U31 is the WatchDog reset. If no keyboard requests (locked up), forces a RESET on peripherals, and sends NMI (0066h) to CPU.

Fluke 9010A Memory locations for various functions.
ROM 0000-BFFF
RAM C000-FFFF
I/O 0 00 Fuse Blown & Probe Sig
I/O 1 Probe Sig
I/O 2 Probe Sig
I/O 3 Probe Sig
I/O 4 Keyboard/Watchdog
I/O 5 Mag Tape
I/O 6 Pod I/O
I/O 7 RS-232

ROM
U21         Display Ram                        DATA
85A4h     F577h 1st position for display R (52)
85A5h     F578h 2nd position display    E (45)
85A6h     F579h 3rd		         A (41)
85A7h     F57Ah 4th		         D (44)
85A8h     F57Bh 5th		        (space - 20)
85A9h     F57Ch 6th		         @ (40)
85AAh    F57Dh 7th		        (space - 20)
The data byte (location of read) is now read from U21 starting at 8623h
in reverse order.

FF95h - Position pointer for display (incremented by 1 for every character added above)