Thank you all for your answers.
I think the architecture proposed is too complicated (also to achieve)
because the majority of comercial sensors do not have the ability to
receive and send a confirmation signal. Some of them have tamper
switches that trips when the cover is removed.
I would like to do a voltage supervised loop. Any links or schematics
are appreciated.

Lucian

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Jack Smith
Sent: 19 octombrie 2003 17:24
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE]:Supervised Alarm Loop

I have searched all the net about a schematic for 3-state supervised
line, but I haven't found anything.
So you say I need 4 wires for a sensor ? 2 for VCC and GND and 2 for the
alarm state ? I was thinking I could do it without an end-of-line
resistor, but with a resistor to ground on the control panel.
What are the most common sensors : normal-open or normal-closed ? In the
case of a normal-closed sensor, when it trips what changes in the loop ?
Is a voltage equal to VCC inserted into the line or the wires are
interrupted ?
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In the burglar alarm business, historically (and alarm systems pre-date
Bell's invention of telephony), alarm systems used a current loop.

Sensors were normally closed, so current flow indicated continuity in
both
the wiring and the sensors themselves.

A sensor that was tripped would interrupt current flow with a specific
make/break sequence (think of a telephone rotary dial type mechanism).

The central station monitoring the alarm would then read the pulses
(originally with paper tape recorders) and determine which specific
alarm
sensor had been tripped.

These general concepts have carried over into modern alarm systems as
well.
A current loop system has much to commend it, particularly with long
runs in
high noise environments. The current loop may also provide power to
remote
sensors as well as signaling.

You might also give some thought as to how one might detect an attempted
compromise of a monitored alarm system such as bypassing a set of
contacts
with a jumper wire. Perhaps smart sensors that could be periodically
polled
by a central monitoring unit to report on their status. And perhaps the
status report is not always the same, such as a pseudo-random number
being
sent as a challenge, read by a smart sensor and a different number being
returned, based on some mathematical computation by the sensor.

The remainder is left as an exercise for the interested student.

Jack

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