[BBC-Micro] Black Teletext

Phil Blundell philb at gnu.org
Tue Jun 16 17:46:19 BST 2009


You could also try patching the teletext ROM to replace &80 with &84 at
the point where it draws to the screen.  I imagine it should be fairly
easy to locate the code in question, and there are probably a few spare
bytes where you could store an extra subroutine.

p.

On Tue, 2009-06-16 at 17:25 +0100, Ian Wolstenholme wrote:
> Oo err, all a bit beyond me, daughter boards and all,  but the VSync event
> might be a goer.  I suspect it would be a bit much to check all 1024 locations
> on a VSync event, but  perhaps I could check a quarter of the screen every
> 50 VSyncs, something like that.  There would be a bit of a delay in updating,
> but it wouldn't matter too much for my purposes.
> 
> Best wishes,
> 
> 
> 
> Ian
> 
> ----- Original Message -----
> From: Mark Haysman [mailto:jumbos.bazzar at btopenworld.com]
> To: bbc-micro at lists.cloud9.co.uk
> Sent: Tue, 16 Jun 2009 15:41:59 +0100
> Subject: Re: [BBC-Micro] Black Teletext
> 
> Hi.
> 
> I don't think so in software. Not quite sure why, but the SAA5050 doesn't 
> get supplied with a regular D7, it seems to come from DEN of the 6845. That 
> and D6 seem to go off via some logic to Pin 10.
> 
> Thinking logically, I'd try replacing the 74LS273 at IC15 with a GAL (You'd 
> need a daughterboad to get the pins in the right places) that looked for 
> &80, and set D2 high regardless of D2s state:
> 
> d2 = d2
>     # d7 & !d6 & !d5 & !d4 & !d3 & !d1 & !d0 & en
> d2.clk = clk
> d2.oe = ttxvdu
> 
> That would also give you an enable pin, "en", when set high, does the 
> modification, when low it's normal. The rest of the pins would be simply
> 
> d0 = d0
> d0.clk = clk
> d0.oe = ttxvdu
> ....etc
> 
> ...I think! I've not tested this, just thought of it now, so don't curse me 
> if it doesn't work, or if it's totally the wrong way of doing it!
> 
> Mark.
> 
> ----- Original Message ----- 
> From: "Ian Wolstenholme" <bbcmailinglist at beebmaster.co.uk>
> To: <bbc-micro at lists.cloud9.co.uk>
> Sent: Tuesday, June 16, 2009 2:31 PM
> Subject: [BBC-Micro] Black Teletext
> 
> 
> At the weekend I wrote a little routine which traps the OSWRCH thingy
> and changes any VDU code &80 into an &84.  Much to my surprise, it worked!
> So if I typed VDU &80,65 I would get a blue letter A etc.
> 
> Except it doesn't work for the Teletext adapter, which must load the pages
> direct into screen RAM!
> 
> Is there a way to trap any write to the Mode 7 screen RAM and do a similar
> fiddle?
> 
> Best wishes,
> 
> 
> Ian
> 
> 
> 
> 
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> 
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> 
> 
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