Addressable Field Devices

7.12.1 Active And Supporting Field Devices

Active field devices are defined as those that can be uniquely identified by the control unit to determine both their presence on the circuit, and their operational status, and which may be commanded to operate or change their operating parameters independently of other field devices sharing a common circuit. Simply put, active field devices can be heat detectors, smoke detectors, manual pull stations, contact monitoring devices (for sprinkler switches), or output ancillary relays, for which the present status can be identified individually at the control unit. Active field devices use serial communication techniques.

Many of these devices (typically up to about 200) can be wired together on a single two-wire circuit (called a Data Communications Link or DCL), reducing field wiring while still providing individual identification (and/or control) of the device in alarm or trouble. Instead of annunciation

messages such as “2nd floor alarm”, an addressable device may initiate a corresponding message such as “smoke detector, room 202 activated”.

The benefits of such finite identification are obvious, and for this there is

"Sampled"

Voltage Levels Annunciator wiring from

Fire Alarm Control Unit8 circuits in

LAMP

Common

To Remote Annunciator (8 zones + common)

SERIAL ANNUNCIATOR WIRING VIA ONE TWISTED, SHIELDED PAIR Note: Extra wiring may be required for power and switch features

in both of the above configurations Predetermined "Sample" intervals PARALLEL ANNUNCIATOR WIRING VIA 9 WIRES

"Start" pulse

an expected cost tradeoff. (It must be remembered that the Building Code contains requirements for ‘summary’ zoning by building areas.)

Thinking back to conventional systems, where each initiating circuit incorporated built-in lamps or LEDs with typed labels to identify both alarm and trouble status per circuit, this would rapidly become

cumbersome in an addressable system having 300 to 400 devices. For this reason, most systems with addressable devices utilize an

alphanumeric liquid crystal display (LCD) that presents a

preprogrammed ‘device/location/status’ message corresponding to each addressable device, upon its activation. In the case of multiple

simultaneous device activations, additional location messages are stored in an internal buffer through which the operator may scroll, one by one, using a built-in keypad. This LCD feature further reduces the size of the control equipment by minimizing the physical annunciation space

required. Note though, that many jurisdictions still require a full LED style annunciation in the Control Unit (in addition to the LCD) for Fire Service use. Also be aware that ULC-S527 requires that, where

sequential displays are incapable of displaying eight input events simultaneously, full LED annunciation is required.

A ‘supporting field device’ is an active field device that monitors other field devices (non-addressable) on a separate supervised circuit and reports the status of that separate circuit to the control unit.

7.12.2 Addressable Device Polling

Each addressable device has a predetermined ‘address’ assigned to it.

Addressable devices are “polled” sequentially by the control unit. One by one, the control unit sends the device addresses out over the Data

Communications Link (DCL). The devices automatically recognize their own address, and upon recognition, they respond to the control unit via the same DCL wiring. In this manner, all devices on the circuit are interrogated in sequence. After all devices have been checked, the cycle begins again, in a continuous polling process.

7.12.3 Automatic And Manual Addressing

Two methods of assigning device addresses have been used - automatic and manual.

Automatic device address assignment (self-configuring) occurs upon system power-up, and does not require the setting of any switches for each device.

Manual address assignment requires the setting of individual address switches on each device. Device addresses do not have to be physically sequential on the Data Communications Link.

7.12.4 Addressable And Analog

As well as there being two types of address assignment, there are two types of addressable device operations: standard and analog. Standard addressables were the first available, and provided one of two responses to the control unit upon being polled – ‘normal’ or ‘alarm’ (no response indicates a trouble condition). Analog addressables can provide a full range of device status values. An example would be the actual smoke obscuration levels of a smoke detector being transmitted back to the control unit in a digital format. This is then interpreted, compared to the

‘alarm parameters’ established by ULC, and processed and acted upon accordingly. These smoke ‘detectors’ have thus become smoke ‘sensors’, along with their thermal heat ‘sensor’ counterparts, and they are

dependent on the system’s microprocessor for support.

Analog addressable devices offer some additional features to assist building owners and maintenance personnel. Among these are the ability for the control unit to automatically compensate for buildup of dust within smoke sensors, adjusting the alarm threshold accordingly, while still maintaining it within the ULC defined window. At the same time, the control unit can initiate a message on the LCD to indicate that smoke sensor cleaning is required, and specifying the individual

sensor(s) needing maintenance.

7.12.5 Data Communication Links

The actual wiring for fire alarm systems serial communication is termed a ‘Data Communications Link’ (DCL). A DCL is the data wiring channel between the CPU and:

• transponders,

• annunciators,

• active field devices, and

• supporting field devices.

A DCL may be configured in one of three ways:

• DCLB wiring, which is similar to a Class B configuration, in that only one end of the circuit is connected to the main control unit,

• DCLA wiring, which is similar to a Class A configuration, connected in a true loop fashion, or

• DCLC wiring, which provides performance superior to DCLB or DCLA.

SYSTEM STYLE System Abnormal Condition

DCLB DCLA DCLC

Single Open T A A

Single Ground (a) A A A

Wire to Wire Short - T A

Wire to Wire Short & Open (Same Link) - T T Wire to Wire Short & Ground (Same Link) - T T

Open and Ground (Same Link) - T A

Loss of Carrier - T T

LEGEND

a = Except as permitted in CAN/ULC-S524-06, Clause 4.2.2.

T = Trouble Indication at the Control Unit

A = Trouble Indication at the Control Unit and Alarm Receipt Capability During Abnormal Operation

- = Not Applicable

7.12.6 Fault-Isolator Devices

Although each addressable device can be thought of as a single ‘zone’, the question must be raised as to how many devices could one afford to

‘lose’ upon failure of a Data Communication Link. For this reason, ULC-S524 specifies that the maximum portion of a system affected by a failure shall be one floor area, to a maximum of 2000 m². It can easily be seen that this is reflective of the original code requirements for the

maximum floor area of any zone. Systems must therefore employ fault isolators between floors, and between each 2000 m² floor area.

Fault isolators are devices which, when wired in a DCLA or DCLC configuration, will literally disconnect a faulty section of the loop, allowing the remainder to operate normally (a trouble condition would also be initiated at the control unit). Fault isolation must also be used on DCLC network wiring connecting panels in a distributed Fire Alarm System.

The installation standard also limits the number of devices that can be served by a DCL, depending on its A, B, or C configuration. Larger systems may fall into the category of a ‘Large Scale System’ and

additional requirements and restrictions will apply such as requirements for ‘Stand Alone’ operation.

7.12.7 Advantages and Disadvantages Of Microprocessor-Based Fire Alarm Systems

Some of the advantages of microprocessor-based fire alarm systems would be:

• greater flexibility for special operational sequences such as timers, counters, logic functions, etc.,

• reduced electronics, which translates to lower cost and smaller physical size,

• reduced power requirements,

• reduced field wiring due to serial data exchange,

• greater ability to provide device-specific information, helping to pinpoint alarms and system trouble conditions, and

• provision for features that are otherwise impossible (event log, password protection for access to certain control functions, etc.).

There are, however, several considerations (disadvantages) to be aware of with microprocessor-based systems:

• they may be more easily damaged by static electricity, power surges, lightning, etc.,