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SCA SCSI options for Macs with SCSI

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A few people have asked how I make use of modern SCA SCSI disks with older machines.


Since high end 10000 or 15000 RPM 3.5" drives are overkill speedwise, and since most of us don't really need 300 gigs on our older machines, I've settled on 70 gig 2.5" disks such as the IBM 90P1316, the IBM 90P1313 and the IBM 26K5158. They're 10000 RPM drives which can communicate at Ultra-320 speeds ,so they're still much more than we generally need, but they're comparatively new, require less power than a 3.5" disk (although they still do require 12 volts) and they're cheap at around $15 USD each.


Next, we have to reliably connect them to our computers. There are common SCA to 50 and 68 pin adapters which look like this:



With this particular type, even though we see terminators, they don't seem to terminate the bus properly. What works consistently and reliably with this kind of adapter is a 68 pin terminator, a 68 pin cable, and a 68 to 50 pin adapter. This has the advantage of letting you use a physically narrower cable which may be easier to route.


Some people don't like using 68 pin cables and adapters, but a common issue is that if the upper bits on a wide SCSI bus aren't properly terminated, drives sometimes won't negotiate down to an 8 bit bus, which means the drives won't show up on your older machine. As I said, with a 68 pin terminator and cable, this will work reliably.


Here's the 68 to 50 pin adapter, which gets installed in the 50 pin SCSI header of your computer:



What this looks like when you're done can be like this, which you can see isn't bad considering the amount of space available:



or like this, which is quite tidy and actually is more out of the way than the usual 50 pin cable:



There are other SCA adapters with proper termination built-in, which I'll cover soon. I hope this helps :)

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Many years ago I used to make terminated adapters that terminated the upper bits of the bus which provided better compatibility with newer drives, annoyingly I can't find good pictures of them anywhere though

I still have a bunch of the PCB's left that I should assemble one day, but I'm thinking of redrawing the board with a better layout


I'm actually surprised the top adapter causes problems, I count 9 elements per resistor pack for a total of 27 termination resistors which should be enough to terminate all the signals on the SCA connector (There are 26 IIRC - at least my adapters had 26 resistors but I guess it's possible I made a mistake on them!) they look to be 110ohm (which is the correct value for active termination) and there is the voltage regulator required for active termination (I can't read the value on that but I have no reason not to think it is the correct voltage)

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I've used this approach (2.5" SCA drives with 50 pin adapters and external terminators) with great success.


I haven't priced drives lately since I bought a bunch years ago, but 36GB or 72GB Seagate Savvio drives were $10-$15 each, and the adapters were $5-$6 each in quantity.


Terminators were a little harder to find, but either in-line or "end of cable" terminators worked fine.

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Thanks to your advice I still have a few 72GB Savvio's tucked away in unopened envelopes! [:)] One day I may go SCSI-to-somethingorother, but that's a way off.


@johnklos Very nicely done. For good measure, I'll tack on a link to the article @trag posted in case problems crop up for others: SCSI Voodoo HowTo


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