"AT&T Home Security System - System Overview"

 By Ron Stultz

21 April 2007

I get many emails from folks who bought a house with an AT&T Home Security System installed but they did not inherit any sort of user or maintenance material with the system. If one downloads the AT&T Home Security System User and Installation Manual available on this web site, most of your questions will be answered, but in the following text, I provide my own thumbnail sketch of system operation.

The AT&T Home Security System consists of a central controller (CC), door or window intrusion detectors, smoke detectors, glass break detectors, motion detectors, keypads and heat detectors. I am not aware that AT&T ever sold either natural gas detectors or carbon monoxide detectors.

The AT&T Home Security System can be wired, meaning that the CC is connected to various system components via cables or wires, but I would guess that most installations are wireless, meaning that various system detectors communicate with the CC via a radio frequency (RF) signal.

The CC is the brains of the system with it being: a very simple computer with both volatile (looses it contents when power is removed) and non-volatile (retains its contents when power is removed) memory, a radio (RF) receiver, a telephone line auto dialer (can dial a stored telephone number automatically) and an audible alarm (actually controls a speaker).

The most basis element of the AT&T Home Security System is what is known as the universal transmitter. This small white, plastic box with a raised button on the front is basically a RF transmitter. Most often a universal transmitter is installed near windows or doors and has a magnetic switch wired to it and operates as a intrusion detector. Universal transmitters are also used inside or with glass break detectors, heat detectors and motion detectors.

When an AT&T system was installed, the front cover of the CC was removed and a house code was hard wired or coded into the system via a bank of small switches known as a dipswitch. The house code dipswitch is in the upper right hand corner of the CC. Defining a specific house code allows for the situation where there are several AT&T Home Security Systems in the same neighborhood. By having a unique house code, and every device in the installation having the very same house code hard wired into it, there is no chance for the detectors of one house alarming the system of another house. Also when the system was installed, the installer used a special programming tool or box to set the telephone number the system auto dials at a system event, such as an intrusion, fire detector, glass break, etc. This telephone number is stored in non-volatile memory such that no matter what happens to system power, the telephone number is never lost. This is a good thing and a bad thing. The good thing is that you never have to reenter the telephone number due to a power loss but the bad thing is that a programming unit is required to set or change the stored telephone number and these programming units are very, very rare and not even available to most current security system monitoring companies.

As I said, the CC is, upon other things, a RF receiver. So when the system is armed and a universal transmitter door or window intrusion detector is tripped by the movement of the window or door, the universal transmitter transmits a signal that the CC detects and based on information the universal transmitter sends out, the CC using its internal telephone auto dialer, calls the monitoring company phone number and relays on information about your home's address and the nature of the security event.

This event detection, RF signal transmit to the CC sequence, is true for glass break, smoke, heat and motion detectors.

In the case of a wireless keypad, it also can and does serve sometimes, as a door intrusion detector but also serves to arm and disarm the system. In this case, again, a RF signal is emitted by the device that is received by the CC upon which the CC takes the appropriate action.

Now the designers of the AT&T Home Security System were quite robust in their design in that, they built each universal transmitter with a pressure type switch aligned to the front cover, such that, even if the system is not armed, if the front cover of any universal transmitter is removed, the system will sound an alarm and the monitoring company will be automatically called. The only way around this alarm when changing batteries, is to place the whole system in the "Test\Demo" mode.

If each system device has the very same house code hard coded or wired into it via a series of small switches or a dipswitches, each system device also has a unique ID code hardwired into it. Think about it. If there was no device ID transmitted to the CC, you could never tell which detector had tripped or alarmed. Additionally, the system designers thought of the situation where the battery in a system device gets so low in voltage that the resulting RF signal is too weak to be received by the CC. This, of course, would not be good, as a detector could sense a problem and try to report it to the CC but the CC could not hear it. To cover this situation, the system designers built into every system device a timer that requires the device to emit a "check-in" signal to the CC every 8 hours or so. Because, when the system was installed or reset, the CC built a table of system devices by ID number, if a device does not check-in within an eight hour period from its last check-in, you get an audible beep from the CC and the device number of the offending system device can be displayed in the CC's LCD status panel.

Finally, as defined above, the CC is a very simple computer and controls all sorts of system parameters such as the delay time from armed house entry until a disarm code must be entered, etc. Unfortunately, changing all these system parameters is not available to the home owner as we do not have the required programming tool or box. I have been told that there is a way to program the CC using a wireless keypad but I have never seen any instructions on how to do this.