[Gllug] re: SCSI vs Serial ATA

[Andrew Halliwell]

And verily, didst Jack Bertram announce to the hordes:
> Indeed.  And at slow data rates, these complications aren't too
> important (hence parallel printer cables being faster than RS232).  But
> for any serious bandwidth, timing becomes so important that it's a
> really big issue.

Yep... Just thought I'd also post a quick clarification for the
non-electronics-aware...

resistance:- pretty much a constant in a medium, only affected by
temperature, the length of the medium, the thickness of the medium and the
relative resistance of the material it's made from. So a wire at room
temperature will retain the same resistance no matter what (of course, when
current flows through it, the resistance will increase slightly, but that's
easily taken care of).

Impedance:- This is resistance based on the magnetic properties of the
electromagnetism. At DC (direct current), there is none. As frequency
increases, impedance increases, because a magnetic field forms, caused by
the changing electric field, which then causes another electic field etc...
These fields cause a resistance to change, so the higher the rate of
change (frequency), the more resistance there is.

Capacitive resistance:- This is basically the inverse of impedance. As
frequency increases, the resistance decreases. Another side effect of this
is phase change between current and voltage. 

When two seperate metal plates are placed close to each other, you might
think there's no circuit between them because there's no physical contact,
but when a DC charge is applied, current briefly flows as the electric field
transfers energy across the plates, the plates become charged, but no more
current flows. With AC, the current constantly changes, as does the voltage,
which induces current on the other plate. So between two close but
insultated wires, the higher the datarate, the more chance you've got of
getting cross-wire induced interference.

Finally, when you get into the GHz, you're effectively in the near-microwave
range, and capable of transmitting, this is the extreme form of inductance,
cos you can affect wires feet or miles away (that's how radios work) so you
need shielding and damping.
-- 
Gllug mailing list  -  Gllug at gllug.org.uk
http://lists.gllug.org.uk/mailman/listinfo/gllug