HP 9845 C with 280 option startup failure

11 Nov 2005

...
   Are you sure
that doesn't also apply to the 98780 enhanced mono monitor? 
 The text and graphic systems are pretty much seperate on that monitor, 
 OK, there is one video timing chain (on the text PCB) that provides 
 timing signals to the graphics board too, but the graphics unit has its 
 own memory, video shift register, etc. The outputs are combinded by a 
 fairly complex mixer circuit on the interface PCB in the monitor. There 
 is no good reason why the 2 can't be displayed at the same time.

     Maybe you're right. In fact, although (according to the boardswapper) 
 the 98780 and the 98770 have slightly different overall video timings, 
 both use one single scan raster for alpha & graphics with just different 
 viewable areas. This is not the case for the standard mono crt. So maybe  
Yes. 

In the 98780, there is one video timing chain, physically located on the 
text (alpha) PCB. It's the traditional chain of counters, decoded by AND 
gates, used to control JK's to provide various timingt signals (you need 
the schematics, OK :-))

Many of the signals are passed across the monitor backplane to the 
procesor (graphics) PCB, but there is no complete timing chain on that 
PCB. The only board that provides syncs to the analogue section is the 
text PCB, and from what I rmember, tbere are no control inputs to that 
timing chain depending on whether it's in text or graphics mode.

I was under the impression that the 98770 (colour) and 98780 (enhanced 
mono) were pretty similar to the user apart from the obvious difference.

-tony

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HP 9845 C with 280 option startup failure