>From your earlier notes it would seem possible that the button wires could
become connected to the phone line. i.e. 50V or so with all the charge
stored in the line capacitance (39-65pF/metre or thereabouts) plus the
current from the line card.
Definitely put some additional protection on the inputs!

Richard

> -----Original Message-----
> From: Dennis Plunkett [mailto:dennis@RDD.NECA.NEC.COM.AU]
> Sent: Wednesday, June 02, 1999 10:24 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [OT] RS485/RS422
>
>
> At 00:50 1/06/99 -0400, you wrote:
> >>> 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.
> >
>
>
> So to put it simply, you have 150 feet of cable or more that
> is terminated
> (Persuming) only at one end by the receiver?
> Humm, sonds like an antenna to me.
> RS232 only has limited ESD, EMI protection on the inputs as
> the clamping is
> to the power source, and assumes that the power source is
> able to cope with
> the current dump delivered by the strearing diodes durning an
> overvoltage
> condition.
> You may wish to look at some form of external clamp
> protection TRISILS on
> the inputs and accross the power sources.
>
> Dennis
>