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Re: Anybody with an oscilloscope as an Apple II display?
Alex Freed wrote:
You need dual D/A's to generate such a display. The speed with which
the outputs
can be changed--which soon becomes the speed with which the outputs
can be
computed, is what limits the complexity of the display.
It would appear that a very simple circuit could be made for an
experiment. The parallel printer card should be able to latch 8 bits
(unless it only uses 7, I don't remember). So a simple R-2R network
or even 1-2-4-8 K resistors can make a dual 16 level DAC that will
have an advantage of being set at once. Add 2 caps and a lot of software...
A bit off-topic, but close enough. I have heard of an interesting
technique of debugging uC systems in the early days when logic analyzers
were not easily available. 2 R-2R DACs, 8 bits each on the low and high
bytes of the 16 bit address bus connected to a X-Y scope allegedly gave
a good idea of what the program was doing. Anybody ever heard this?
Yes, it worked fine on relatively slow processors.
The setup you describe produces a "raster" of addresses, and
shows line segments for sequential execution or data accesses,
with brightness representing relative frequency. If the clock
is run slow enough, a light pen can be used to trap accesses
to particular regions!
I enjoyed this enough to incorporate this type of display into
a trace routine. It would trace code you pointed it at and
display the code and/or data addresses on a DEC point-vector
scope. It ran at a few thousand instructions per second, so
the light pen "breakpoint" worked very well. ;-)
As I noted in a reply earlier in this thread, I also used a
single-channel D/A driven by the PC value to display "flow
charts" of code execution by driving the Y-axis from the
D/A output and the X-axis from its derivative (a simple R-C
differentiatior).
Fun with scopes is probably what led me to interactive graphics
as my first excursion into interactive computer programs (around
1964). The graphical capabilities of the Apple II were one of
its strongest attractions to me in 1980.
-michael
NadaPong: Network game demo for Apple II computers!
Home page: http://members.aol.com/MJMahon/
"The wastebasket is our most important design
tool--and it's seriously underused."