[Steve Rosenthal]

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

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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.