Little challenge for HP-15C experts...

[J-F Garnier]

Further analysis of the 'bank display' behaviour:

Manually doing -99.5 x! on the 15c LE (instead of the 15c CE) very clearly shows the blank display during the x! calculation on the LE.

Looping 100 times this short program with initial X=-99.5

01 LBL A
02 x!
03 LSTx
04 DSE I
05 GTO A
06 RTN

takes about the same 16-17s on both the 15c LE and CE, so the difference of blank display is not caused by a speed difference.

Explanation (attempt):

At the 15C microcode level, the display on/off state is managed by the DISOFF and DISTOG opcodes (resp. "display off" and "display toggle") as on the 41C.
There is no "display on" opcode, if the display state is unknown, the sequence DISOFF, DISTOG ensures that the display will be on.

My interpretation is that the blank display is filtered out by the 15c CE simulation firmware (that is, the X display is maintained even after the DISOFF opcode) to some extend, around 0.1s:

                key pressed                  end of key processing
                  DISOFF                        DISTOG
                  v                                 v
15C, 15c LE: |<---      dsp off      --->|
15c CE:        |<- 0.1s ->|<- dsp off ->|

Very likely, this has been done to help solving the issue of interaction between the blank display state and the blinking display in the 15c LE.
The two states are managed by different means: the blank display is managed by the 15C software with the DISOFF and DISTOG opcodes, whereas the blinking display is managed by hardware and just triggered by the DISBLK opcode (a Voyager-specific opcode).

Unfortunately the used solution completely masks the brief blank display used for keystroke visual feedback.

J-F

[Edit: replaced "15C-specific" with "Voyager-specific" regarding the DISBLK opcode - Thanks Mike! ]