[volt-nuts] Prema 6048 Cal switch confusion

[David C. Partridge]

I've got a Perma 6048 that I've trying to put into Cal mode.  It just won't!

The switch on the rear panel works OK, AFAICT when in the Cal position it
connects ~WE on the DS1220 (pin 21) and ~W on the $864 SRAM (pin 27) to a
4K7 pullup to +5V and to the output of an LS00 which should be strobing the
Write lines low as needed.   I can't see any way in which the processor can
be informed of the switch setting!  In measure mode it just connects ~WE on
the DS1220 to a 10k pullup to +5V.

 The schematic is on page 120 (Figure 11.4) of the user manual and also page
9-7 of the service manual. 

Clearly I'm missing something - as a friend reports that his does go into
cal mode.

Confused ...
Thanks
Dave