14 Dec
2008
14 Dec
'08
7:07 p.m.
[Adding a video display]
I think I have one of those somewhere. I thoguht the video outptu was
analogue RGB at TV rates, but I could well be mistaken.
[...]
On 68K machines it;'s more ocmmonly done by delaying DTACK until the
video hardare has completed oth its memory cycle and allowed the CPU to
do one.
-tony
A sup[rising
amount of logic.
Way back I had some sort of RS232-capable graphics box called Pluto - I Whilst that's not quite what the OP's after, I
think there's some potential
there in that a modern PC with a serial port could be initially and quickly
used to do that task, with the 'real' hardware built later. (I suspect it's
the route I would take, purely as offloading the graphics ability elsewhere
seems to have a certain 'cool factor' about it :-)
Another possibilty would be to add some slots compatible with HP's DIO
(strictly now DIO-1) bus. This uses the same connectors as the S100 bus,
but it's basically the 68000 bus (23 address lines, 16 data lines) with a
few extras that yuu could mostly ignore. About the only thing you'd have
to have on your processor board would be the prioirty encoder from 7
interrupt lines to the CPU's IPL signals.
There is at least one DIO monochrome video board set -- the 98204 IIRC.
It comes in 2 versions, for differnt horixontal scan rates (the 98204A is
TV-rate I think). It's all standard chips (maybe the odd PAL), I am
pretty sure I have scheamtics.
And of course there are DIO RAM boards, I/O boards, etc.
I think there's some info on the DIO hardware spec in one of the
_Pascal_ OS manuals on bitsavvers. I can dig out the URL if anyone can't
find it.
Basically you need an area of memory that
can be accessed both by the CPU and by the video hardware. Typvially RAM
with multiplexers to switch it between the 2 devices.
I think quite a few systems did it on the falling edge of the clock signal,
didn't they? i.e. the memory was only accessed by the CPU on the rising edge,
so (if fast enough) was essentially free on a falling edge. Elegant, but
probably hell to debug and get working as a first project (particular if DRAM
refresh is thrown in there)