Re: [N8VEM: 3914] Re: [!! SPAM] [N8VEM: 3908] Use of bus termination cards?

["Bill Beech (NJ7P)" <nj...@nj7p.org>]

Michael,

That opamp and transistors form the regulated supply that can sink and 
source.  In my S-100 system the processor board is at the front, and the 
terminator is at the back on the backplane.

I have looked at the signals and the 2.6 V supply on a scope back in 
1981 when I built it.  It does clean up the signals on the bus reducing 
ringing and over/undershoots.

I also notice all my Multibus I backplanes have terminations at the 
opposite end from the main processor card.  Looks to be all passive 
resistors only.

Bill

Michael Haardt wrote:
>> In the S-100 bus realm we used an active termination - I believe it was 
>> 470 ohm resistors to an approximately 2.0 volt stiff DC source.  
>> Termination only on one end of the board.  This forces all boards to 
>> actively drive both ones and zeros but did match the impedance of the 20 
>> inch long S-100 bus lines.  It really cleaned up the signals especially 
>> with the higher speed boards.
>>     
>
> If the termination card is at one end, what's at the other?
>
> The other thing I wonder about is the "stiff" DC source.  Doesn't it have
> to be a very fast regulator for being able to improve things?  The design
> I am looking at uses an opamp and two complimentary darlington transistors
> with the second transistors being BD139/BD140 - low frequency transistors.
> Five 10uF and four 10nF capacitors are supposed to help there and the
> layout shows the autor was worried about this.
>
> I am aware of the fact that active termination usually draws less power,
> but that's just "usually".  I wouldn't want things to get unstable if
> I have extreme bus signal patterns.
>
> The design uses a voltage of 2.6V, connected with 270 Ohm to all bus
> lines.  Poor bus drivers, that almost pushes them to the limit.
>
> Finally: I believe these cards do help, but did anybody ever look at the
> signals? Could it be that this is called termination, but in fact only
> improves signals to a certain extent, but does not really terminate the
> backplane ends?
>
> Michael
>
> >
>
>