Re: strange cursor... any idea ?

I think I have a clue...

I added some wires to power my own extensions  I was taking 5v and GND 
from one of the 74LS... near the video connector

and from another chip near the keyboard connector

one of these extensions convert the video signal to composite

and the other one was for a ps2 keyboard... (not used anymore and 
replaced by a led + resistor to see if the 8032 is on or off)

I discovered that by disconecting the led I get the standard bloc cursor


I have to remove my wires and find a better point to get 5V and GND


Le 12/16/2022 à 8:31 AM, Baltissen, GJPAA (Ruud) a écrit :
>
> Hallo Didier,
>
> The interrupt routine only inverts bit 7 at $e476. My question is: do 
> you see a checker board all the time? I think answer is “yes” because 
> the shown checker board is $7F. And I assume that 1) the checker board 
> shows up when you move the cursor over an existing character and 2) 
> that this character reappears when you move the cursor? Added later: 
> does it “blink”?
>
> If so, here are my thoughts (so far):
>
> - This is a software cursor, the hardware cursor of the 6845 is not 
> used, so we can rule out the 6845 itself.
> - The character is read at $E46e, “lda (CurScrLine),Y”/”B1 C4”. This 
> is a two-bytes instruction. If this instructio is replaced by “lda 
> #$FF”/”A9 FF”, then you have a possible answer for your problem. 
> Problem here: this answer does change the contents of the screen 
> memory but does not restore the original value when you move the 
> cursor. Just check if your E-ROM is OK and that should rule out this 
> answer.
>
> - Broken CPU? No, because: what could be broken that it only shows up now?
>
> - Broken RAMs? No, that would show up in other characters as well. And 
> you did replace that.
>
> - Broken buffers between the video RAM and CPU, in the direction form 
> RAM to CPU? No, then all four ICs have to be broken.
>
> - Bad signal that steers the mentioned buffers? That could be but: if 
> the buffers don’t output anything, $FF is read. Inverting bit 7 means 
> it becomes $7F. Question, you didn’t mention that the cursor “blinks” 
> so: does it? If not, that explains a steady checker board. The bummer 
> of this answer is the same as above: it does change the contents of 
> the screen memory but does not restore the original value when you 
> move the cursor.
>
> So the question remains: what does change the read value into $7F but 
> does not change the contents of the video memory? No solution from me 
> but maybe it triggers someone else to have another idea.
>
>
> Met vriendelijke groet,
>
> Ing. Ruud Baltissen
> Senior Beheerder TI
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