AHA-2940 Ultra/Ultra Wide

[Klaus Steinberger]

Taras M. Dowhaluk wrote:

> On advice from an Adaptec FAQ, I disabled "Initiate Sync
> Negotiation" (what ever the hell that is) and set the "Max Transfer
> rate" down to 10mbps on all devices, and rebooted.

On SCSI bus, transfers could be asynchronous and synchronous.
Async transfers are slower, maximum is 5 Megatransfers per second
(that's 5 Mbyte/sec on narrow SCSI).

The "Max transfer rate" setting is only useful and active for
synchronous transfers.

> Errors went away, beaudy !
You should check for cabling and or terminator problems. Async
transfers are more forgiving against such problems (like cable
length, too much or too less termination, bad cables).

Also you need definitly active terminators for 10 Mbytes/sec and
more.

> To prove a point I then re-enabled "Initiate Sync Negotiation" and
> they did NOT come back ! What the ???
> I then bumped the speed back up to 20mbps, and lo ! still no errors
> at boot.
As I said above, "Max transfer rate" has no effect with asynchronous
transfers.

> If anyone has got some 'real' scsi bus exercising (exorcising ?)
> software, pls email the location.
Cable problem could not be exercised with software. You should check
for the following things:

1. Cable length: A maximum of 3 Meters for up to 10 Mbyte/sec, and
   better much less on higher speeds.

2. Check that the bus is __only__ terminated on its far ends, so
   that you have only 2 Terminators on the bus, and they must be
   on the end!

3. Check that you don't have "T" structures on the cabling.

4. Check that you have active terminators on both ends. Some drives
   have passive terminators built in, even if they could transfer
   at Fast rates! Your adaptec controller has active termination,
   and does the right things even if its in the middle of the bus,
   as long as you let the default (automatic termination) enabled.

5. If you have a mixed wide/narrow cable you have to be very careful
   about termination of the upper 8 bits, as they must be terminated
   at their ends too, but not all adapter connectors or cables do
   this for you!

Sincerly,
Klaus

-- 
Klaus Steinberger               Beschleunigerlabor der TU und LMU
Muenchen
Phone: (+49 89)289 14287        Hochschulgelaende, D-85748 Garching,
Germany
FAX:   (+49 89)289 14280        EMail:
Klaus.Steinberger at Physik.Uni-Muenchen.DE
URL: http://www.bl.physik.tu-muenchen.de/~k2/