Date: Wed, 16 Jul 1997 19:14:20 -0600 From: vanmaren@fast.cs.utah.edu (Kevin Van Maren) To: ade@demon.net Cc: smp@FreeBSD.ORG Subject: Re: HEADS UP: EISA cards. Message-ID: <199707170114.TAA28520@fast.cs.utah.edu>
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> On a related note, can someone explain to a PC-hardware ignoramus > (ie, me :) why no-one makes completely PCI-based motherboards, say > with 7 or 8 slots? [perhaps this question should move over to > -hackers?] Because you need ISA slots :) Seriously, there are very few PCI sound cards, and no PCI modems. (Of course, you can go external modem). But the integrated serial ports are `isa', and adding a connector is cheap, especially when most people `need' them. (For a PCI sound card, look at ASUS's site, which uses the ISA connector onthe mediabus, or ensoniq's site at http://www.audiopci.ensoniq.com/) As for more PCI slots, there are a few motherboards with 6+ slots. Usually in high-end servers. A few motherboards support 5 PCI slots, on one PCI bus. But beyond that you need multiple PCI busses which involves a bridge chip. Award BIOS can only deal with one bridge. Cards like the Adaptec 6944 (quad 100Mb ethernet) and the 3940/3985 also use bridge chips to get multiple PCI devices in one slot. Symbios logic has a dual function SCSI controller that has two SCSI controllers in one chip; they also have a SCSI/Ethernet combination chip. Both of those are only one PCI load, and don't require a bridge. If you really want a lot of PCI, there are currently three choices: 1) Buy a very expensive server. You can get 8 or so PCI slots by doing that. 2) Buy a PCI expansion Box. Magma and Bit3 both make them. For ~$1000, you get a computer case that will hold 7 PCI cards, and plugs into a single PCI slot. However, they use bridges three levels deep, which causes problems with some software (and many BIOSes). 3) Buy a SBC (single board computer) and plug it into a passive backplane. There are a lot of choices for pasive backplanes; you can get them with as many as 17 PCI slots (they are bridged, of course). However, since 4-5 PCI slots are enough for 95% of the PC market, all three of the previous options are rather expensive. There are several motherboards that have integrated UW SCSI as well as the 5 PCI slots (ie, www.airwebs.com/p6kdi.html/) AMI (www.ami.com) has announced a dual motherboard with 6 PCI and two ultra-wide SCSI (NCR875) using a bridge. It is likely more than the $480 for the AIR. Annother option would be the SuperMicro I2O motherboard. Also, I wanted to point out that there are known problems with the PCI 2.1 coherence protocol in the presence of PCI bridges. It remains to be seen how much of a problem it will be with the I2O spec in practice. Kevin
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