Adaptec 29160LP and Ultra160/320 hdd sync to 40 only

Why does this combo not sync to 160?

If I exchange the 29160LP with an 2940U2W, they sync at 80.
HDDs tested are a Hitachi U160 and a brand new Maxtor Atlas 15K U320.

I've read that for the 29160 controller 40 really should mean 4 x 40 or 8 x 40 for 160 and 320 respectively but I don't think this is right.

This is a single chain. No other drives or devices are connected and the cable is double checked and is the original with fixed terminator.
The controller setup is default but, of course, nearly any other setting has been tested with no success.
The controller has been moved to different slots with no change in performance.
Machine is IBM IntelliStation with 32 bit PCI bus. It runs very fine except for this issue.

Any ideas?

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Gustav BrockCIOAsked:
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Duncan MeyersCommented:
The cable and terminator are the most likely source of the problem. You may have seen cables that have sections where the pairs are twisted - they should support higher transfer rates. Make sure, also, that you have an Ultra160 spec terminator.

>I've read that for the 29160 controller 40 really should mean 4 x 40 or 8 x 40 for 160 and 320 respectively but I don't think this is right.
You're right - that's wrong.... This may be a hangover from SCSI2 where the higher transfer rate was achieved with 16 bit transfers rather than 8 bit. You'll still see some controllers offering the option of terminating the high-order or low-order bits depending on the attached devices.

In short - try a cable and terminator that is specced for the transfer rates that you're trying to achieve and you should be right.

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Gustav BrockCIOAuthor Commented:
OK, thanks.
The cable is twisted by pairs.
I will check the terminator later. What should it read?

I have another cable at hand, original Adaptec and twisted, but the terminator is marked LVD/SE while all the connectors are marked "Ultra2 LVD/SE".
Is that a cable _not to be used?
I did test with this cable once. Also only sync at 40. Shouldn't that be 80?

Duncan MeyersCommented:
I wonder if the SCSI bus is operating as single-ended (SE) as opposed to Low Voltage Differential? SE will run at an absolute maximum burst speed of 40MB/sec. LVD will run at 80MB/sec. You need Ultra-3 to run at 320MB/sec. 80MB/sec is as fast as your current setup will go (which, incidentally, is faster that you can get data off the disc in most circumstances):

My guess is that either a device (such as a disc) or the terminator is forcing the SCSI bus into SE mode. Since you have tried two different discs, try a new LVD terminator and see if that helps.
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Duncan MeyersCommented:
You may find this interesting as well:
Gustav BrockCIOAuthor Commented:
The cable in use is marked.
This is not an Adaptec but an IBM cable which I can locate by Google as:


Looks pretty OK to me ...

Duncan MeyersCommented:
I suspect that the terminator is at fault here. You can try removing it alltogether (purely as a test - leaving it off during normal
operation invites poor performance and data corruption).
If the SCSI card then negotiates to 80MB/s, then you can safely say that the terminator is probably at fault.

The other thing you can look at is that, usually, terminators have a LED on them to indicate that they are active, and often the LED
changes colour to indicate if the bus is operating in single-ended or Low Voltage Differential mode. If that is the case, the colour code
should be printed on the terminator itself. If not, I'd chase down another termninator and try it.
Gustav BrockCIOAuthor Commented:
Unfortunately the terminator is fixed to the cable - and it carries no LEDs.

I'll get a new cable and test with that ... it may take some days though.

Gustav BrockCIOAuthor Commented:
Got a new cable with "320 LVD/SE" stamped on the terminator and that cured the problem!
Now Sync is reported as 160 by the controller.


Duncan MeyersCommented:
Excellent! Thanks for the points...
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