Question about choice of PCI bus speeds.

[Patrick]

I've overclocked my C900 to 1116 MHz on a CUBX board and it seems very stable. I can't get it to boot at a FSB of 133 so it's limited to 124 MHz tops. However, there are two different 124 settings. One underclocks the PCI bus speed to 31.0 MHz and the other overclocks it to 41.33. My question is this: What are the pros and cons of underclocking and/or overclocking the PCI bus speeds?

[Wombat]

Underclocking: Everything works fine, but you get stuff ( &lt10% in your case) slower from the HDs and stuff. But most things couldn't saturate the PCI bus too often.

Overclocking: At 41MHz, your hard drives will probably not run in DMA mode, if they run at all. High risk of data corruption.

I strongly advise you to take that mild underclock.

[Patrick]

Don't like the sound of that!

High risk of data corruption.

Although my hard drives appear to be working correctly at the present bus speed of 41 MHz, you've now put the fear of gawd into me.

My boot drive is the master on the primary IDE, my two video capture drives are on a FastTrack/33, and my extra drive is the master on the primary of the onboard CMD controller. I haven't noticed anything unusual...yet. Maybe I'm just living right.

Any other opinions, in agreement with Wombat or not?

[Nuno]

They may work just fine or they may fail at the precise moment you´re reading this

Now for serious, if they work fine, maybe you´re lucky and your hard disks/IDE controllers can keep up with the high pci speed. But noone can assure you that you system won´t fail tomorrow - That´s the joy of overclocking...

If you feel unconfortable with it, just use the lower pci speed (much less prone to cause malfunctions) or run all your system within spec. There you go.

[Wombat]

If you're getting DMA transfers at 41MHz, I'm very surprised. If you're not getting DMA, then DMA at 31MHz may be faster than PIO at 41MHz.

Even more than the HDs, I'm surprised the Promise card is working. Promise cards seem to be good, but they usually don't tolerate out-of-spec PCI.

[spazm_1999]

According to Intel Application Accelerator and the Asus app... my drive run at UDMA-4 at 41.5MHz it can also run at 42Mhz... but at 42MHz my DVD gives read error once in a while.

Spazm

[Bohrn]

that 41 mhz fsb has always been a thorn in my side too. Go with the lower one. It will be fine, and like someone said, prolly faster overall anyway. I had my primary drives data give it up on me once when I tried to do an 83 mhz fsb overclock of a 300a Celeron. everything else was great, and fast, but no-go with the drives. And even if you get it to work reliably, forget trying to burn CDs. They hate that sort of thing.

[Wombat]

You may even be getting data corruption now, but you won't know it. It can be silent corruption, and will only show up when you try to open that really important document you saved, and you find out that the document is unreadable.

If this machine has any important data on it, I wouldn't think it's worth the risk.

[Patrick]

Thanks guys.

Ok, ok.... you've convinced me! I'll use the lower setting!

I wish I could get this chip to overclock ONE step higher. It would then be at 133 FSB and the bus speed would be right on spec. However, it's just not meant to be...

[Indiana]

You better believe what Wombat says. I oc'ed my rig to 160 MHz FSB (40MHz PCI) and while my IDE was working correctly (with UDMA) my SCSI totally freaked out and destroyed my boot partition (cost me about two days to reinstall the most important things on the machine...) and I only noticed the corruption when Win2k suddenly refused to boot.
Not one single error on my IDE HDs, not even on the Raid, but then this also depends on the make of your HDs.

And as said you might be lucky and never get problems but when the drives are working at that speed correctly for now that doesn't mean there won't be situations (heavy DMA contention - think about SBLive) where there are errors and with such sporadic errors you're in VERY high risk of noticing the corruption only when it's too late.

Edit: I didn't see your reply. So we convinced you already....