Apparatus for improving the reliability of an alarm circuit

An adapter module for converting a Class B alarm system to Class A operation. The module includes a first pair of terminals connected to corresponding terminals of a Class B control panel, and second and third pairs of terminals to which Class A alarm loops are connected, one terminal of each of the second and third pairs of terminals being directly electrically connected to a separate terminal of the first pair of terminals.

The present invention relates to alarm circuits and more particularly to a 
system for converting a class B alarm circuit to more reliable class A 
operation. 
BACKGROUND OF THE INVENTION 
Various types of systems are called upon to signal an alarm condition when 
a specified event occurs. Examples of such systems are fire alarms and 
burglar alarms. Because of the reliance placed on such systems, steps must 
be taken to insure the reliability of the alarm circuit. One step which is 
commonly taken is the use of separate supervisory and alarm modes. In 
supervisory mode the alarm circuit is monitored to insure that it is in 
working order, i.e. has no broken wires or other open circuits. When an 
alarm condition occurs, the circuit shifts from supervisory to alarm mode, 
causing the alarm to be signaled. 
Such alarm circuits may be divided into two general groups. These are known 
as "class A" circuits and "class B" circuits. A class A circuit has 
certain fail-safe features not present in a class B circuit. If an open 
circuit condition occurs in a class B circuit the alarm will not function 
until the open circuit is repaired. In a class A circuit, however, an open 
circuit, while detectable by the supervisory mode of the system, will not 
prevent the alarm from sounding should an alarm condition occur. Class A 
circuits, therefore, have significant advantages over class B circuits in 
providing increased safety, by insuring that alarms will be sounded if an 
alarm condition arises before an open circuit can be repaired. 
Alarm systems commonly are designed with a central panel which acts as a 
controller and various remote sensors and alarms. The design of the 
central panel will determine whether a class A or a class B alarm circuit 
may be connected thereto. In some situations, however, it may be desirable 
to upgrade a system which has been previously installed with class B alarm 
circuits to class A alarm circuits. Therefore, a system for converting 
class B alarm circuits to class A is desirable. 
SUMMARY OF THE INVENTION 
In the present invention an adapter circuit is provided which allows a 
central control panel designed to accept a class B alarm circuit to be 
upgraded to support a class A alarm circuit.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
FIG. 1 illustrates a typical class B alarm circuit. The portion of the 
alarm circuit enclosed by box 10 would typically be provided on a central 
control panel, while the remainder of the circuit would be external to 
such a panel. 
The portion of the circuit on panel 10 includes terminals 11 and 12 which 
connect to the external alarm circuitry via conductors 13 and 14 thereof, 
respectively. The output 15 and 16 have capacity 1, 2 and 3, 4 connected 
thereto. In supervisory mode circuit nodes 15 and 16 are electrically 
biased such that an electric current flows from node 15 to node 16. In so 
doing, it will flow through load resistor 17, but diode 18 will prevent 
current flow through circuit branch 19 thus preventing the activation of 
annunciator 20. In supervisory mode, if there is an open circuit 
condition, such as that which is indicated at point 21 on the circuit, the 
current will be unable to flow and the presence of an open circuit will be 
indicated on the control panel. 
When an alarm condition occurs, nodes 15 and 16 will be electrically biased 
so that an electrical current will flow from node 16 to node 15. When this 
occurs, diode 18 will be forward biased allowing current to flow through 
circuit branch 19 thus activating annunciator 20. Annunciator 20 may be a 
bell, a horn, a light, or any other signaling device which will indicate 
the existence of an alarm condition. As may be seen, if an open circuit 
condition occurs, such as at point 21, no complete circuit will exist, 
preventing the signaling of an alarm condition. 
Turning now to FIG. 2, the portions within box 10, i.e. the portions on the 
central control panel, match those of FIG. 1. The portion within box 22 is 
a module which may be connected to terminals 11 and 12 in order to convert 
the circuit to class A operation. In a preferred embodiment the circuitry 
in box 22 would be provided on a printed circuit board which could be 
attached to the central control panel. The circuitry in box 22 includes 
terminals 23 and 24 which connect to terminals 11 and 12, respectively. 
The circuitry in module 22 further includes terminals 25, 26, 27 and 28 
for connection to alarm loops. The circuitry also includes diodes 31, 32. 
and 33 and resistor 17'. Diodes 31, 32 and 33 are not required for proper 
operation of the circuit, but are preferred. Particularly, the reasons for 
preferring the presence of diodes 32 are described in my co-pending, Ser. 
No. 880,625, and filed July 1, 1986. In operation loop 29 is electrically 
connected to terminals 25 and 26 while loop 30 is electrically connected 
to terminals 27 and 28. Alarm branches 19' and 19" run between the two 
loops include diode diode 18' and annunciator 20' and 18" and annunciator 
20", respectively. 
As in the previous example, when the circuit of FIG. 2 is in supervisory 
mode, nodes 15 and 16 are electrically biased so that an electric current 
flows from node 15 to node 16. This current may be used to detect open 
circuits at locations in the alarm loops. For example, open circuits at 
location 34, 35 or 36 would be detected. When an alarm condition occurs, 
the biasing on nodes 15 and 16 is changed so that current will flow from 
node 16 to node 15. As before, this will cause the annunciators to be 
activated. Unlike the circuit of circuit 1, however, an open circuit which 
has not been repaired at locations such as 34, 35, 36 or at similar 
locations on loop 30 will not prevent activation of the annunciators, 
therefore, producing a fail-safe system.