To be honest, the easiest and most sensible answer to that is: TRY IT OUT !!! Switch the multiplier to the lowest setting and then put the bus speed jumpers in all the different configurations. For non mathematical people, there are 2 to the power of the number of jumpers configurations. 4 configurations for two jumpers, 8 configurations for 3 jumpers, and so on. Just boot up to the DOS prompt and run ctcm, or a similar program which will tell you the CPU speed. You then only have to divide it by the multiplier setting to find your bus speed. Again, if your motherboard uses SoftMenuTM BIOS, you only have to look in your BIOS setup menu to find all of the different bus speed settings from which you can choose.
There is also, however, a different approach:
Motherboards with the PLL chip PLL52C59-14 can run at up to 75 MHz and they also support the 'turbo frequency' feature, which increases the bus speed by 2.5% (officially approved by Intel's CPU specifications).
For 75 MHz Pin 8 via 2.2 k Ohm to 0 V ('0 V' means 'ground' and NOT 'disconnected'!) Pin 12 via 10 k Ohm to 5 V Pin 13 via 10 k Ohm to 5 V
For 68 MHz, the 'turbo frequency' for 66 MHz Pin 8 via 2.2 k Ohm to 0 VPin 12 via 10 k Ohm to 0 V Pin 13 via 10 k Ohm to 5 V
For 61.5 MHz, the 'turbo frequency' for 60 MHz Pin 8 via 2.2 k Ohm to 0 V Pin 12 via 10 k Ohm to 5 V Pin 13 via 10 k Ohm to 0 V
Measured on the Abit boards IT5H, IT5V, PR5, which all use the PLL52C59-14. The PR5 also comes in a version with the PLL52C61-01, however, the setting below seems to set the board at 61.5 MHz instead of 83 MHz.
Motherboards with the PLL chip PLL52C61-01 can run at 83 MHz bus speed as well and also theoretically support the 'turbo frequency'. There obviously is, however, a way of configuring this chip (circuitry), so that it would not run at 83 MHz. I'm working on that.
These are the conditions:
For 83 MHz or 61.5 MHz 'turbo frequency' for 60 MHz. Unfortunately this depends on the circuitry on the motherboards. Pin 5 via 10 k Ohm to 0 V ('0 V' means 'ground' and NOT 'disconnected'!) Pin 12 via 10 k Ohm to 5 V Pin 13 via 10 k Ohm to 5 V
For 75 MHz Pin 5 via 10 k Ohm to 5 V Pin 12 via 10 k Ohm to 0 V Pin 13 via 10 k Ohm to 5 V
For 68 MHz, the 'turbo frequency' for 66 MHz Pin 5 via 10 k Ohm to 5 V Pin 12 via 10 k Ohm to 5 V Pin 13 via 10 k Ohm to 5 V
I have measured this on the Asus P/I-P55T2P4 rev. 3 board and was able to verify the exact same settings on the FIC PA-2006 board. The FKI SL586VT II or Magic Pro MP-586VIP board also use this chip, but you can't get to 83 MHz. Instead you get the pathetic speed of 61.5 MHz. In terms of jumpers, this means that you will have to find out which of the three jumpers is connected to the particular pins. It's circuited via the pull up/down resistor of 10 k Ohm. In case you only have jumpers with ON/OFF positions instead of 1-2/2-3 positions, the ON condition is the condition for 0 V, and the OFF or open condition is for 5 V.
This information is only for real freaks who want to get their boards to 75 or maybe even 83 MHz by all means. This way, you might be able to use the higher bus speeds on boards that only have 4 bus speeds, as long as this board uses one of these PLL chips. You do the necessary soldering at your own risk and should only attempt this if you know exactly what you are doing!!!
There are several motherboards that use one of the two PLL chips, depending on what was available when the motherboard was assembled. The Abit PR5 is such a fellow (of course I was unlucky enough to receive a board with the PLL52C59-14) and it seems the shuttle HOT 557 is another. There are a few reports of 83 MHz bus speeds with these boards, but most simply can't run at this faster bus speed. This is a real pity, because both of these two boards can use SDRAM - the best RAM for the 83 MHz bus speed.
Special Precautions for 75 and 83 MHz Bus Speed
Using this higher bus speeds includes some important restrictions which you should be aware of.
The PCI bus runs at 37.5 or even 41.6 MHz. This can lead to several problems with PCI devices. Typical trouble makers are SCSI controllers, some video cards, and network cards. SCSI controllers and network cards often refuse to work at the faster speed, but some video boards just get much hotter than usual. If you find a way to cool these video cards, you shouldn't have any trouble. My Diamond Stealth 64 Video VRAM isn't affected at all by those higher bus speeds. I hope the 75/83 MHz bus speed survey will help us find out which PCI devices run at higher PCI bus speeds.
The speed of the EIDE interface included with the chipset is not only determined by the PIO or DMA modes, but is also highly dependent on the PCI clock. This is one reason why the EIDE interface is always slower in systems with 60 MHz bus speed or less. This is also valid in the other direction, meaning your interface will be faster when you are running at 75 or 83 MHz bus speeds than at 66 MHz. At first this sounds fine, but often either the interface or, in more cases, the hard disk isn't up to the faster bus speeds. My HDDs work fine at 75 MHz bus speed, but at 83 MHz I have to reduce the PIO down to 2. The same is valid for EIDE CD-ROM drives. This could be the cause if you are running into strange lock-ups in windows.
The Asus P/I-P55T2P4 is one example of a board that does not allow you to adjust the ISA bus speed. It seems to be a fixed divider from the PCI clock. This can cause sound cards to run into trouble if they don't like the higher ISA bus speed. I haven't come across this problem myself yet, but I've heard of one fellow whose AWE 32 produced strange sounds when running at a faster speed. If you run into this, increase the ISA wait states in the BIOS setup to try to remedy the problem.
Let me say again that the RAM type and quality is of great importance. Most 60ns EDO will run fine at 75 MHz bus speed, but for 83 MHz you'll need high-end EDO or SDRAM (as long as the motherboard supports it).
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