Signals to scope on a PET?

[ayates]

I was wanting to show some teenagers how an oscilloscope can be used, so I was looking to see what signals I could view from my PET I just resurrected. I can see the Phi1 and Phi2 clock signals (though they are showing in phase which I don't think makes sense). Are there any other signals that are easily accessible which would be interesting? I was hoping for two signals that were related. i.e. an interrupt that comes every three clock cycles. I have a two channel oscilloscope that is even older than the PET


Thanks,

Allan.

 

[MikeS]

I can see the Phi1 and Phi2 clock signals (though they are showing in phase which I don't think makes sense).

Are you sure you're triggering correctly?

Are there any other signals that are easily accessible which would be interesting?

Video and sync signals and their relation to each other and what's on the screen can be interesting.

 

[dave_m]

Allan,
You might write a tight loop that reads a location in memory. Then use the PET address select to get a nice sync for the scope to look at timing signals.

For instance:

10 peek(36864) : goto 10

will read address $9000 which will assert the SEL9 output for you to use as a sync on channel 1 to look at other signals like "read/write" on channel 2 of the scope. BASIC is slow so if the scope display is too dim, you may want to use the machine language monitor to do the same thing in machine code for a brighter display (higher repetition rate).

 

[fachat]

This is what I wrote on 6502.org ...:

You could scope some of the keyboard inputs - maybe triggered by a specific keyboard output (note: the 4 PIA( VIA?) pins that act as keyboard scan select are expanded to 10 separate lines where only one is active at a time. You may want to use these instead of the PIA pins)

You could then let someone press the appropriate keys to show them how the keyboard output changed - but only when the correct keyboard input is active. Give a little matrix keyboard explanation up front, that should be a great show

You could also scope the video output synced with vsync and 20ms total time, i.e. you see the whole screen on the scope. If the timing resolution of your scope is high enough you might even see the cursor blinking. You could then let someone fill the screen with character "online" and see how the display changes. You could also do a single rasterline (i.e. one of hte eight (graphics mode) or 14 (IIRC, upper/lower case mode) horizontal lines per char. i.e. sync with hsync, 100us total time). You see an overlay of all rasterlines on the scope. With a blank screen you should see mainly a flat line, but with a lower intensity line going up flashing for the cursor. Now again let someone put some chars on the screen. Again, before showing this, give them some explanations, let them decide on which characters to try for which reason. hint: vertical bars....) Again, should be a good show.

You could also save something on tape, and scope the tape while it reads the file back, but that is more sophisticated. Would probably require some knowledge on the encoding...

Hope this helps, let us know what you did and how it worked out!
André