The 75/83 MHz Bus Speed Project


The first time that we all heard of 75 MHz bus speed was, when the Cyrix 6x86 P200+ was launched. Before this time there had been the Intel Pentium, which had been pushing the bus speed from 25, 33, 40 and 50 MHz from 486 times to 60 and 66 MHz.

Shortly after the announcement from Cyrix about the 75 MHz bus, boards with the Intel 430 VX chipset became available, which could be jumpered to 75 MHz bus speed. This certainly was and still is kind of strange, because Intel doesn't officially support the 75 MHz bus at all and neither do the Intel chipsets. At present there are only a few chipsets on the market that officially support 75 MHz bus, which are the VIA chipsets 580VP and 590VP , the VLSI Lynx and the SiS 5571, but motherboards that would carry these chipsets are still very hard to find. We also have to take in consideration, that the performance of these rare chipsets yet has to be seen. The high performance of the Intel 430VX but particularly of the 430HX chipset however, are well known to us. Since the beginning of October you can now even get an Intel 430HX chipset carrying 75 MHz board, the Asus P/I-P55T2P4 . This board used to be a more or less average kind of performer in the field of the HX chipset boards, but since it's possible to jumper this board to 75 MHz bus speed, it has become the screamer of the HX boards .

I know for sure now that the Asus P/I-P55T2P4 even supports 83 MHz bus speed !!! Markus Sundstrøm was so nice to share this little secret with me. It's the following jumper setting: JP8 1-2, JP9 1-2 and JP10 2-3 . 83 MHz bus speed means 41.5 MHz PCI clock, and the following CPU speeds: 125 MHz, 166 MHz, 208 MHz and 250 MHz. However do I have my doubts that FPM or EDO RAM will be able to put up with this speed. IMHO 83 MHz bus speed will be reserved for SDRAM, which unfortunately isn't supported by the HX chipset.

Here the comment of two visitors, who've been using the Asus P/I-P55T2P4 successfully at 83 MHz bus speed with EDO:

Thought you would like to hear about the 83 MHz bus speed on the Asus P/I-P55T2P4. I had thought it wasn't possible without instability, but after hearing of a friend's success I tried it. I am running a B-step P133 at 83 MHz x 2.0, with 2 8x32 60ns micron EDO, and have yet to experience any sort of crash, even in win95. Guess SDRAM is not a requirement . My memory speed is set on 60ns in the system BIOS. If you'd like I can post quake, ctcm or whatever benchmarks you like. My overclocked system is a good bit faster than a true p166.


Hello, I would also like to tell you that I have successfully overclocked by P-133 using an 83 MHz BUS speed, from a 133 MHz to 166 MHz without any problems at all. In fact, the chip runs cool and the heatsink doesn't even get warm. I am using plain old OKIDATA 60ns EDO RAM (nothing expensive like Micron). I can use the 60ns RAM setting on my P55T2P4 board and everything is completely 100% stable in Win95 and DOS. Interesting though, is that I had to change my CPU voltage from Standard to VRE to get this to work successfully. When it was at Standard, my Windows 95 would get wierd errors occasionally even at 75 MHz BUS, and it wouldn't even run any programs at 83 MHz BUS. But when I put it to VRE to give it a little bit extra voltage, everything runs 100% at all the BUS Speeds.

It looks like both my SC-200 SCSI card and Matrox Millennium both agree with the 83 MHz bus speed because I have never had a problem. I have heard bad things about the Adaptec 2940UW card though, like the older revisions don't even work with 75 MHz, and you would be VERY lucky to get one running on 83 MHz BUS speed.
Just thought I'd let you know.

Darren Shady

Sounds like this is the way to go. I've been ordering one of these boards myself and can't wait to run my CPU at 208 MHz (2.5 * 83 MHz).

Now as already said, running a 430VX or HX board at 75 MHz bus speed or more is something which isn't supported officially, and that is why no motherboard manufacturer ever would announce the support of this bus speed. The reason for this seems fairly simple, they probably don't want to get in trouble with giant Intel, because after all does Intel not even have one Pentium CPU, which officially would run at 75 MHz bus speed and neither do their chipsets. Hence an announcement of a motherboard manufacturer, which says that their boards support 75 MHz bus speed, would be something like an invitation to overclock Pentium CPUs - something that Intel certainly won't appreciate.

There are some more things to consider, if you would like to run your motherboard at 75 MHz bus speed. In this case you'll certainly have to overclock your Pentium CPU and just as well you are overclocking your Intel chipset. This is of course invalid, if you should be using a Cyrix 6x86 P200+ and/or a VIA 580/590VP chipset. However in both cases you are also running your PCI bus at busclck/2 = 37.5 MHz. This certainly is a nice thing, as long as your PCI devices are able to put up with it. The new PCI 2.1 specifications anyway define the PCI bus to be able to work at up to 66 MHz.

Another problem is your main memory. Will it be able to put up with 75 MHz? It most certainly will run just fine, in case you are using SDRAM . Should you be using EDO or FPM RAM however, you should try to get the fastest version of it. Literature claims that FPM, EDO and even BEDO are unable to run at anything more than 66 MHz. My visitors however contradict to that, as you can see in all of my surveys. I do believe my visitors more than any magazine or book, so obviously you ARE able to use FPM and EDO RAM at 75 MHz bus clock.

Hence for most of the users, there are these things to be considered:

  • you are overclocking your (Pentium) CPU
  • you are overclocking you (Intel) chipset
  • you are overclocking your PCI devices
  • you might overclock your RAM

I'd say that for us overclockers (the typical visitor of my site :)) this is sounding just great - three dimensional overclocking in one go - let's go for it!

In case you are unfamiliar with overclocking, you should visit my Overclocking Guide and my Overclocking Survey .

But don't forget:
You are overclocking your system at your own risk!!! I'm not responsible for any damage, that might be caused by using 75 MHz bus clock at your system.

Now what are the benefits? Well, there are even 3 of them.

  1. Your CPU runs faster than before.
  2. Your memory performance has increased by 12.5 %, that's more than switching from 60 to 66 MHz!
  3. Your PCI (video) performance increases too.

The best benchmark to show how much you are actually gaining by switching to 75 MHz bus speed is the Quake Frame Rate Benchmark on my site. You will see that e.g. the same system is performing considerably faster at 187.5 MHz CPU speed (75 MHz x 2.5) than at 200 MHz CPU speed (66 MHz x 3)!

Does My Board Support 75 MHz ?

I got a very interesting mail from Jason Langhoff concerning this question. I think, it's best, to just put it up here:

There is a way you can tell if your motherboard can run at 75 MHz. If the clock generator has either of the following brands, it is overclockable. The two brands are PLL52C59-14 and PLL52C61-01/21 . If you are unfortunate enough to have ICS9159-14 , it can't be overclocked (this includes me).

I'm not completely sure about the following:

The jumper settings for the PLL52C59-14 are DS3-OFF, DS4-OFF, and DS7-ON.

The jumper settings for the PLL52C61-01/21 are DS3-ON, DS4-OFF, and DS7-OFF.

If you already know this, no loss, but if not it should be able to help others find out if they can overclock their MB to 75 MHz. I do plan on getting a different motherboard, if I can successfully overclock it, so all might not be lost. I enjoy your page and think you do a great job. Thanks for your time.

Jason Langhoff.