x1mb SIMMs. I put it in a suitably retro AT case from eBay (one of the slim desktop ones that has been for sale for a while).

So far I have two questions someone here might know about!

1) Any suggestions on how I might upgrade the cache? It appears to be 64k upgradable to 256k. However, I can't find any documentation online about what types of cache chips might work.

2) The turbo switch dupont cable on the case has three pins, labeled "LCH". But the board only has a two pin connector. Can I assume the "C", which is a black wire, is a ground?

Thanks!

 

[wesleyfurr]

2) The turbo switch dupont cable on the case has three pins, labeled "LCH". But the board only has a two pin connector. Can I assume the "C", which is a black wire, is a ground?

If memory serves, just stick it on one way and see how it works...if it is opposite of what you would expect, switch it to the other end. The switch shorts 1/2 or 2/3...depends on whether turbo is normally open or normally closed...that sort of thing.

Wesley

 

[cmc]

Hmm, I'll try that out, thanks. By the way, do you think it is a safe assumption that it will cut the mhz from 66 to 33? Once I can get an OS installed (still trying to find a few components) I will test the performance with a benchmark. Until then, I wonder if there is a way to confirm what the turbo is doing. I may not even attach it if it does something odd, such as alter access to the cache. I am not sure if this is dependent on the motherboard, CPU, or both.

 

[Maverick1978]

So sad to see a VLB board using a non-VLB desktop riser....

Not a negative comment, just sad

So far as your Turbo, it'll likely drop to 33mhz assuming that it's not just a "cache cutter" (as I call them) - though worst case, use a slow-down tool.

So far as teaching the x86 assembly language on modern PC's... you might want to test out the PCE PC emulator - I have no personal experience, but I've heard good things about it's ability to more accurately mimic the old hardware than DOSBox, which has a goal of mimicry for gaming purposes.

 

[SpidersWeb]

If memory serves, just stick it on one way and see how it works...if it is opposite of what you would expect, switch it to the other end. The switch shorts 1/2 or 2/3...depends on whether turbo is normally open or normally closed...that sort of thing.

Wesley

This ^
It's so you have the option of the of the button being clicked in for full speed. If you hook it up, and want the turbo button to be the other way around (clicked out for turbo or clicked in for turbo) you just flip it around. It'll stand for Low Common High.

No idea about cache sorry.

 

[smeezekitty]

Its kind of anybodies guess what the turbo button does.
With only 8 MB RAM 64 KB of cache may be sufficient.

 

[cmc]

So sad to see a VLB board using a non-VLB desktop riser....

Not a negative comment, just sad

It is sad, but only for now! I am working on a DIY solution because I can't find a VLB riser that will fit the case. I will report to this forum if I get a workable answer.

So far as teaching the x86 assembly language on modern PC's... you might want to test out the PCE PC emulator - I have no personal experience, but I've heard good things about it's ability to more accurately mimic the old hardware than DOSBox, which has a goal of mimicry for gaming purposes.

I have not tried this emulator but will check it out -- thanks. So far QEMU has worked just fine for my purposes, but there is something that just "clicks" better for the students when they can DD their image to a floppy and see their own mini-OS boot on real, "breathing" hardware.

 

[cmc]

Thanks for all the replies. On the point about the using an ISA riser for a VLB board, I managed to rig a DIY solution using a PCI riser cable. VLB riser cards seem to be rather rare and expensive. And anyway might not fit a small case. Fortunately PCI and VESA seem to be identical physically, except that PCI has four "extra" pins.

If anyone else has this problem, you can use a standard PCI riser cable like this one ($7):
http://www.ebay.com/itm/231117456142

And *carefully* cut away the four extra pins and pin slots using a fine-toothed hacksaw. I cut right down the middle of the third and fourth "extra" pin. The width of the saw blade ended up cutting away most of the rest of the fourth pin, leaving just the right amount of material left.

Then you can use one of the ISA card spots for the ISA pins, and the now-miniturized PCI extender cable for the VESA connection. Here are some pictures:









So far so good... here's a video of the motherboard and video card being booted for the first time in 20 years!

 

[cmc]

On the cache question, does anyone know what the "R" stands for in ISSI IS61M256-12NR, such as these:
http://www.ebay.com/itm/IS61M256-12NR-QTY-10-28-DIP-ISSI-/290905821224

These seem to be compatible with the configuration for 256k on my board, but I can't find anything about this particular version. Plenty of datasheets out there for the IS61M256-12N and similar, but none with the "R". So, are these a real product or just badly-labeled knock-offs?

Thanks!