From: "Alejandro Desalvo" <[EMAIL PROTECTED]> Subject: Re: [PCI] CPU/bus ratio Date: Wed, 30 Jun 2004 11:04:44 -0300 Message-ID: <[EMAIL PROTECTED]>
Then, speed limitations are given by the CPU processor and not by the PCI card? I could get the ZIFCarrier and put a hight ratio CPU processor and it should work at higher speed?
May not be possible simce XLR8 carrier ZIF cards were made before high speed G4s came out. I suspect the cards are limited to 10X max that its DIP switches would permit any higher ratios.
The supported bus ratios depend on the CPU chip installed. In the case of ZIFs, the contacts on the CPU which are used to tell the CPU which bus ratio to use, are brought out as pins on the ZIF card (in most cases). So when you set the bus ratio on a ZIF Carrier, you are telling the carrier to bias those pins on the ZIF in a certain pattern. So even if a Carrier card's documention does not mention a certain bus ratio, it may still be possible to get that bus ratio.
For example, suppose that when the Carrier card came out, all ZIFs gave a ratio of 6:1 when their ratio pins were biased 0101. But then, suppose a new ZIF/CPU card comes out and when you bias this new ZIF with 0101 on the same pins, it's CPU interprets that as 14:1. In that case, with the newer ZIF/CPU, the setting that the instructions say is 6:1 would actually yield 14:1.
In practice I don't think this is likely to come up, as I believe that the newer faster ZIF/CPU cards (PowerLogix 800 MHz & 1 GHz G3) have the bus ratio settings as switches on the ZIF card itself and they ignore settings on their pins.
And what about Sonnet 800MHz cards? do they work at rated speeds? or are also limited by 10x?
The bus ratio supported depends on the CPU chip on the card. The original G3 had a maximum supported bus ratio of 8:1. When speeds got faster, IBM added support for 10:1. They did this by taking one of the pin combinations for a lower bus ratio and building the chip so that this little used combination tells the chip to do 10:1.
So most G3s support 8:1 or 10:1.
Most G4s support 9:1 which was a little bit of a step backwards, given that 10:1 was already available on G3s when the G4 came out. However, IBM released the 10:1 G3s and Motorola built the G4s, so 9:1 was a bit of an improvement over Motorola's previous high of 8:1.
When clock speeds got higher, IBM and Motorola started building chips with higher bus ratios. I haven't followed the G4s in a while, but I think some of the later chips supported 14:1 and then 16:1 or 20:1. The G4/700 and G4/800 cards use 14:1 and 16:1 bus ratios respectively. That is how they are able to support those speeds. In fact some folks experiments with clock chipping the G4/800 up to a 60 MHz bus speed to yield a 960 MHz clock speed. I don't remember how this worked out, but it's fairly easy to do, as the oscillator on the G4/800 is a half size metal can.
IBM released the PPC750CX and then the PPC750FX. The FX has a top bus ratio of 20:1. PPC750 = G3.
So, if you get a G3 card with a 750FX you can use a 20X bus ratio and (in theory) hit speeds as high as a GHz or higher if your machine can take higher bus speeds.
But the vast majority of G3 and G4 cards out there were sold earlier (earlier versions of the chips) and so they support 8:1 or 10:1 for G3s and 9:1 for G4s. It's only the high end cards (with newer versions of the PPC chips) of the last couple of years that support the higher bus ratios.
Jeff Walther
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