The LED Display and Keyboard mode knows the following key command:

AD    sets mode to modify current addres 
DA    data  sets mode to modify data in current location
+     increments to next location
RS    system reset, a hardware key
GO    run  a program (via GOEXEC)
ST    stop nmi hardware key
PC    display current pc

After startup give the NMI and IRQ/BRK vector a known value

NMI Initialization for Single Step and Stop
17FA 00
17FB 1C

IRQ Initialization for BRK
17FE 00
17FF 1C

Make sure you are not in decimal mode:
00F1 00

Machine Context saved/restored by ST/GO
00EF PC low
00F0 PC high
00F1 Status Register (flags)
00F2 Stack Pointer
00F3 A
00F4 Y
00F5 X

Breakpoints
– Write BRK instruction ($00) at desired address, program will be stopped here.
– ST button: Pressing will invoke NMI interrupt.
– Single Step: Set SST switch to on, press GO button to step one instruction.

Cassette Load and Save

12 Volt power source is only required when reading tapes

Save to tape
– Store $00 in $00F1 (to ensure CPU is in binary mode).
– Save start address (low/high) in $17F5, $17F6.
– Save end address+1 (low/high) in $17F7, $17F8.
– Write tape ID ($01-$FE) in $17F9.
– Start tape in record mode.
– Run address $1800 (DUMPT) to save.

Load from tape
– Store $00 in $00F1 (to ensure CPU is in binary mode).
– Write tape ID ($01-$FE, $00 loads any ID, $FF loads using start address values (low/high) in $17F5, $17F6) to $17F9.
– Run address $1873 (LOADT) to load.
– success of load: current address is 0000, error is FFFF