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Conventional alarm facility

Discovery devices should be polled at regular

intervals to maintain communication with the

control panel and therefore enable correct

monitoring of the protected premises.

However, if the polling mechanism fails, for

example because of a processor failure in the

panel, the internal operation of the Discovery

device will be unaffected as long as a DC

supply is maintained. After 108± 4 seconds

without protocol, the device will

automatically switch to its conventional alarm

mode. In this mode it will operate as if it were

a conventional detector (or manual call point)

and will impose an alarm signal on the loop if

an alarm condition is detected by the internal

processing.

The alarm signal takes the form of periodic

current pulses, which can be detected by

simple hardware in the control panel. A full

description of the signal can be found in the

Discovery Protocol PIN Sheet PP2027.

Flashing LED

All Discovery detectors have two integral LED

indicators, which can be illuminated at any

time by the control panel to indicate devices

in alarm. When activated, the LEDs will draw

an extra 3mA from the loop. In addition to

this mode of operation it is possible to enable

a flashing LED mode by writing to one of the

memory locations. In this mode the LEDs will

flash each time the device is polled. The

device does not draw extra current in this

mode since the LED current is part of the

normal current pulse reply from the device.

This facility is available on all Discovery

detectors and the manual call point.

Discovery detectors and call points are

factory set to non-flashing mode.

Remote test feature

This feature, available on all Discovery

detectors and the call point, is enabled from

the CIE by changing the state of a forward

command bit. On receipt of the command

the detector is forced by electrical means into

an alarm condition. After a delay of

approximately 10 seconds due to signal

processing, an analogue value of between 54

and 120 – nominally 85 – is returned,

provided that the detector is functioning

correctly. This value is sustained until the

forward command bit is changed back to its

original state, after which a period of 40

seconds is required for the detector to return

to its normal analogue value.

The manual call point is different in that the

receipt of the command bit will cause the call

point to generate the interrupt sequence,

followed by a sustained value of 64. The call

point resets when the forward command bit is

changed back to its original state. For further

details of this function, see Discovery Protocol

PIN Sheet, PP2027.

Rejection of transient signals

All Discovery detector algorithms are

designed to give low sensitivity to very rapid

changes in the sensor output, since these are

unlikely to be caused by real fire conditions.

This is achieved by digital low-pass filtering

of the sensor values which optimises the

rejection of false alarm sources while

maintaining the response to fire.

The filter parameters depend on the mode

selected and for some modes the filtering is

minimal. The filtering has no significant effect

on the response to fires but does affect the

way in which detectors respond to transients

and to step changes of smoke or heat. This is

seen in the “minimum time to alarm” given in

individual detector specifications. These times

represent the time taken by the detector to

reach the alarm condition when responding

to a large step change in input.

Servicing Note

The “minimum time to alarm” referred to

above is important when detectors are

tested in situ, for example using aerosol

test gas. A delay in response may be

apparent.

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