28 Feb
2013
28 Feb
'13
11:59 a.m.
This will be little help, and I can't recall the exact details, but a
magazine I used to have described a circuit that IIRC put MDA on a
composite monitor. It was Micro something, and common enough but just
wasn't BYTE or PC Mag. And you built the thing by wire wrapping
w/machine pin sockets and pins that at the time IIRC you could only get
from Mouser. They were named as the source anyway.
It's not hard to do, basically, XOR the suncs and combine with tehe video
signal. I have used a slgihtly modified versio nof the video output
circuit of the TRS-80 model 1 onoccasion.
BUT this will not coverrt scan rates. You end up with a composite signal
with an 18kHz horizotnal scan frequency if you start with an MDA singal.
Some monuitors can be tweked ot handle that, many cannot. Of those that
can, they might not like it too much,m you are stressing the horizotnal
ouptu transisotr and flyback transoformer a bit.
Have you tried beeping? I think rom BASIC supports
beeping or some
The cassette mtoor control commands (is that MOTOR ON and MOTOR OFF?) are
suitable too.
kind of rudimentary sound generation. With no floppy
disk or h/d it
automatically loads BASIC. Hallelujah. It can be a useful diagnostic
tool.in some instances.
When I had a floppy disk controller problem on mt PC/XT, I put the board
on an extender, booted to ROM BASIC and used port acccess commanedst to
fiddle eith the control registers on the FDC board. A bit of work with a
logic probe and I found the faulty IC.
-tony