>Hi All
>My PIC based alarm system has been blown up with a High Voltage
>zapper; those handheld units for shocking people/animals...
>Apparently this is a new craze among burgulars. Thy kill home alarms,
>car alarms and cct video with these zappers.
>
>Short of using screen cable for every sensor, is there other devices
>or goodies that can protect alarm systems from this assault????
>
>Regards
>Cassie

Yikes! And I thought I'd heard everything!  What do those zappers put out?
Several kilovolts, I'm sure.

By "screen cable" do you mean shielded cable? I have doubts how effective
plain shielded cable would be against this type of attack.

Hmmm...I'm not sure what could be done about this except to use either
fiber optics for any cable runs or optoisolation on the controller board,
though 5300 KV seems to be tops for cheap optoisolators. If the zappers
produce more than this, you're sunk.

Oh, what else...clamping diodes & current limit resistors on every I/O.
The sensors would all have to be hardened in some way, too.

Defensive software techniques would be helpful, too. Zilog has a really
great appnote on their website: "Using Software Techniques to Maximize Z8
System Software Noise Immunity."

        http://www.zilog.com/frames/z8otp/pdf/z8_app.pdf

It's for their Z8, but the techniques are applicable to most any MCU. These
guys are _really_ paranoid (for example, they suggest scanning your hex
files before programming your MCU to make the WDT reset opcode doesn't
appear anywhere but one place, the place where you call it).

Intel appnote AP-125 "Designing Microcontroller Systems for Electrically
Noisy Environments" is also a good reference Sorry, I don't have an URL for
this one; maybe somebody else out there does...

-BN

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