VOGONS


First post, by starhawk

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I have a stack of weird reasons stuck together that makes me want to do this. Please don't ask unless you want a response so long it will have numbered chapters, AND you're willing to read something that's that long. No, I can't condense it, don't ask for that -- I will cackle, tell you that you were warned, and hand you a novel that you can expect to be quizzed on.

I have a datasheet for an Intel 82335SX chip that (theoretically) can take a 16MHz or 20MHz i386SX CPU and i387 FPU, and make it work in a system that expects to greet an 80286 when it gets up in the morning. I've attached it. Unfortunately, while I can get the chip from a questionable source in China (for the usual caveats, including a price that makes me wonder just how high the odds are that I'll be getting an experience similar to Sparkfun's early 2010 experience with Not Alibaba... especially considering that there are *four* *listings* *on* *all* *of* *eBay* and of the four, this is the odd one out at *half* the price of the others!) -- I can't find ANY schematics for how to use it, nor can I locate any modules/daughtercards/etc that are prebuilt that use it.

To be completely honest, the track record I have with putting circuitry together and having it actually *work* means that whether or not that chip is actually a chunk of bare copper flattened hard and stuck in a 132pin QFP probably doesn't actually matter much.

But I want to try.

Specifically what I want to make is a daughterboard that will adapt a 12MHz 286 motherboard to a 20MHz 386SX + a matching 387 coproc. I don't mind bodge-wiring the existing motherboard to heck and back if I have to, and I have a spare for it, but I basically want to "286-Overdrive" (as if such a thing ever existed by that name) the system in question. If it absolutely matters, the machine I'm upgrading (or trying to) is a Compaq Portable III, and I want to do it in a way that respects the era and heritage of that machine. This is *NOT* getting a RasPi inside; those people make me as close as I ever get to violently angry. A 20MHz or 25MHz 386 is about perfect for me -- my very first system of my own rocked a 25MHz AM386SX and that turns out to have been an incredible coincidence, because that's right about at the sweet spot for me where computing just makes sense. (Funny how that works.)

Laugh at me if you want, but can anyone find me that ref design... or *any* schematics that will let me build what I describe?

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Reply 2 of 9, by Oetker

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Have you looked into existing boards that do the same thing? These devices exist and aren't as special as you seem to think they are. One of them might be using the same chip, maybe someone that owns one could provide you with more info.
Hyper386sx 286 Upgrade CPU
Re: 286-to-386...or...286-to-486 Adapters...GOT ONE??
Looking for 286 CPU upgrade "kit"

Reply 3 of 9, by wiretap

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You likely won't find implementation schematics, because those are property of the OEM who designed boards with them and probably still under proprietary release exemptions. Looking at the datasheet, it is actually very detailed and goes into all the necessary voltage requirements, current requirements, capacitance, resistance, AC/DC timings, wait states, logic timing charts, etc. For interfacing other devices, you need to also look at their datasheets for the respective requirements so you can design a circuit that meets the electrical spec.

Your best bet is to reverse engineer a schematic from an existing board if you don't know where to start based on the data sheet. These Intel 82335-SX chips did come on some early Shuttle motherboards from what I see on CPU registries.

Regardless, if you don't know where to go from the datasheet, a hardware design of this level would be a monumental task that would have an extremely high chance of failure. You won't be able to do it with bodge wire.. it is way too complex. Even simple CPU upgrade boards for the Amiga that interface with the 68000 take years to develop from very skilled individuals that know hardware engineering like the back of their hand or have been in that industry for many years. A 286 to 386 interface with coprocessor will more complicated than that.

Here is what a 8088 or 286 to 386+coprocossor upgrade board looks like -- The Intel InBoard 386: http://www.cpushack.com/2019/08/14/how-to-386 … inboard-386-at/

edit: as the post above mine, there are also the later-era upgrade boards that adapt to the existing socket.

Circuit Board Repair Manuals
EGA 120 Column Project
MHz Display Project
Dual Socket 8 Project

Reply 4 of 9, by starhawk

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Hmmm...

Right, they're not so uncommon... sure... off to eBay, then, since that's where nearly all such stuff always is -- at least in my experience! (Note, ignore the quote marks, they're used here to delineate search terms, they weren't part of the actual search because I have more intelligence than an empty can of store brand baked beans. Duh.)
"hypertec 386sx" --> No matches found. (FWIW, the *only* HyperTec brand /anything/ of even remote relevance is an ISA-8 RTC card. Meh.)
"evergreen technologies" --> One result, not relevant (an upgrade for non-super Skt7 motherboards to support a 333MHz K6-II CPU, if I understand it. I didn't look that close because I didn't care enough.)
"SX/20LCM" --> No matches.
"SX/25LCM" --> No matches.
"SX/33LCM" --> No matches.
"kingston 286 386" --> No matches.
"intel inboard" --> No matches.

Right... exactly as I'd pretty well figured. @Oetker, I know I'm "special" and I'm starting to think that you might be as well 😉 but I'll not issue summary judgement. Do me a kind favor, if you would, please -- since you're apparently privy to knowledge I've not got, where is a place I can find these things, that more ordinary folks normally would not go? Clearly you know better than I do!

Reply 5 of 9, by megatron-uk

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The kit isn't exactly rare, but it's not common either. The IBM PS/2 range has a whole suite of proprietary 286 to 486slc modules from Kingston, for example. (I have one myself, but most of them won't work in anything else than the system they were designed for)

The devices so come up on eBay, but we are talking about 30 years old kit that would have sold in the tens of thousands range, rather than hundreds or millions, so you won't see them everyday.

Reply 7 of 9, by starhawk

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@Megatron-UK -- OK, three questions. One, is it reasonably possible (or even UNreasonably possible without swapping the entire freaking PCB or programming a whole new ROM -- I do some crazy stuff with electronics and I'm kinda known for it in my more usual circles, but it keeps me in the black bank-wise) to make eg a Kingston PS/2 unit work in a different system? Two, is there anywhere even a little less exclusive than chip-collecting sites? and three, about how much should I expect to pay? 😒

I've never even actually seen one listed on eBay.

Reply 9 of 9, by Tiido

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The short answer is that this info will never come / doesn't exist.
Longer answer is that when you know how the 286 and 386 buses work, the datasheet of that chip provides enough info to build the thing you're after. Most of it made sense when I went though the datasheet and there shouldn't be too much difficulty getting things going.

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