Exceptional Event Handling in ADB Keyboards

Documentation

Little is documented of the "Exceptional Event" bit (bit 14) in ADB except that it is specified to be set to 1 if not used[^1] and that it is set to 1 if the reset/power key has been pressed on the ADB keyboard and 0 if no exceptional event has occurred[^2].

[^1]: Apple Guide to the Macintosh Family Hardware, Second Edition, page 322 [^2]: Apple IIgs Hardware Reference, page 144

Observations

On some models of ADB keyboard, the Exceptional Event bit is set to 1 by default and only set to 0 while the reset/power key is held down; it does not latch the event, so if a Talk 3 command is not issued while the key is being held, the Macintosh will not see the exceptional event.

| Model Name | Model Number | Clears Exceptional Event Bit | | -------------------------- | ------------ | ----------------------------- | | Apple Keyboard | M0116 | While reset/power key is held | | Apple Extended Keyboard | M0115 | While reset/power key is held | | Apple Extended Keyboard II | M3501 | While reset/power key is held | | Apple Keyboard II | M0487 | While reset/power key is held | | AppleDesign Keyboard | M2980 | Never |