Theory of operation
A normal C64 will check for the presence of a cartridge at power-up but once it has determined there isn't any, it will no longer try to access the areas which may be mapped to !ROMH and !ROML if you leave the C64 idling with only the cursor flashing and no software running. After the initial check, !ROMH and !ROML should remain completely inactive if the PLA is glitch free.
Using the tester
To use this circuit you must make sure you use the original C64 kernal (no JiffyDOS or things like that) and that no cartridge is attached to your cartridge port.
When the C64 is first turned on, it will check for the presence of a cartridge and this will trigger the red LED to turn on. Press the button to turn it off and see what happens. If your PLA is not generating glitches, the LED will stay off indefinitely. If there are glitches, they will trigger the LED back on.
Normally, !ROMH and !ROML will always stay above 3V after the initial check and the set/reset latch will stay inactive. The 74LS279 will detect any glitch which brings the voltage below the threshold voltage for TTL logic circuits, about 1.5V. If you used the 74HC279 then the threshold voltage is about 2.5V and the circuit will detect any glitch which brings the voltage below the threshold voltage for 5V CMOS logic circuits.
