Diagnostic Switch

Project: Switch for entering builtin monitor program.
Target : CBM 2/3/4/8000 series (read note!)
Time   : 5 min.
Cost   : ~ 3 US$
Use    : start monitor from within an arbitrary program

Summary

Details

Note: This switch is said to work on PETs in general, I only tested it on my 8296D, though. So to be absolutely safe that you don't damage your computer in case you don't have the pinouts of the user port for your specific model, attach the switch via a 470 Ohm resistor!

Monitor commands

G xxxx

Go. Executes a program starting at xxxx.

L "filename",dev

Loads 'filename' from device number 'dev'.

M

Displays memory contents: M start_address end_address

R

Displays processor registers.

S "filename",dev,start_address,end_address

Saves memory from start_address to end_address to 'filename' on device number 'dev'.

X

Exit TinyMon.

:

Modify memory (in output of 'M' command)

;

Modify processor registers (in output of 'R' command)

Ingredients

• 1 single-pole push-button switch (normally open)
• 1 user port connector (edge connector, female)

Instructions

1. Solder push-button switch to pin 1 and pin 5 of the edge connector. Be sure to not shortcut anything, insulate the wires between the connector pins and the switch. I recommend using hot glue to both insulate and fix the switch.
2. Now test the switch:
   • Plug the user port connector into the user port
   • Press and hold(!) the diagnostic switch
   • While still pressing the button, switch the machine on The machine should boot up without the characteristic beep sequence and immediately enter the builtin monitor.
3. If everything seems fine, you successfully made your own ultra-complex diagnostic switch :-) Note that instead switching the machine on while pressing the diag switch, you can also press the reset button instead (while pressing the diag switch!).

Possible failures

• Are you sure that the push-button switch is normally OPEN?

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