2940 & G3 & CDRW

[foleym]

Having only recently been introduced to the
MAC, I have a CDR (which may or may not be compatible, that's a seperate issue) which I
want connect to my G3.
I am running ProTools and have a 2940UW SCSI
accelerator as the interface to my ProTools
hard drive.
The MasterList CD documentation explicitly states that I should not connect my CDR to the SCSI accelerator. I sould connect it directly to the MAC SCSI bus.

I don't have a way to do that. The Drive and
CD drive are not SCSI. There is no SCSI connector.

Does this mean that I need another SCSI card?

Should I have bought a 2 channel SCSI card to begin with?

Thanks!

[Steve Rosenthal]

The MLCD documentation is out of date. You can hook up your CDR to your SCSI card, but you need to observe the following guidelines:

All wide devices need to come before your CDR on the SCSI bus. Between the last wide device and your CDR, you need a 68 pin to 50 pin terminating adapter. Your CDR (if it's the last device on the chain) will need to be terminated.

Make sure you use the best SCSI cables you can, and try to keep the chain as short as possible.



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--Steve Rosenthal, Digidesign ETS

[KymataDigital]

Steve,
Wouldn't hooking up a narrow, slower device to a wide connector with a fast device slow the performance of the wide drive?

Adaptec's 2940U2W has 68-pin external/internal and 50-pin internal just to avoid this problem.

[Steve Rosenthal]

Kymata,

No, actually it won't. The way SCSI works, only one device can control the bus at a time, and each device negotiates it's own separate transfer rate with the initiator (SCSI card). This means that the wide drive will perform at the maximum it's negotiated with the initiator, regardless of the presence of the narrow drive on the bus. Now, where this changes somewhat is in a scenario where you're attempting to access both a wide and narrow device "simultaneously." Since the narrow drive is transferring data at a rate at least half as much as the wide device, the wide device will have to wait longer to gain access to the bus. In this sense, your overall system performance be lower than if you were using all wide devices.

You are correct that the 2940XXX cards have a narrow connector on them. Since it's a single-channel card, the same bus usage issues would apply, but you'd have a cleaner connection by avoiding the adapter required to go from wide to narrow.

Hope this clarifies.



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--Steve Rosenthal, Digidesign ETS

[KymataDigital]

Makes total sense, Steve.

Hence, the 64-bit path of the Adaptec 3940 dual-channel card.

Thanks.

[Steve Rosenthal]

Kymata,

The 3940UW is actually -- in a manner of speaking -- 32 bits (two 16 bit busses), but you clearly see the distinction between the two cards.

I appreciate your posts. They bring up points that really help to clarify the confusion about SCSI.

Thanks.



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--Steve Rosenthal, Digidesign ETS

[KymataDigital]

Oops! I was thinking about the Adaptec 3950U2B....it's 64-bit dual-channel.

[Steve Rosenthal]

Oh, I getcha. When Adaptec refers to 64 bit with that card, they're talking about the PCI bus interface.

The SCSI bus(ses) are still 16 bit (wide SCSI).

So many bits, so little time.

--Steve Rosenthal, Digidesign ETS