>> I am using RS232 drivers/receivers to poll a 10X10 matrix of buttons.
>
>  Can you clarify?  Does *every* remote (button sender) have a unique
>line to the base?  What do you mean by "Matrix"?  Are you sharing RS-232
>lines (paralleling)?


Keep in mind I am not actually transferring 'data' per say..  The matrix is
so I can get 100 different combinations from only 20 I/O lines.  One side of
the first 10 buttons are all connected to output 1.  The other side of the
10 buttons each go to 1 of 10 inputs. (i.e. the first button goes to input
1, the second to input 2...) So now if I were to put output 1 high and I
polled the 10 inputs, I would be able to tell if any of the buttons were
pressed because the input would be high.  Now set up 9 more outputs (each
with 10 buttons) and connect the other side of the buttons to the original
10 inputs.  Now I can go 1 output at a time and poll the 10 inputs.  This
would check all 100 buttons.  For every output there is a RS232 driver and
for every input there is a RS232 receiver.  The MC1488s and MC1489 have 4
driver and 4 receivers, respectively, per chip.

>> each button is installed in an apartment up to 150 feet away.  They
>> are all connected with 24 gauge twisted pair telco cable.
>
>  Does this cable also carry the power, or is it locally supplied by,
>e.g., a wall-wart?  Wall-warts have the advantage of isolating the
>ground lines.


The whole system is in a central location.  The only things in the apartment
is the actual button and the 2 wires going to it.

>> I put 1 output bit high and check the 10 inputs one at a time.  Then
>> I move on to the next output bit. I  scan all 100 switches about 3
>> times a second.
>
>  Does this imply that one transmitter at "base" sends to ten remotes in
>parallel and each of these comes in to a separate receiver?


Hmm, yes and no.  As explained above, one driver does drive 10 lines, but
they are not terminated unless someone is pressing a button, then the signal
would be sent back to a receiver.  Both transmitter and receiver are at the
'base'.

>> I cant tell exactly how good it is because I'm constantly dealing with
>> people ripping the buttons off the wall, grounding them out or
>> connecting phone lines to them.
>
>  ROTFL.  Sounds par for the course.  Are the offenders users or
>tradesmen?

Both.  Its actually not an apartment building, its a state run homeless
shelter in the Bronx.  The buttons are mounted on the phones as the phone
isn't supposed to be moved.  But the temporary tenants sometimes insist that
they hook up their own phone and take it upon themselves to rewire it. Or
they get mad and just knock the thing clear off the wall...Since it was such
a custom install, when the phone company comes they don't have a clue whats
going on and have screwed things up in the past.

The whole reason for the system was because the phone based intercom system
we were installing did not include a way for the tenants to contact the
guard.  That was one of the requirements of the contract.

Regards.

-----------------------------------------
                 Mike Montalvo
          G-FORCE Motorsports
http://www.g-forcemotorsports.com
                516-794-0858