Date: Thu, 22 Apr 2004 00:50:33 -0700 (PDT) From: Gregg Eshelman <[EMAIL PROTECTED]>
--- Mark Benson <[EMAIL PROTECTED]> wrote: <clip>Nubus Powermacs are a bit of a grey area, I thought they were all single SCSI bus but others have told me that (at least) the 7100 is twin, which makes me think the 8100 probably is too. Dunno about the 6100, but the 9150 is for certain.
Yup, the 7100 and 8100 (and clones like the Radius 81/110) are dual SCSI bus. What's different about the 9150 other than it was only sold as a server?
Picking nits here--the 7100 is single SCSI bus. The 8100 and 9150 have dual SCSI busses and were the first machines to have fully supported dual busses on the motherboard. The 9150 has four NuBus slots instead of three and includes a different AMIC chip (Fat AMIC) to support the additional slot (and probably supports up to 5 NuBus slots).
Power Computing's Power 80/100/120 machines were 8100 clones but they had spots on the motherboard for 5 NuBus slots though only three connectors were installed. And the AMIC chip in those machines is on the I/O card, not on the motherboard, so in theory, Power Computing could have built a Fat I/O card with the Fat AMIC chip and then five NuBus slots would have been supported on the motherboard which was ready for them. I suspect that Apple wouldn't sell them the Fat AMIC chips.
The Q900 (I think) and Q950 (definitely) were the first machines to have dual busses on the motherboard, but there was a hitch. There are two 53C96 SCSI controller chips on the motherboard, so there are two electrically seperate SCSI busses each with its own requirements for termination and each separate and independent from the other SCSI bus.
But the two busses are treated as one bus logically, meaning that you can't duplicate SCSI ID's between the two busses and so are limited to a total of 7 SCSI devices instead of the 14 you would expect. That is the software and ROM of the Q9x0 thinks that there is only one SCSI bus there. SCSI Manager 4.3 added support for multiple SCSI busses.
If you load SCSI Manager 4.3 or one of the later OSs which have it built in, then the two busses are (may be?) treated as separate logical busses and the full complement of 14 drives is supported with the additional exception that at boot time, any drive you are booting from or otherwise need access to before the OS loads needs to have a unique SCSI ID amongst both busses. This is because this old machine does not have SCSI Manager 4.3 support in the ROM and so the two physical busses/one logical bus limitation holds until the OS has managed to load a copy of SM4.3 and everything that happens before that needs to follow the old rules that applied when the Q9x0 shipped.
An interesting experiment here is to install a JackHammer SCSI card in the Q9x0. Reputedly, the JackHammer has SM4.3 in its on board ROM and so the question is whether having the Jackhammer present will lend support for both built-in SCSI busses before the OS loads.
Jeff Walther
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