HP 1000

[Christian Corti]

On Sun, 18 Dec 2005, Bob Shannon wrote:
> The 12555 generates its Z axis signal based (indirectly) on the I/O backplane
> timing.
> In a faster machine (E and F series) a new set of X/Y values can be loaded 
> into
> the registers before the Z axis signal is done.  This will produce a glitch.
> If your using this board in an E or F series processor, I'd recommend one of
> two approaches to work around this issue.
> 1. use programmed I/O rather than DCPC (DMA) transfers.
> 2. Double up each point sent, sending the same X/Y values at least twice.

I'm not using DMA, nor do I use the Z axis signal for blanking. We've 
scoped some signals on the card, and I really don't know what is going on 
on this card. The Z signal appears just when the glitch starts and stays 
active just before the new value arrives. So either you have a long Z 
signal and always the glitches at the beginning, or you only have a very 
short pulse which is not long enough to have something useful on the 
screen. (The 12555 is used on the HP 2100S)
The glitch I am talking about depends from the distance the point has to 
go, and is opposite of the direction! That means e.g if the dot is 
at the center position of the scope and I let it move to the bottom center 
position, I get a very strongly visible glitch to the top, the longer the 
distance the longer the glitch (verified by Y-T scoping the signals using 
the Z signal as trigger). This must come somewhere from the digital part 
of the D/A card, and that's why I need the schematics to understand how it 
works.

I hope the following will help a bit
(O=current position, | and - transition, X=final position, .. glitch)
    .
    .
    O
    |      ..O----X    X----O..
    |
    |
    X

Christian