On Wed, 8 May 1996, Don McKenzie wrote:

> >On the design of a PIC ( probably 16c65 or 16c74 ) 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...

We're missing some design details, so we make them up.

If the system is going in a public space, then the biggest cost is in wiring
the switches, since the wires have to be hidden.  In that environment a 2
wire system is probably best.

If the wires are short enough, they can use Dallas Semiconductor serial
number chips.  The chips use a 1 wire plus ground connection and there can be
many of them on a single connection.  Connect the chips to normally closed
switches and have the PIC poll them all and report the chips that don't
answer.

Possible problems:

  1. I think the serial numbers are 48 bits which means a table of 128
     chips would take 768 bytes.  Memory is scarce in PIC's.
  2. There are length restrictions on the wiring, plus limits on total
     number of chips per wire pair.  It might take 4 strands for 128
     chips.
  3. A lightning strike would be bad.

--
Paul Haas paulh@Hamjudo.com