On Jul 28, 10:12am, "Henry B. Hotz" wrote:
>I know which way the terminators are labeled and I had them in with 
>the little pin-1 mark next to the arrow on the disk controller card. 
>But just for the hell of it I tried putting them in backwards and it 
>works now.
>
>The terminator pulls the bus lines to a reference voltage with a 
>certain resistance in order to damp out signal reflections from the 
>end of the wire.  Since the resister pack is connected to both Vterm 
>and Ground reversing the pack should have the effect of shifting the 
>reference voltage and increasing the load on signal drivers, but 
>should have no effect on the damping, I think.  So maybe this makes 
>some kind of sense?
>
>Or maybe these resister packs are labeled backwards.  ;-)

It has to do with logic levels.  In TTL, < 0.8V is a 0, and > 2.4V is a 1.
Resistor packs are normally 220/330 ohms, which would put the termination
voltage at ~3V.  (TTL pulls down more easily than it pulls up.)  With the
packs reversed, the voltage would be ~2V, no man's land.

~Steve


-- 
Steve Allen - wormey@eskimo.com   http://www.eskimo.com/~wormey/   ICQ 6709819

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