SCSI rules

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doug-doug the mighty's picture
Joined: Apr 14 2004
Posts: 1355

Okay, here is the deal. My knowledge of SCSI is pretty basic - I hook up some devices, make sure each has a unique id.

But when it comes to adapters, I get lost a bit.

In my 8600 (G3 upgraded), I have one hard drive, a 68pin 9G Cheetah. It is, of course, connected to an 50/68 adapter. I also have a ribbon cable for 68 pin drives with a couple of connectors on it.

The thought crossed my mind: can I connect this ribbon cable to the adapter and then hook up more 68pin drives (jumpering each drive accordingly)?

Or will I let all of the magic smoke out of several core components of my only working desktop?

Can someone knowing better than I, please chime in?

*In my mind, I think I know the answer to this, but I just want to make sure before I do anything considering the two 950s I killed last year (serial port incident, not SCSI-related), still waiting for me to find time to fix them.

TIA
--DDTM

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--DDTM ('Fritter Critter' since Apr 26 2004 - 18:16)

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dankephoto's picture
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Yes, but . . .

All 68pin drives connected to that bus need to have all lines terminated, you can't just term the 50pin part.

I'm not exactly clear on your setup's topography however . . .

I've had no problems using a simple 50-68 adapter - type what has the female 50 on one side (plugs into a host, such as an internal 50pin Mac MLB SCSI port), with a female 68 on the other side (same connector as a 68pin host or HDD.) I plug a 68pin multi-connector cable into that and attach my drives. I just have to ensure the final drive on the cable is properly terminated or that the final cable connector contains a separate terminator.

Speaking of SCSI drives, let me mention my current favorite economy drive, the Seagate Seagate 181GB ST1181677LCV. It's a 1/2 height 3.5" 7200rpm server-class drive and sells for ~US$50-75. A terrific bargain, IMHO. Here's an eBay search.

dan k

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doug-doug the mighty's picture
Joined: Apr 14 2004
Posts: 1355
what I mean is...

what I mean is that in my 8600, the 50 pin SCSI cable inside the box has a few devices connected and a few open terminals. One of the devices connected is a 68 pin drive, via a 50/68 adapter.

Can I take my ribbon of 68 pin plugs (would hold about 4 devices) and connect this chain directly to the 50/68 adapter? The end picture would be 4 68pin hard drives connected to a single 50/68 adapter.

8600 ==> 50pin internal SCSI ribbon ==> 50/68 adapter ==> (1x) 68 pin drive

would become

8600 ==> 50pin internal SCSI ribbon ==> 50/68 adapter ==> 68pin internal SCSI ribbon ==> (4x) 68 pin drive

* note that the 50pin internal SCSI ribbon would also be carrying a CD ROM and zip in both scenarios.

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--DDTM ('Fritter Critter' since Apr 26 2004 - 18:16)

'If it ain't broke, take it apart anyways. If you can't take it apart, break it so that you can fix it.'

dankephoto's picture
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re: what I mean is...

Sure, that should work fine. Just remember (as always) only the end device gets terminated. A 68 terminating the whole chain will term all the 68pin devices as well as the 50pin devices below it.

dan k

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doug-doug the mighty's picture
Joined: Apr 14 2004
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just for the sake of argument...

...and since I am asking stupid questions, for the sake of any other readers out there, tell me again how I terminate the last device on the chain (remembering that this is a compound chain connected to the internal bus of an 8600 where the last device on the 68pin chain is a 9G Cheetah).

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--DDTM ('Fritter Critter' since Apr 26 2004 - 18:16)

'If it ain't broke, take it apart anyways. If you can't take it apart, break it so that you can fix it.'

dankephoto's picture
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oh ye terming o' yer bus

Assuming the last drive is a pukka 68 pin device (ie: not an adapted SCA (80 pin) jobbie), just set that drive's term jumper to 'enabled'. Lacking a termable device (eg: an SCA drive on a cheapo term-less SCA-68 adapter), you'd need an actual discrete wide terminator (an 'active' terminator if possible) plugged into the last connector on the cable.

example:
50pinHost--50PinCable--Untermed50PinDevices--50PinTo68PinAdapter--68PinCable-...
-UntermedDrive1--UntermedDrive2--TermedDrive3

or similar:
50pinHost--50PinCable--Untermed50PinDevices--50PinTo68PinAdapter--68PinCable-...
-Untermed68PinDrive1--Untermed68PinDrive2--Untermed68PinDrive3--68PinTerminator

Your Cheetah has active termination builtin. It's also a hot little sucker. You sure you want to run one of those lil' 10K barnburners inside your Mac? If so, I strongly suggest you rig up some sort of breeze-maker to keep it cool(er). I've got a pair of stacked 9gig Cheetahs inside my server 9600, but I built a whole internal cooling duct system with a 120mm fan to keep their temps within reason, otherwise it'd get hot enough in there to fry the proverbial egg.

dan k

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doug-doug the mighty's picture
Joined: Apr 14 2004
Posts: 1355
a quick update...

Well, in my supreme ignorance of the different flavors of SCSI, I mis read what I had. The installed 9G drive is an 80 pin, as are all the other 9G Cheetahs I have. I do indeed have several 68pin drives and a 68pin cable. Now that my memory has been suifficiently jogged, this cable ans all 68pin drives were destined for my dormant 950 project and the NuBus SCSI card I have for that which supports the aforementioned 68 pin collection.

So what I have is a couple 80 pin Cheeteahs, one 50-80 adapter and no 80 pin cable. To compromise, I threw a pair of 50 pin drives in the 8500 (a 4G and a 2G) and slicked them. I have an extra two 50 pin drives (1.2G each) I would like to add, but I would need to build some brackets for that to happen.

Incidentally, I bumped the OS all the way up to 9.2.2. It seems to boot MUCH slower now, but I am also running the OS off of the 2G 50 pin drive as oppossed to the 9G 80 pin that it originally was. Since both drives are pretty much bottle-necked by the 50 pin bus they are both attached to, I really think it is the OS straining a bit. The draggin is only noticeable (right now) during boot up.

So, if any one has an 80 pin cable, or a half doze 50-80pin adapters they would like to part with, I will have to wait until I come across one or save up a few bucks and break down and buy one.

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--DDTM ('Fritter Critter' since Apr 26 2004 - 18:16)

'If it ain't broke, take it apart anyways. If you can't take it apart, break it so that you can fix it.'

davintosh's picture
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Don't forget...

... the most important part. Sacrifice a goat (or two just to be sure) and repeat the magic incantation three times -- no more, no less -- while facing the north with one foot in the air and your left hand behind your right ear.

The incantation? "1, Fast, Fast-Wide, Ultra, Ultra Wide, Ultra2, Ultra2 Wide, Ultra3, Ultra-320, Ultra-640. Know it. Live it. Embrace it.

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Jon's picture
Jon
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I don't think you're going to

I don't think you're going to find an 80 pin cable. The 80 pin SCA spec was designed to be used with backplanes and allows for hot swapping drives. The only machines I've seen with internal 80 pin SCSI are my Sun Sparc 5 systems and my SGI IRIS Indigo. The Suns take two 80 pin SCA drives in special drop in brackets that fit into a two drive backplane that itself is then connected to the main board via a regular SCSI cable. Most systems with SCA have the drive available to be pulled out from the case for easier maintenance. Those typically have the drive chassis with the SCA backplane that is attached to the SCSI controller via a 68 pin cable. These are where the common short two plug 68 pin SCSI cable is used.

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dankephoto's picture
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re: find an 80 pin cable

They're out there but I couldn't turn up an example in a 5 min search.

I'd not bother with an SCA cable anyway, it'll be limited to one device, which isn't all that convenient. Best choice is to get several cheap(ish) SCA-68 adapters and run those SCA drives along a 68-pin cable, the end of which is terminated.

Untermed SCA-68 adapters are inexpensive (US$4-6) and usually pretty reliable. In my experience, it's those fancy-schmancy SCA-50-with-term jobbies that cause all kinds of problems. If you're just running a single SCA drive on a 50-pin bus then I reckon they're the way to go, but for multiple wide drives (which includes all SCA drives of course) you're better off stringing them along a 68-pin cable.

dan k

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