🚀 go-pugleaf

In article: <qN$EETAUs6G3EwwD@dbqa.demon.co.uk>  Tom Bird 
<hamster@nospam.demon.co.uk> writes:
> 
> In article <KNhhHLA1DbG3EwEV@jasper.demon.co.uk>, Mike Tomlinson
> <mike@jasper.demon.co.uk> writes
> >In article <h7KmZCAAjMG3Ewzx@dbqa.demon.co.uk>, Tom Bird
> ><hamster@nospam.demon.co.uk> writes
> >
> >>Any idea where I can get a DD50 SCSI terminator?
> >
> >Just about anywhere, it's a very common item.  Make sure you get 
an
> >active one (if it has an LED in it, it's probably active.)
> 
> Thanks.
> 
> What's the difference, active, passive and so on?
Passive, has two resistors for each line:

+5v (TERMPWR)---220ohm---signal---330ohm---GND

This 'pulls' the signal up to 3v, and gives a characteristic 
impedance of 132ohms. The 'problems' are that 132ohms is at the upper 
end of the required impedance range (this could be fixed with smaller 
resistor values), and (worse) that any noise or 'droop' on the 5v 
rail will be imposed on the signal line as well. Active instead has a 
voltage regulator and a single resistor:

+5v(TERMPWR)---2.85v regulator---110ohm---signal

(There are 'other' values for active, with slightly lower voltages 
and resistors, but the shown ones are the most common).

This gives a large degree of immunity to noise and droop on the 
termpwr line, and a better impedance 'match'. Active is _required_ on 
any bus using the 20MHz signalling rates (ultra), and recommended 
even at the 10MHz 'Fast SCSI-2' rates. I can make a suprising 
difference to bus reliability...

Best wishes

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