Andrew Warren wrote:
>
> I wrote:
>
> > Now tie 12 of the shift-register outputs to one side of each
> > thumbwheel switch.  Tie all 12 pins on the other side of the
> > switches together, run them to a single I/O pin on the PIC, and put
> > a pull-down resistor on that pin.  Configure that pin as an INPUT.
> > ....
> > push [a] single "1" bit through the shift-register, applying it to
> > each switch in turn.  After each [clock], check the INPUT pin; its
> > state indicates whether the corresponding switch is open or closed.
>
>     Uhh... A small, but important, correction:
>
>     The above method will only work if there's a PIC-facing diode
>     between each switch and the PIC's INPUT pin.
>
>     If you want to dispense with the 12 diodes, you need to find a
>     shift-register with open-collector outputs and invert the 1's and
>     0's (that is, fill the shift-register with "1" bits and clock a
>     single "0" bit through it).
>
>     Such a shift-register DOES exist, but as I recall, it was very
>     hard to find.  I'll go through my notes and see if I can find a
>     part number (or if anyone else knows of a shift-register with
>     open-collector outputs, please post the part number to the
>     list).
>
>     -Andy

Hi,

Maybe I've missied something, but, is it not easier (no diodes and
standard shift-register) to use the shift-register to shift IN the
switch positions.

In a normal setup the BCD outputs of the switch present a logic 1 or 0.
Therefore, if they are connected to the 12 parallel shift register
inputs, the value of each input would be serially shifted in.

Peter.
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Peter Homann   email: peterh@adacel.com.au       Work : +61 3 9596-2991
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