This post could also be in the PPC Forums, or Other Technology, maybe even Hardware Hacks. Admin, please move if necessary. This should have been done a long time ago (and it should be repeated for ATA/IDE interfaces). There are so many different drive interfaces, and most descriptions are incomplete, and/or confusing. What I'm posting is merely a starting point. Hopefully others will post their knowledge, and eventually enough info could be put together to request an entirely comprensive scsi reference info page for Macs here at Applefritter. The main problem I see is that "scsi" is a catch all term for any scsi interface, usually meaning a particular interface, but possibly meaning something different than the usual. What I'd like to see is all Mac models listed with the type of interface including the number of pins for the interface, its actual scsi designation and the multiple names meaning the same thing, a treatment of scsi termination (all terminated? or just one of multiple drives terminated?), as well as a treatment of NuBus/PCI cards available (discontinued or otherwise) with their scsi multiple-interface/pin designations outlined. Better... a sort of glossery, starting with the # of pins, scsi designation, other names and references back to them in the glossery, data transfer rate, Mac models that used it, and different NuBus/PCI cards that supported it, software drivers necessary, etc. I realize there will be redundancy, but that would make it easiest to look up anything in particular. This sounds like a big project, but technically there are a finite number of scsi designations, so I don't see any problem including all this information on a single reference page. I've seen other reference pages on the internet, but they are never fully comprehensive.
Here is the quick breakdown (please feel free to correct errors/add info):
- 25-pin SCSI-1: Uses an 8-bit bus, and supports data rates of 4 MB/s
Any models use this internally? Are all the 25-pin external connectors on Macs SCSI-1?
- 50-pin SCSI-2, Fast SCSI: Same as SCSI-1, but uses a 50-pin connector instead of a 25-pin connector, and supports multiple devices. This is what most people mean when they refer to plain SCSI.
Isn't this identical to Fast SCSI? Lets make it clear what the multiple scsi names are for the same damn interface!. Aren't there SCSI-1 interfaces that also use 50-pin?
The PowerMac 8600 has both a SCSI-1 & a SCSI-2 bus, both are 50-pin on the logic board, but the SCSI-1 interface has an external 25-pin connector.
- 68-pin SCSI-2, Fast SCSI
- 68-pin Wide SCSI: Uses a wider cable (168 cable lines to 68 pins) to support 16-bit transfers.
How can you tell the difference between Wide interfaces and Fast Wide? What's the data rate?
- Fast SCSI: Uses an 8-bit bus, but doubles the clock rate to support data rates of 10 MB/s.
- Fast Wide SCSI: Uses a 16-bit bus and supports data rates of 20 MB/s.
Are there NuBus cards that support this in 68k Macs? What are they? How many pins? Is this the true interface inside an Apple Network Server?
- Ultra SCSI: Uses an 8-bit bus, and supports data rates of 20 MB/s.
Which is superior, Ultra SCSI, or Fast Wide?
- 68-pin SCSI-3, Ultra Wide: Uses a 16-bit bus and supports data rates of 40 MB/s. Also called Ultra Wide SCSI.
Are there SCSI-3 50-pin drives, or are they all 68-pin?
- Ultra2 SCSI: Uses an 8-bit bus and supports data rates of 40 MB/s.
What is the purpose of the different bus speeds? Why did the technology advancing like this, with both 8-bit and 16-bit bus speeds, yet sometimes identical data transfer rate?
- Wide Ultra2 SCSI: Uses a 16-bit bus and supports data rates of 80 MB/s.
What are the cards that support this?
What effect do adapters have on scsi data transfer rates? For example, the 80-pin IBM SCA type connectors will always need adapters... which are made for 68-pin and 50-pin... what is happening here? ... effectively turning a drive from one scsi to another? Are faster SCSI drives always backwards compatible with earlier scsi interface technologies so long as you have an adapter? For instance, can you put a Ultra2 SCSI SCA drive with an adapter in a machine with 50-pin SCSI-1?