The 486DX-50, unstable or speed demon?

(Image from Wikipedia commons)

The Intel 50mhz DX chip, released in the summer 1991 was the first 486 CPU designed to run on the then blazing fast 50mhz front side system bus. For a very quick and simple explanation, front side bus or FSB is in a very simplified sense the speed at which the various components of the motherboard such as the chipset, RAM and so on communicate.  If the CPU is to be thought of as the brain of a computer then the FSB would be the nervous system. The higher the number the faster the machine is overall. Generally the CPU ran at the FSB of the motherboard, thus if you had a 486DX-33 CPU running at its stock speed your FSB would be 33mhz as well. the 486DX-50 promised blazing fast speeds in a time when most 486 motherboards were running a FSB of 33 or even 25mhz. Unfortunately the majority of motherboards at the time simply could not cope with a 40mhz FSB let alone 50mhz and PC systems running a 486DX-50 quickly gained a reputation for being a very unreliable setup. This reputation was compounded by the fact that the faster VLB or VESA slot cards especially had a hard time running on the 50mhz FSB since this slot was tied directly to the CPU.

With that said I wanted to to take a look at the nowadays uncommon 50mhz DX chip and see if I could put together a stable running PC. I also wanted to compare it to a few of its contemporaries, especially the 486DX2-50 which also ran at 50mhz but on a much more stable 25mhz FSB via “clock doubling”.

So due to that last sentence I think we need to take a very quick look at what “clock doubling” is to help us better understand this era of CPU’s. I’m going to quote the Red Hill guide here on how they explain clock doubling.

“While it is relatively easy to make a CPU run faster, it’s much more difficult to do it for a whole motherboard…. you will remember that the IBM AT (286) decoupled the expansion bus, so that the video and I/O cards could run at a safe, conservative 8MHz even though the motherboard and CPU were zipping along at 16 or 33MHz. But even the best motherboards were limited to about 40MHz in those days, so to make a 50 or 66MHz CPU work reliably, the motherboard had to be decoupled as well. This is a mixed blessing. It allows a faster CPU, but looses performance because access to anything off-chip (RAM in particular) is limited to motherboard speed — in this case, 25MHz.

So CPU clock doubling is really motherboard clock halving. In itself it doesn’t make the CPU run any faster, it just lets the motherboard run more reliably with a fast CPU.”

So in a nutshell a 486DX-50 is running both the motherboard AND the CPU at 50mhz where for example a 486DX2-50 is running the motherboard at 25mhz and the CPU at 50mhz but how great of a difference does this really make? My reading suggests a DX-50 in a well setup system can rival the famous DX2-66 which is a 66mhz CPU running on a 33mhz FSB but that remains to be seen.

First off lets look at the setup I will be using.

Specs are as follows

Motherboard – FIC 486-GVT U2?

The motherboard is a slight mystery to me. It identifies itself on the board as a 486-GVT or on some chips a 486-GVT U2 but from looking on the internet the jumper layouts of similar boards don’t seem to quite match up. There seems to be a few layouts for the 486-GVT 2 but they don’t exactly match this board though I was able to figure out were the jumpers that control FSB were and discover the settings via experimentation. This board is actually pretty nice featuring both 30 and 72 pin RAM slots and two VLB slots as well as a lithium coin battery for the CMOS so I would guess this is a later model motherboard for the era.

CPU’s – for testing I will be using all Intel CPU’s. a Intel 486DX-33, Intel 486DX-50, Intel 486DX2-50 and lastly the legendary Intel 486DX2-66.

(DX-50 on left with heatsink I added)

(bottom side of Intel 486DX-50)

RAM and cache – for RAM I’m not using anything special just 24MB total of 30 and 72 pin RAM, for L2 cache I’m using 15ns chips totaling 256KB

Video and sound – for video I’m using a 1MB VLB Diamond Speedstar Pro card based off the Cirrus Logic CL-GD5428 chipset. Cirrus Logic was mostly known for their mid range graphics card offerings but near the end of the 486 era did put out a few of the fastest video chipsets. the GD5428 is no slouch and is a very competitive 2D DOS video card.

 the sound card really doesn’t come into play here with the CPU benchmarks but for the sake of telling it is a sound blaster 16 Vibra model.

I/O – for I/0 ports and IDE control I’m using two separate cards. for serial and parallel ports I’m using a generic 8-bit SIS controller card and for my floppy and IDE controller I’m using a SIIG CI-1050. One problem I’m having with the SIIG card is my CD-ROM drive is not being detected which I haven’t taken the time to trouble shoot since I did not need it for these tests. It also does not work with other CPU’s installed so I’m chalking it up to an issue with the SIIG card. for  a hard drive I’m running an old 1.1GB Quantum fireball 1080.

 The main thing one needs to watch out for on 486 machines if your using 40mhz or higher FSB is the number of VLB cards you have installed. Since they are tied to the CPU they can be very sensitive to the bus speeds. The general rule is the less VLB cards the more stable. Your usually safe running two cards on a 33mhz bus system but on a 40mhz bus system you will likely hit instability using two or more VLB cards. It’s not uncommon to have one or another card refuse to work, corrupt data ect. On a 50mhz bus system even getting one VLB running reliably can be a challenge. The safest bet for the type of card to use in a VLB slot is a video card as this is the type of card the slot was originally intended for.

With this in mind I limited myself to strictly one VLB card to use for video. Many times PC builders had to cherry pick cards to find one that ran reliably in these high FSB system but luckily my Diamond Speedstar Pro has run perfectly at 50mhz.

Which leads me to the reliability of my machine. Although I don’t use it daily I did run it extensively prior to writing this article and played quite a few games on it without any issues. I don’t doubt the frustration I’ve read about with 50mhz FSB 486 boards but it appears I got very lucky with this build. I haven’t had any of the usual issues such as data corruption, crashes or refusals to post. The one problem I did encounter though is that I cannot confirm my L2 cache is working. Cachechk crashes when attempting to run it and Speedsys also detects no L1 or L2 cache to test. This seems to be a usual issue to very fast 486 systems. Faster cache RAM may help solve this issue.

I have compared benchmarks though with multiple other users of 50mhz 486 systems on Vogons and my numbers seem spot on with those systems with slight variations.

So, does the 50mhz Front Side Bus speed pay off in the end? Well, no, not really.

I ran several benches on the exact same machine with the only differences being the CPU and FSB and in the end the results were usually the same regardless of the test. The 66mhz DX2 running on a 33mhz FSB always smoked the 50mhz DX chip. Also the 50mhz DX and DX2 were usually somewhat close in performance despite the FSB of the DX2 being half that of the DX version. In theory programs only using the CPU’s internal cache should run about the same speed wise but those that are I/O intensive should see a noticeable speed bump and my VLB video card running with 0 wait states SHOULD be blazing fast under the 50mhz bus speed. keep in mind results seem to vary depending on the motherboard and chipset as well.

here’s two graphs to illustrate. First is 3DBench a popular 486 era benchmark test.

3DBench

Results are mostly as we would expect. the 33mhz DX lags well behind while the 66mhz DX2 beats the 50mhz DX by roughly 7 points while 4.8 points separate the 50mhz DX and DX2.

This is largely repeated in the Doom speed test as well.

Doom Speed Test

In the Doom speed tests the DX and DX2 are even more evenly matched it appears with the 66mhz DX2 pulling well ahead again and the 33mhz DX falling well behind. I made a comparison video of both these tests running here as I think a video better represents the various CPU’s.

So in the end, yes you can get a 486 system with a 50mhz DX chip and a VLB card running reasonably stable with a little luck and the right parts. Is it worth it? Again, my answer is no, at least in my opinion. Sure having a 50mhz system has a kind of retro cool factor but despite what Red Hill seemed to say I don’t think it comes close to the 66mhz DX2 at least from what my testing and running both CPU’s has shown me. I have also looked at benchmark results as I said earlier from other users that compared systems and the 66mhz always seems to come out on top in these other tests as well. I’m sure there are instances where the 50mhz bus system may run faster but those instances seem few and far between. When looking at both 50mhz 486 chips and comparing to the DX2 version, sure, it’s a little faster but its really not that significant especially when put up against the hassle of finding the right parts to make a DX system run reliably.

Tandy 425 SX

When I first saw this machine my first thought was that it was a later Tandy 1000 PC but on closer inspection I discovered it was an IBM PC clone machine. It seems every company made IBM clones back in the day and I shouldn’t of been surprised Tandy did either but I had never seen a straight clone from them, besides the later Sensation! machines which I think all used the Pentium and had some kind of backwards compatibility with Tandy sound. The price was right so I present the Tandy 425 SX.

The closest PC I have that I can directly compare the 425 SX to would be my Packard Bell Legend 115. I say that because they roughly use the same small desktop form factor and feature somewhat similar motherboards and the same base Intel 486 SX CPU. The 425 SX is a little wider but the expandability you get is worth the extra size. The machine I have here is bare stock and as far as I could tell was not upgraded in any way from the factory configuration. It almost seemed like NOS. There’s a power button on the far right and a reset button on the far left along with a key lock for locking the case shut I think which was kind of a thing back in the day. There is two 5 1/4 bays and two 3 1/2 bays one being fully internal and meant for a hard drive. Mine came with a 170MB IDE hard drive installed as well as the stock 1.44mb floppy in mine that had “1.44” printed on the eject button which I think is kinda cool. I like the two 5 1/4 bays because this gives you the option of a CD-ROM drive and something like a 5 1/4 floppy drive or a HDD caddy unlike my PB Legend 115 which only has one 5 1/4 bay which goes to the CD drive.

Here we have the back of the unit. The PSU that came with this machine has the monitor pass through which is always convenient. As you can tell right away from the expansion slot placement this machine uses a riser card but It has ISA slots on both sides of the card allowing for five cards which is really nice though the two on the left have to be short in length and you need to pull the power supply to get them in but at least you have the option. As far as ports from left to right we have Keyboard and mouse PS/2 ports, two serial ports a printer port and the VGA port for the built in video.

One thing to note about the VGA port is that it uses an older pin configuration and so one of the pin holes is blocked. This was an issue for me with my flat screen Gateway CRT monitor I use as it had this pin on the connector. One option is buying a VGA extension cable and snapping off the pin on the PC connection side or you can do what I did and drill out the hole so newer monitors will connect. this won’t hurt anything.

Opening this PC is very easy and just involved the two screws on the back and lifting the top away. So here we have a look at the internals. jumper sheet here.

1) CPU – The CPU for the 425 SX is, surprise, an Intel 25mhz 486SX. The SX designates that this CPU has either no math copro built in or has the math copro disabled. The CPU on this PC’s motherboard is soldered onto the board so it can’t be removed but luckily the machine had an extra socket to allow for upgrades. The 25mhz 486 SX is a lower end CPU and although does the job fine for older games it’s really suboptimal for later DOS stuff and an upgrade is highly recommended.

2) CPU upgrade slot – This is the CPU upgrade socket. This is a cheaper LIF socket so insertion and especially removal of the CPU is a little more difficult then a ZIF socket with a handle. This is where you would install either a 487 math coprocessor or the better option, a DX CPU. There are jumper setting to set the FSB to 25mhz and 33mhz so upgrading to a 66mhz DX2 or even an AMD 133mhz 5×86 should work fine. Once a CPU is inserted in the socket and the jumpers set the soldered on 25mhz 486 becomes disabled.

3) RAM – The RAM on this PC is a little strange. There are eight slots for 30 pin RAM as well as one 70 pin RAM slot but this machine seems very picky about what RAM it will accept. I couldn’t make it post with any of the 30 pin RAM I had and only one 8mb 70 pin RAM SIMM as well as the 4mb SIMM that it came with would allow it to post without error. All the other 70 pin RAM I installed allowed the machine to POST and was detected but they tripped a parity error. This board will supposedly support up to 40MB of RAM by using all available slots ( 1 x 8mb 70 pin and 8 x 4mb 30 pin ) but I suspect the right kind of 32MB 70 pin SIMM will also work, but as I said the 32MB stick I tried tripped errors on POST.

4) L2 cache – these are the sockets for adding L2 cache ram. This model did not come with any L2 cache installed which isn’t surprising as it was pretty pricy back when this machine came out. The 425 SX supports 64, 128 or 256kb of L2 cache. even though this isn’t as much as the Legend 115 which supports 512kb of L2 (but NOT 256kb) the chips needed for 512kb on that machine are very hard to come by and after 256kb you start getting diminishing results anyways. The downside is the number of chips and the size of the tag RAM chip needed is not the same as the rest. All cache configurations need either a 8k x 8 or for 256kb a 32k x 8 11 pin TAG RAM module which seem to be a little uncommon. All configurations also require an extra ninth 64k x 1 14 pin module. I don’t know why this ninth module is required, possibly for parity checking. It’s a little bit of a hassle considering that in many other boards all the cache modules you need are the same physical size or like the PB 115 where you just installed five modules of the same size for 128kb.

5) Video – On board Video uses a Western Digital WD90c31A-LR chip with 512MB of video RAM. This is a decent chip and unlike the Headland chip in the PB 115 does not suffer any timing errors. I believe this chip is running on the ISA bus rather then a local bus. The chip can be expanded to a full 1MB of video ram by means of sockets next to the chip. The RAM should be easy to find especially if you harvest it from an old  generic or Trident VGA ISA cards.

6) Riser card supporting five ISA expansion cards (three full length).

7) One on board floppy connector and one on board IDE connector.

8) AT power connector.

9) external CMOS battery.

10) PC piezo beeper speaker for PC speaker sounds.

I like the Tandy 425 SX. Its a small machine but not to small so you have room for expandability. The only downside is the picky RAM on this motherboard as well as the need for extra L2 cache chips. I originally had plans for the PB Legend 115 for a special project setup but due to the ability to cache 256kb as well as the added expandability I think I may use this PC instead.

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ICL Ergolite DT486/66

In my last article I covered the Pentium Compaq Prolinea 5120e. This time will be taking a look at the other computer I picked up alongside that one, the ICL Ergolite DT486/66. I had never heard of ICL when I picked up this PC. ICL  from what I can tell was a rather large British computer hardware, software and service company that operated until 2002 before being bought out and absorbed into the Fujitsu company. From those I’ve talked to the DT486/66 seems to of been found mostly in office settings in its day and used primarily in business settings rather then the home market.

You can see right away that the case isn’t a standard design and it somewhat resembles in form and function a 90’s Macintosh machine. It’s quickly evident that the case design does not leave much room for expansion, this problem is actually worse once we look inside. We have two bays, one 3 1/2 inch and the other a 5 1/4. I received this machine with a 3.5 inch 1.44MB floppy drive installed. Below the large round power button we have lights for power and hard drive along with another unlabeled led which I have yet to identify. Possibly for a turbo button enabled by a key combination.

On the rear of the PC we have the power plug on the far left as well as a pass though plug for a monitor. I really like the port label with the various ports and what they are, especially the little mouse for the mouse ps/2 port. So we have two ps/2 ports for keyboard and mouse which is nice for a 486 machine. This is followed by two serial ports a printer port and finally the built in VGA port.

another interesting thing is

I do believe this is the first time I’ve ever come across anything manufactured in Finland. we also have the date of manufacture as 1993. The case as you can see is screwless. Whatever holds the case top on must of broken off because mine just slides right off. This is very much like the screwless Macintosh PC’s of the 90’s that would become brittle with age and have the various retention tabs snap off. And now a look inside.

Here is the machine on the inside. The SB16 card is something I added. As you can see this machine uses a riser card with four 16 bit ISA slots. Two things stand out. The mounting bracket that holds the drives and the power supply.

Nothing to special about the PSU except that its rather long.

So on to the mounting bracket.

So as I said before there are no screws so the whole bracket sits in some grooves and just lifts out. This actually works really well except for one thing. On the rear of the bracket there are two 3 1/2 inch mounting points so one could easily install two hard drives. The front on the other hand only has one 3 1/2 mounting point for the 3 1/2 inch 1.44mb floppy drive. The problem is there is no way to mount any 5 1/4 drives which means no 5 1/4 1.2mb floppy drives or more importantly no CD-ROM drive. Therefore going back to what I said about the outside case and that bezel that pops out for a 5 1/4 drive is very misleading and rather pointless. I suppose you might be able to rig up something with twist ties and maybe gluing something to the bottom of the case for the drive to sit and balance on but honestly its not a practical option. Perhaps they use ice troll magic in Finland or something to mount hovering drives but other then that it’s really a downer that you can’t stick a 5 1/4 drive in even when the case allows it.

While were talking about hard drives the one currently installed in this machine had a amusing message on it.

Damn turbulent supply situation.

Now will get down to the motherboard itself.

The motherboard all around is an okay design. There is a complete lack of L2 cache memory which hurts things overall but it looks like there is a space in the corner where cache could be an option. There are a lot of jumpers on this board and unfortunately no way to identify what they are for so I did not change this machine from its default configuration.

1) CPU – The CPU is a classic 66mhz DX2 486. The socket is not a ZIF socket but a cheaper LIF socket so there is no handle or easy removal of the CPU. Regardless of the rest of the machine the 66mhz DX2 is possibly the most iconic 486 and is an excellent all around CPU.

2) Speaker – a cheaper Piezo beeper speaker as opposed to a full speaker though if your relying on PC speaker for sound on a 486 you don’t care about quality anyways.

3) RAM – eight 30 pin slots for FPM RAM. This machine came to me with 8MB of RAM but I have no idea what the maximum amount of RAM that can be installed. I would think a safe bet for maximum RAM would be 32MB and 64mb being a less likely max amount.

4) CMOS – I cant be totally sure on the CMOS battery of this machine. There is a DALLAS chip which in my experience is usually a real time clock battery although this one is soldered directly to the board and does not appear to be a clock battery. Next to it is a lithium battery, which happens to be seated in a vertical position rather then a more common horizontal seating. This is the prime suspect for being the CMOS battery.

5) slot for the riser card

6) Video – The on board video chip is a Cirrus Logic CD-GL5422. Not a terrible chip as CL can be pretty good. I don’t think this is running on a local bus but just the ISA bus as Wikipedia states the CD-GL5422 chip was only used on ISA cards but I could be wrong.  My tests reported 512MB of video RAM on board and I assume the sockets next to the chip are for expanding video RAM to 1MB.

7) Finally we have the standard AT power connection along with a built in IDE and floppy controller.

Finally this PC does have a kind of weird non standard BIOS.

You can access the BIOS by hitting Control + ALT + Escape on POST. It’s rather limited in what you can do but at least it’s there.

So final word on this PC. It does come off as a rather budget 486 for the office and lacks nice expansion options found in better 486 machines like a VLB bus. the lack of any way to mount a 5 1/4 drive (in the absence of Scandinavian ice troll magic) despite the misleading removable bezel is a major downside in an era of emerging CD drives. On the plus side the small, light weight and screwless case allows fast and easy access to the machines innards and  PS/2 ports for both mouse and keyboard are also very convenient. You also really can’t go wrong with a 66mhz 486 DX2 CPU and in a lack of any more capable machine this PC can make a good DOS machine, provided either that all your games are on 1.44MB floppy or you can attach an external SCSI or parallel port CD-ROM drive.

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Benchmarks (Intel DX2 66mhz 486, No L2 Cache, 8MB FPM RAM, Built in Cirrus Logic CL-GD5422)

3DBENCH – 27

PCPBENCH – 8.0

DOOM -13.93

Quake – N/A