[Gllug] re: SCSI vs SATA

[Andy Farnsworth]

Pete,

-- You said --
>   In it's simplest form, serial needs only one wire.  However, this is

> never used in practice as the way a signal is read is that the voltage

> is compared to "ground".  Ground is theoretically zero volts, however 
> in reality it often has a voltage associated with it.  This is why 
> serial uses two wires, one for signal, one for ground so that both 
> ends of the communication has the same "common ground" to compare the 
> signal line to determine if it is a one or a zero.

As we all know, electricity works by having electrons flow in a circuit.
With only one wire, we must use a "ground return" system which means
that we have current flowing through our ground - not really a good idea
for many reasons, and basically equivalent to our two-wire situation.
This is why we use two wires per signal.
-- /You said --

Yes, I agree, absolutely :) That is why I said "in it's simplest form".
I should have included the obligatory "Don't try this at home"
disclaimer :)

-- You said --
>   SATA uses four wires but if you compare that to Parallel ATA which 
> uses 40 or 80 wires you still see a  cost savings.  Also, with only 
> one wire used for transmit (ok, plus the ground) there is no 
> syncronization issues.  In theory, if you have 10 transmit lines and 
> 10 receive lines (with associated grounds -> hmm... 40 wires sounds 
> familiar :-) then you would have 10 times the throughput possible, 
> however, in reality the time it takes to syncronize the signals from 
> these lines causes the whole system to slow down so much that just 
> pushing data as fast as you can down a single line is faster, hence 
> Serial is faster than Parallel.

I'm not sure about SATA, but 802.3 uses manchester encoding which
basically allows one signal to provide data and synchronisation, totally
eliminating the whole problem of getting the synchronisation signal and
the data
signal(s) to arrive in sync.  Hence one can ramp up the speed a bit.
Other techniques to help speed up serial communications include using
twisted pairs of wires to remove the effects of crosstalk, and even the
need for shielding.
-- /You said --

I am not sure either, however I am fairly sure it works like 802.3 in
that it includes the data and sync on the same signal.  I was trying to
state the work it takes to get the 40 lines of PATA working in sync
here.

-- You said --
> Speed of light in a vacuum (not a hoover, a vacuum)
> 	300,000,000,000 milimeters / second

c = 299792458 m/s exactly.

This is actually how we define the second these days.

-- /You said --

My seconds are are Microsoft Seconds(TM) and as such are just a hair
longer than yours :) but be aware that Microsoft Seconds(TM) are
dynamically sized as needed by Microsoft.

That said, the actuall speed of the processor is more like
3,370,000,000[1] cycles / second :)

[1] I did not look this up, just guestimated.

--You said --

> Speed of today's fastest processors (for this lesson, pretent that 
> faster = More cycles per second)
> 	3,400,000,000 cycles / second
> 
> (300,000,000,000 mm/s)/ (3,400,000,000 c/s) = 
> 88.235294117647058823529411764706 milimeters / cycle
> 
> Therefore, if the distance the signal has to travel is more than 
> 88.235 mm, the process will take longer than one cycle of the 
> processor. That is a little over 3.5 inches and shrinks each and every

> time the clock speed increases.

You're forgetting about pipe-lining.  Still, at least on earlier intel
processors, a multiply did indeed take 30 or 40 cycles and some
instructions took over 100 cycles!  Also, waiting for memory/io takes
time too.

-- /You said --

Pipelining only works if your system has something to put into the
pipeline other than "your process" so I gave the simplest example here.
You are correct that most systems will let something else run that is
ready to run and has it's data in RAM and the CPU cache while waiting
for Disk Access to finish.

-- You said --

> P.S. I have never seen any of my posts appear on this list, are they 
> getting through?

Yes. :-)

-- /You said --

Thanks, of course the moment I state this everything works as it should.

Andy


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