[rescue] Macs & IDE vs. SCSI

Dave McGuire mcguire at neurotica.com
Sat Apr 12 17:12:01 CDT 2003

On Saturday, April 12, 2003, at 03:06 PM, Curtis H. Wilbar Jr. wrote:
> I usually try to stay out of IDE vs SCSI wars... as I usually try to
> stay out of Mac vs PC wars, and occasionaly this unix vs that unis 
> wars....
> However, there are a few points I haven't seen anyone mention...
> 1. IDE drives are only now starting to support tagged command queing 
> type
>    features for optimizing access across a busy drive..... this is a
>    definate advantage for SCSI drives vs the majority of IDE drives.
> 2. Most SCSI drives come with 5 year warranty... most IDE drives came 
> with
>    2 to 3 year warranties but have not been reduced (with some 
> exceptions)
>    to 1 year warranties.

   I assume you mean "have NOW been reduced"?

> 3. SCSI is an internal and external buss.  There is no real external 
>    buss.

   ...and it's not really possible either, within reason.  IDE was 
designed as an extension of the ISA system bus, to move the drive 
controller electronics off of a controller board (typically a Western 
Digital WD1003 or similar) and onto the drive.  As such, it's not a 
real storage interconnect at all; its more akin to a microprocessor I/O 

   Because of this, looking at the electrical interface, there is no 
attention paid to termination to deal with transmission line effects.  
Long cables will result in signal degradation, reflections, noise, and 
even resonance in worst-case scenarios.  A properly designed (vs. 
quickly thrown together) storage interconnect will have well-defined 
target impedances for the lines, termination on both ends of those 
lines, and well-defined restrictions on things like stubs.

   This is an extreme example, but in other words it's not much more 
practical to have an IDE drive dangling outside a case on a cable than 
it would be to plug a cable into a DIMM socket and have a DIMM on the 
end of it outside the case.  Actually it's not such an extreme 
example...modern memory buses DO have transmission line considerations, 
more so than the early low-speed ISA crud that IDE is based on.


Dave McGuire           "She's a cheek pincher.  I have scars."
St. Petersburg, FL                          -Gary Nichols

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