[BBC-Micro] Help with 'B' video output (only mode7 working)
pete at dunnington.plus.com
Sun Apr 6 17:00:19 BST 2008
On 06/04/2008 16:26, Andy Ford wrote:
> Additional info...
> I have just this moment tried swapping IC5 and IC6 with ones from a
> issue3. This "spares" issue3 machine does work (it has some faults but
> the video is ok)
> But there was no improvement. I don't mind desoldering if needed but I'd
> like to be (as much as possible) that I would be doing the right
> component if someone thinks it is item X etc.
>> I have here a nice (tape only) issue 4. However, only mode7 appears to work.
>> When tested with the composite video (either with or without the colour
>> jumper enabled) , I do get a very vague picture (it is offset to the
>> left and almost inverse)
About the only differences between Mode 7 and the other modes, is that
different address bus drivers control the RAM access, the video
processor gets bit data directly from the memory instead of from the
SAA5050 - and if you see some sort of picture, however distorted, that
much is probably working - and some versions of the video processor
control the polarity of the video signal differently. It sounds like the
video processor is not setting up its outputs correctly in Modes 0-6,
giving an inverted picture, so check whether IC6 is a Ferranti ULA or a
later video processor (2069), or an early video processor (2023). If
it's a ULA or later vidproc, there should be a jumper on S26 and no wire
linking S26 centre (or IC6 pin 27) to the inverted TTX signal (usually
from one of the bus buffers, IC10 pin 1). If (and only if) it's an
early vidproc, there should be no jumper on S26 and that wire should be
fitted, which is a modification specifically for early VTI chips.
It sounds as if somebody has got a mismatch between the video processor
and the modification.
Pete Peter Turnbull
University of York
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