>On the design of a PIC ( probably 16c65 or 16c74 ) security system,  we need to
> be able to identify one of the following situations;
>1> A unique closed switch out of 128 SW's ( normally open contacts)

I missed the original post of this question so I hope I don't quote it
out of context, but it seems to be gathering that much steam that perhaps
the required answer lies in the fact that it is for a 128 station
security system.                             ^^^^^^^^^^^^^^^^^^^^

If so, it may well be a large office, factory, wharehouse, or Airport
Terminal and will need some long line drivers that will rule out most of
the simple approaches that would be satisfactory for a simple keyboard type
matrix.

Don...

Don McKenzie donmck@labyrinth.net.au
DonTronics Tullamarine, Australia
http://www.labyrinth.net.au/~donmck

PIC Basic Compiler available now. PIC Programmers starting at $15US
PicoSaurus, the 40 pin ETI PIC Basic with 8K EEPROM Free Windows Dev Sys