[Gllug] re: SCSI vs SATA

[Tom Fairbairn]

OK, time for a hardware engineer to contribute :-)

Parallel busses (e.g. SCSI) are beginning to run out of steam.  It's all down to
how well the signals travel down the cables you need to connect things together.
As you increase the speed of data transfer, the tolerances you have for
imperfections in the cables and connectors (which cause signals to arrive at
slightly different times, etc. get smaller and smaller.  This means you need
better quality cables and connectors, which get not only more expensive but
bigger and heavier as well.  Otherwise, your least significant bit might end up
arriving in a different clock cycle to your MSB.

With a serial protocol, you can embed the clock in the data signal.  This means
all the information can travel down a single wire (or two, if you're using
differential signalling to reduce noise).  This means the cable doesn't have to
be quite so high spec - as long as it doesn't distort the signal it doesn't
matter when it gets there, you only have one conductor, and you don't have to
worry about matching the delays between connectors.

Of course, the transmit/receive circuits are far more complicated and hence
expensive than the parallel equivalents, but over recent years this cost has
reduced compared to the cost of cables and connectors.  It's not getting to a
point where people are using serial connection systems on PCBs

In summary, serial systems are cheaper these days because the serial cables are
cheaper for a given speed and cable cost dominates system cost.

Hope that helps?
     Tom





"t.clarke" <tim at seacon.co.uk>@gllug.org.uk on 06/09/2004 10:47:47 AM

Please respond to Greater London Linux Users Group <gllug at gllug.org.uk>

Sent by:  gllug-bounces at gllug.org.uk


To:   gllug at gllug.org.uk
cc:
Subject:  [Gllug] re: SCSI vs SATA


One thing that always puzzles me is why SCSI isn't inherently at least 8/16
times faster than SATA, bearing in mind that the data is transferred on SCSI
over more wires in parallel than SATA (as I understand it !).

Can anyone explain/point to an explanation ?

Tim

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