Re: Anybody with an oscilloscope as an Apple II display?

[Charlie Springer <RAM@regnirps.com>]

On Tue, 1 Apr 2008 01:30:20 -0700, Alex Freed wrote
(in article <vtmdnWVrgcCAb2zanZ2dnUVZ_gadnZ2d@comcast.com>):

> 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...

To get good A/D from this use 1% resistors, which are practically the norm 
these days, and buffer with a 74HC part (unless that is what the output 
driver is) so that you have a good voltage source. TTL parts are not speced 
for this. The CMOS parts work great. I used them plus discrete R/2R for the 
video output on the ImageWorks cards.

> 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. You can see a definite pattern when your program is running and another 
one for stuck in a small loop or when waiting for user input, etc. Some of 
the logic analyzers had this as a "feature"  and there was a name for it. 
Vector mode?

-- Charlie Springer