Jump to content
Keatah

Putting together an Am5x86 rig.

Recommended Posts

Posted (edited)

It's one of the best of the Socket 3 CPUs, but the Pentium Overdrive might be better for some things (like running things that need floating point processing, like Quake). But generally anything that would be better with the Pentium Overdrive would be way better on boards in the Pentium class or better (Socket 7 being the best but still slow enough for DOS). 

 

https://ancientelectronics.wordpress.com/2015/09/26/socket-3-motherboards-and-pushing-their-limits/

Edited by DragonGrafx-16
  • Thanks 1

Share this post


Link to post
Share on other sites

For this project I think I want to stay strictly in the 486 architecture. I will be sticking with the AM5x86-133 unless there is some fundamental reason not to. I hear OverDrive chips in the higher performance range are tricky and sometimes troublesome - and that this Am5x86 seemed to be a popular defacto upgrade path. I also don't want to do anything exotic that results in me having to set things just so, just perfectly, in order to get things going. Like overclocking. That's a no-no. Don't want to feel like my rig is ready to fly off the cliff as it 'rounds the bend, you know..

 

I don't know how much interested I am in building a real Pentium 60 -> 200MMX rig right now. And I'm not too concerned with FPU performance, or lack of it. If I want FPU speed I've always got the P3 and of course the i7 & i9 beasties.

 

I've got enough core parts to put together a Pentium II 266 rig, similar to what I had back in the day. Actually those ARE the parts. It started out being P2-266 on 440LX chipset but over the years I gradually upgraded it to a P3 1.4GHz on BX chipset, on a legacy style mobo with real serial and parallel and ISA slots. I just threw all the old stuff in a box as the years rolled on.

Share this post


Link to post
Share on other sites

I had an AM5x86 back in the day.   I think I still have it too.   Nice chip,  it could be overclocked too, but mine got unstable if you set the bus to 50mhz,  I think it worked at 40

  • Like 1

Share this post


Link to post
Share on other sites
Posted (edited)

VLB is very sensitive to FSB timing, if memory serves.  Stick with the standard 33mhz for FSB.  Very important.

 

Here we go.

https://en.wikipedia.org/wiki/VESA_Local_Bus#Limitations

 

Quote

 

Limited scalability
As bus speeds of 486 systems increased, VLB stability became increasingly difficult to manage. The tightly coupled local bus design that gave VLB its speed became increasingly intolerant of timing variations, notably past 40 MHz. Intel's original 50 MHz 486 processor faced difficulty in the market, as many existing motherboards (even non-VLB designs) did not cope well with the increase in front-side bus speed to 50 MHz. If one could achieve reliable operation of VLB at 50 MHz, it was extremely fast – but again, this was notoriously difficult to achieve, and often it was discovered not to be possible with a given hardware configuration.[6]
The 486DX-50's successor, the 486DX2-66, circumvented this problem by using a slower but more compatible bus speed (33 MHz) and a multiplier (×2) to derive the processor clock speed.

 

 

  Edited by wierd_w
  • Like 1

Share this post


Link to post
Share on other sites

I plan on running the Am5x86 chip at 33x4. I learned early on it isn't worth the time and effort to overclock. I strongly prefer stability above slight increases in speed. To get more performance all I have to do it get next year's upgrades.

 

Questions for running at 33MHz.

1- I plan on doing 60ns SIMMS.

2- I plan on using 20ns 256K cache chips.

... couldn't find a datasheet for the SiS 85C461 and SiS 85C407. So I'm guessing that those are sufficient speeds.

 

Share this post


Link to post
Share on other sites

As long as you stay inside normal specs, and are extra careful on card insertion and removal (and assure that your board is very neatly centered on the AT standoffs), you should be fine.

 

VLB was notorious for having boards not want to insert into (or stay in) the slots.  Just baby them, they were real bitchy things. You will get much better disk performance with VLB IO controllers and video hardware than you will get with ISA slot native stuff.

  • Thanks 1

Share this post


Link to post
Share on other sites

I remember the write back cache mode of the 5x86 being a big deal back then and not all motherboards supporting that mode enabled.  Otherwise, it defaulted to write thru mode.  I don't have the pinout handy, but I ended up placing a little jumper in the processor socket to successfully enable write back.  Two pins needed to be bridged if the board didn't have BIOS support.  Perhaps a non-issue if you are running 486 era software.

 

This might not be a concern in your case, or anything you want to mess with either way.  I was probably trying to make Quake run a bit faster in those days.  That cpu did indeed have a reputation for being rock solid.

 

  • Thanks 1

Share this post


Link to post
Share on other sites

Cache memory was and still is a big deal to me so I'mma wanna be sure it's as fully functional as it can be. Messing with a makeshift jumper isn't a problem, provided the BIOS doesn't support it.

 

In my PC builds I like all the features to work. If they are not in use, that's ok. If they offer only marginal but "factory spec" improvements, that's ok too.

 

Saying that reminds me of the early apartment days where my neighbor had come across a number of mainboards that had the onboard IDE chip blown. Since I was his helper and technical advisor he wanted me to fix them.

 

He balked at the idea of taking the time and money to desolder and replace the chip. So in went a multi-I/O card. It worked till a customer got steaming mad and pissed off.

Share this post


Link to post
Share on other sites

Have some parts on order. So in the meantime we wait. Questions..

 

1- Does the motherboard (in the first post) support 32MB ram or 64MB? I have two conflicting sources and can can't find a 3rd or 4th source to break the tie. I would guess 8 pieces of 8MB x 8 or 8MB x 9 would make it so.

 

2- Is it safe to assume that this board has 3-VL slots. I mean real slots, and not having the 3rd slot wired as a memory expansion slot in disguise. I've head of that, and people have plugged in VLB cards, and the damn thing blows up. I would guess this has 3 vl-slots - and the name supports that.

 

3- Related to another board, running at 25 MHz, or DX2/50, would changing the memory from 70ns to 60ns result in any performance difference? Personally I don't think so. But without datasheets for the chipset, anything goes!

 

4- What is the difference between 3chip SIMMS and 9chip SIMMS? Logically I assume they're the same. But they may differ electrically? And is any one faster than the other? Someone said the 9pin modules are slightly faster and more stable.. but no hard evidence was given.

 

I may (or may not) find the answers to these ?'s in those Bible-sized "Upgrading & Repairing your PC" books. But for the benefit of other vintage builders and upgraders I thought I'd ask here and have a discussion.

 

Share this post


Link to post
Share on other sites
Posted (edited)

I'll see if I can find documentation on this board.

 

Technically, all VL slots are hooked to the memory bus directly. Otherwise they would not be a local bus. (they would instead be an expansion bus.)

 

The board might only support so much memory in the SIMM banks, but I bet you could throw more in using a VLB memory expansion solution. (they certainly existed.)

 

The 70ns vs 60ns RAM is a question I would have to revisit old literature for.  Give me a few to see if I can find this board.

 

----

EDIT

 

Tularc confirms 3 VLB slots, and 32mb onboard DRAM.

https://stason.org/TULARC/pc/motherboards/G/GENOA-SYSTEMS-CORPORATION-486-TURBOEXPRESS-486VL.html

 

Actual motherboard documentation in pdf form:

http://www.motherboards.org/files/manuals/47/486vs8a.pdf

 

 

Edited by wierd_w
  • Thanks 1

Share this post


Link to post
Share on other sites
11 hours ago, Keatah said:

What is the difference between 3chip SIMMS and 9chip SIMMS? Logically I assume they're the same. But they may differ electrically? And is any one faster than the other? Someone said the 9pin modules are slightly faster and more stable.. but no hard evidence was given.

I can only offer anecdotal evidence on this.  BITD working in a tech shop and retail store, we saw dozens of computers which came in with stability problems.  We replaced the two- or three-chip SIMMs with proper eight-chip and sanity was restored.  Now, to be fair, I cannot recall if they were "name brand" SIMMs or fly-by cheap crap, but this trend continue from the 30-pin era into 72-pin.  The only odd-configuration SIMMs I remember working were the flat-chip RAM on the 72-pin boards which came 'round during the early 133-pin SDRAM-style EDO DIMMs.  These were low-profile and had a lower power draw.

 

As for the VLB stability at higher speed, you just have to be careful what you choose.  I have two VLB cards which were designed to work in systems running 40MHz to support CPUs like the DX2/80.  One is a multi-I/O card (IDE, floppy, serial, and parallel,) the other is a Trident video card.  I was rather proud of this configuration as it was close enough to the performance of a friend's DX4/120 he bragged about so much.  (CLOSE ENOUGH, that is, not exactly on par.)

 

Gotta say I kinda miss those days.  I remember we ran his DX4/120 at 160MHz one day.  It was pretty unstable but still, a '486 running 160MHz?  Amazeballs!  I have a '486 board with PCI slots here in an anti-static bag.  Need to run it up sometime.  I keep a couple of old systems like this around for legacy recovery efforts.

  • Like 1

Share this post


Link to post
Share on other sites
Posted (edited)

I'm sticking with a 33MHz bus. I could OC for fun, but that's down the road

 

I found in the "Upgrading and Repairing PCs" books that the 3chip vs 9chip logic is the same, but the 3chip parity variety have the third chip which has slightly different electrical specs. Different enough to cause the instability. Another source said there were subtle timing differences too. They mostly worked fine on older chipsets. But eventually chipsets supported both versions and reliability came back up.

 

The 3chip design was strictly a cost-cutting measure, a sweet spot of density and price.

 

My old original 486 is full of the 3chips and has no issues. But this "new" rig will have the 9chip design.

Edited by Keatah

Share this post


Link to post
Share on other sites

Also bear in mind nine chips are not necessarily necessary.  The ninth chip holds a parity bit for systems which perform memory error detection.  If your system does not support it (which would be most desktop chipsets,) then eight chip SIMMs are just fine.

  • Like 2

Share this post


Link to post
Share on other sites
7 hours ago, wierd_w said:

The board might only support so much memory in the SIMM banks, but I bet you could throw more in using a VLB memory expansion solution. (they certainly existed.)

 

7 hours ago, wierd_w said:

Actual motherboard documentation in pdf form:

http://www.motherboards.org/files/manuals/47/486vs8a.pdf

Close. It's not the exact manual. It describes what is likely another board from the same lineup with the SiS 471 chipset. The board I'm working with has the SiS 461. Note the position of the BIOS and Keyboard/timer chip. I suspect the 461 supports 30-pin SIMMS and the 471 supports a mix of 30-pin and 72-pin.

 

But I bet most of the BIOS options are going to be same however.

 

Share this post


Link to post
Share on other sites
3 minutes ago, OLD CS1 said:

Also bear in mind nine chips are not necessarily necessary.  The ninth chip holds a parity bit for systems which perform memory error detection.  If your system does not support it (which would be most desktop chipsets,) then eight chip SIMMs are just fine.

I'm not 100% sure if the SiS 461 chipset supports that, or not. I looked seemingly everywhere for a datasheet. Can't find a damned thing.

 

I'd also like to know if it has a direct-mapped cache scheme. If it's not then I could potentially upgrade the cache to 512K. The cache chips are cheap enough at 10$ a set on ebay.

 

Finding chipset datasheets is a pain in the ass these days. Seems like they only published them while the part was being manufactured. And we all know that was a short time of just a year or two. Then on to the next! Not like the 74LS series or something which is 50 years old and still in use.

 

Hot on my "datasheet want" list is:

MIC 461A

MIC 462A

SiS 85C407

SiS 85C461

 

Now, for intel products. I've got practically everything. These have been archived many times over on various sites.

Share this post


Link to post
Share on other sites

Sadly, I dont think those exist out there...  I have seen several other people looking for the same thing, but no PDFs with chip data for that chipset.

 

I will hunt on archive.org

  • Thanks 1

Share this post


Link to post
Share on other sites

Picked up an SBAWE32 for $20. Guy said it didn't work. But I know better!

 

It's the huge full-size one like in my other thread. So I will be using that. It looks like it came with 2MB of SIMM RAM for the "soundfonts". I understand this board will take 2x 16MB SIMMS, and utilize 28MB of that memory directly. Not sure what/why happens to the other 4MB unless its space is mapped to "stuff on the card".

 

Now I have to decide on a graphics card. I'm not paying more than $50 absolute tops for one. I've seen VLB cards go for literally 300 or 400 on ebay, especially some name brand cards that are still based on reference designs and totally not deserving of premium prices. So that's not happening.

 

Not sure how to start the selection process. I know most cards of the era sported 1 or 2MB of RAM. So 2MB it is, but what chipset?

 

 

Share this post


Link to post
Share on other sites
On 4/9/2020 at 4:39 PM, Keatah said:

Anyone see any reason why this pair wouldn't work?

Won't work. Could have blown up stuff had it been powered it on.

Share this post


Link to post
Share on other sites
Posted (edited)

Think I found another solution. It will have to wait for parts to get here in June. Covid19 and all that.. So while we wait I was thinking about CF or SD card interfaces.

 

What would be a good way to do this? There seem to be 6 or 7 ways. And I've narrowed it down to:

 

https://www.ebay.com/itm/SD-SDHC-SDXC-MMC-Card-to-IDE-40Pin-3-5inch-Male-Adapter/123457150544

https://www.ebay.com/itm/SODIAL-R-44-Pin-Male-IDE-To-SD-Card-Adapter-DT/252470736918

https://www.ebay.com/itm/XT-IDE-Deluxe-Bootable-ISA-CF-IDE-Interface-Card-with-IBM-XT-Slot-8-Support/124122830301

 

I plan on doing one on this machine and my older original 486 Gateway machine. On this "new" build I suppose I would need the XT-BIOS card for the BIOS.

 

On my older 486 I have a PROMISE MAX IDE card w/bios. And it has 1 IDE connector, thus adding in a second IDE channel. The system currently has three drives. And this would be the forth. And all I think I need I need is the XT-IDE card's bracket, which I have in my parts box.

 

I also have a network card in there to host a different bios if needed.

 

I'm also wondering about 486 boards with PCI slots. It seems VL-BUS was made specifically for the 486 and that a PCI bus would not be efficient. PCI might even have too much overhead. And I can't imagine the first implementations (on 486 boards) were stable or reliable.

 

 

 

Edited by Keatah
I have got to read my "Repairing your PC bibles!"

Share this post


Link to post
Share on other sites

There *WERE* some 486 class systems with PCI slots, but very late in their product lifespan.

 

Personally, I would look for something like an IDE hardcard, strip off the (probably dead) IDE drive, and replace it with a DOM.

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...

  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...