"Gary Tait" wrote in message
...
"robb" wrote in
:

and button lines also feed into the SN75518N a "VFD driver chip" which
does not make alot of sense to me either ?

They use the same I/O lines to multiplex the display and the keypad. Not
at
all uncommon.


Thanks Gary,

Could you elaborate on that a bit more ? as i am a mere amateur

i have traced one side of switches back to the Q22-Q30 lines ?

the general layout is ...
that two button/switches {increment, decrement} will share a line to one
Q## output and repeat for 12 pairs of buttons (total 12x2 or 24
button/switches)

the other side of the switches get bundled/connected together into two
groups of 12, that each group shares one line down the ribbon to the main
controler board (with 12k resistor just before going down ribbon wire)

so the Q## pins on the SN75518 are Outputs ... So i guess there is some sort
of timed pulsing and controler board has to figure out which button was
pressed ?

thanks for your help,
rob