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Old 04-13-1999, 01:11 PM
Techsupt2 Techsupt2 is offline
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Default Wide drive, narrow bus

Heya Steve,

I brought one over from the Project II forum. I thought going from wide to narrow was done by a simple adaptor but seem to recall more complex posts on this in the past (or was that something else?). I'd also like to know once and for all what is the best method.


<BLOCKQUOTE><FONT SIZE="1" FACE="Arial,Helvetica,sans-serif">quote:<HR>beautyfish
posted 04-12-99 04:28 PM PT

Here's the question: Being a normal musician guy, I really don't understand the difference between Ultra Wide, Wide, SCSI II, etc. I now have a G3/266 desktop and just got a Project II card. What I would like to do is get a good 9 gig drive that I can try using on the external G3 SCSI bus WITHOUT an Adaptec 2940 Power Domain card, and see what kind of track count I get. Then, if I have to, I would like to be able to get the Adaptec card WITHOUT changing drives. Is it simply a matter of having the right pin adapters (50 to 68, etc.) when the time comes, or do I have something else to worry about?

Thanks <HR></BLOCKQUOTE>

Thanks,

-Bryan
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Old 04-14-1999, 10:48 AM
Florian Florian is offline
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Default Re: Wide drive, narrow bus

Steve,
I have 2 questons for you:


1. you write

>>>"It is also important to remember that the wide drive must come after any and all narrow devices on the narrow bus. You would then use a wide terminator."<<<

Question:
Do you mean physically "after" or in terms of Scsi ID. If i put an 18273 LW internally in my 9500 and i get the granite adaptor, can i just put it on the open connector of the internal harness??

2. Does the 2940U2W wide/lvd card work with protools?? Or is it overkill to do that and one should better go with a "regular" 2940UW since we have to throttle down in the control panel anyways??
What about in terms of coming higher samplerates/storage demand. Is it better to get the faster card and the fastest drive now instead of investing in a "regular" wide setup??

thanks, I appreciate your posts on this forum.

Florian Ammon
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Old 04-14-1999, 01:38 PM
Steve Rosenthal Steve Rosenthal is offline
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Default Re: Wide drive, narrow bus

Hi Florian,

The wide drive has physically come after the narrow drives -- SCSI ID is not a factor (I'll explain below). Since the wide device has signal lines that the narrow bus doesn't, and since the pinouts for narrow are not a direct-mapping subset of wide, you'll get signal and termination problems if you try to run narrow-wide-narrow. The only ways you can use narrow ane wide devices on the same chain are as follows:

1) Wide and narrow drives on a narrow bus:

Narrow-Narrow-...-Wide

2) Wide and narrow drives on a wide bus:

Wide-Wide-...-Narrow (There must be a terminating adapter between the last wide and first narrow device that actively terminates the high byte of the wide signal.)

RE: Adaptec 2940U2W...

We haven't officially qualified this card yet, but many of our customers are using it without problems. You are correct that we recommend throttling it down to 20MB/sec to mitigate any PCI bus flooding problems, but there are advantages to running an LVD subsystem. For one thing, think of LVD as somewhat analagous to balance audio lines. You get better signal quality, less noise and cross-talk. Also, you can run much longer cable lengths (up to 12.5 meters).

Hope this answers your questions.

--Steve Rosenthal, Digidesign ETS

========

How SCSI IDs work:

On a narrow SCSI bus, you have 8 data lines -- which is why narrow SCSI is referred to as 8-bit SCSI. These lines also serve the purpose of SCSI IDs. This explains why narrow SCSI can only allow 8 SCSI IDs (0-7). When devices identify themselves and arbitrate for the bus, they assert the data line that corresponds to the SCSI ID for which they are set. (Note that when data is transferred, all 8 lines are used.)

For example, you have a drive set to ID 5. It lets the initiator (your SCSI card; the Mac's native SCSI bus) know where it is by asserting (sending a voltage over) data signal line 5. When two devices are set to the same ID, they are then both trying to assert the same data line, which is why only one -- or none -- of them will show up, hence a SCSI ID conflict.

Wide SCSI behaves exactly the same way, only since you have 16 data lines, you can support up to 16 SCSI IDs (0-15).

What SCSI IDs CAN'T do is predict the physical location of their corresponding devices on the bus. They are entirely independent in that regard.



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