On 10 Mar 2016 20:20:46 -0800, you wrote:
David,I agree but make it three switching approaches as I have thought of
another outlined below.
Putting it in the external keyboard is not obvious or intuitive, and some 7854s may not have keyboards. I would personally prefer a small mod in the mainframe and a rear panel switch like the original. Others may prefer the easier keyboard change.Linking the backup memory function to the calculator keyboard has a
certain engineering elegance that appeals to me but I suggested it
knowing that it is not really feasible do to operational concerns. One
easy mistake and the contents of memory are lost. I thought of a
better way however.
Replacing printed circuit board A32 to duplicate the uprated 7854
design will add considerably to the cost and work involved so I think
this should be avoided if possible. I think the best option for this
kind of solution is to air wire the necessary circuit or include a
small circuit board section as part of the memory card which can be
snapped off and affixed to the inside of the rear enclosure. That
will duplicate the original stock look and operation.
There is another way though. All of the necessary signals for
emulating the backup memory function switch are present on the D
connector where the cable for the calculator keyboard attaches. A D
connector dongle could be made with the backup memory function circuit
inside of it. If the dongle is attached, then the firmware boots in
memory backup mode.
The circuit needed for any of these options just requires a transistor
inverter comprised of a series connected resistor and diode connected
to the base of a NPN transistor which will operate in place of a
74LS05 gate just fine.
On an unrelated subject, it might be fun to change the keyboard id and see what mnemonic the firmware brings up for each key / shifted key. Some day when I have nothing better to do...I have only used my keyboard long enough to verify that it works but
decided to take it apart for inspection. The ID jumper is soldered
into place and on my late model keyboard, it is still the first
jumper. It would be easy enough to alter with a soldering iron. I
suspect Tektronix did this to support keyboard revisions if it ever
became necessary to release an incompatible keyboard or maybe one with
a different key arrangement. I assume this never happened.
I also noticed that the serial number on my keyboard matches the
serial number on my 7854. The cable is marked with the keyboard part
number as well as "C.A.I. 01/88 REV.C". The middle part is obviously
the production date but I do not know about the rest.