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Re: poor AHA-2940UW performance under Linux/Alpha?



With regards to your message at 06:23 PM 07-23-99 -0400, Andrew Hobgood.
Where you stated:
>> however, ULTRA scsi is a signal difference.  ALL devices on the channel
have
>> to support ultra scsi signalling, and if they don't then the 'lowest common
>> denominator' for signalling is used.
Kaka.
The SCSI initiator negotiates with each device for width ( 8 bit or 16
bit), data rate ( 5,10, 20, etc.) per request asserted. As a result it
matches each devices capability.
A "slower" devoce will not hurt anything. Each transfer is negotiated and
handled on a custom basis.
There is ONE exception, and that is with UltraSCSI2 also known as LVD (Low
Voltage Differential). On an LVD bus if ALL devices are LVD it runs at
80MB/sec data rate. IF any are Single-Ended SCSI it drops the whole bus
down to the SE maximum, which is 40MB/sec.

>Ahh... interesting.
>
>> in your case, the wide channel is going to report 20 Mb because that is the
>> physical width of the channel.  the narrow device obviously doesn't support
>> the electrical signalling for ultra so it is slowing up your bus.
More Doo-doo. There are both narrow (8bit) and wide (16bit) UltraSCSI Devices.

>Well, I removed all non-Ultra devices from the bus, and tried to get it to
>negotiate at ultra speeds... but, no such luck.
>
>/me wonders if perhaps this is because he's using a differential drive on a
>single ended bus through a bus converter...
>
>Would that kill the ultra signalling on the way to the drive?
>
>/Andrew
>
>-- 
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>

Again a SE device makes the whole bus run SE.

What CAN screw things up is the use of the worng type of terminator. There
are special terminators for SE, LVD, and UltraSCSI3/aka Ultra160. Use the
wrong one and it all gets messy..

Best regards,

  Maurice W. Hilarius                   Telephone: 01-780-456-9771
  Hard Data Ltd.                        FAX:       01-780-456-9772
  11060 - 166 Avenue                    email:maurice@harddata.com
  Edmonton, AB, Canada  - T5X 1Y3       http://www.harddata.com



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