Innovative communication protocols used in addressable analog fire alarm systems.
Timely and reliable detection of a fire is one of the most important tasks in the field of fire safety. This task is solved by a fire alarm system. There are two main classes of such systems: traditional (non-addressable) and addressable. In non-addressable systems, each peripheral device, whether a fire alarm or a siren, is connected to the control panel (CP) individually, via its own pair of wires. In addressable systems, several devices can be connected to each other by one communication line. The most advanced systems are addressable-analog. Their main difference from simply addressable systems is that peripheral devices transmit not only already generated notifications, but also measurement results, for example, the level of smoke in the room. This allows using the measurement results of several detectors at once to make a decision on issuing an alarm notification.
In order for several devices to exchange information over one communication line without interfering with each other, a certain set of rules is used — a communication protocol. It should be noted that in most cases the communication line is also used to power addressable devices, and the length of the line can be hundreds of meters. At the same time, there are quite high requirements for the noise immunity of devices and communication lines of fire alarm systems. All this imposes significant restrictions on information capabilities, and, as a rule, such protocols have low bandwidth. Currently, communication protocols for fire alarm systems are not standardized, so each equipment manufacturer is forced to independently develop its own solution.
The preferred topology of addressable communication lines is a ring, as this increases the reliability of the connection. The system remains fully operational when the communication line is broken; in this case, the ring simply turns into two radial loops, and the addressable devices continue to work with the control panel. Special devices, short-circuit isolators, are used to protect against short circuits. When the line is short-circuited, the isolators closest to the fault location open the circuit and cut off the shorted section of the ring. In some systems, isolators are built directly into the addressable devices. This, of course, slightly increases the cost of the products, but the system is fully protected from short circuits in the communication line.
Modern communication protocols are usually fully digital, i.e. all data is transmitted in binary form. Earlier solutions may contain analog inserts in the transmitted information packet, where the quantitative value of the data is specified by the pulse width. In general, digital protocols are more reliable in terms of ensuring the integrity of the transmitted information, since digital data is easier to protect, for example, with a checksum of the packet data.
For several devices to operate in one communication line, each of them must have a unique network address that uniquely identifies the device in the network. Usually, an addressable device has a unique factory serial number, but in most cases, a shorter number assigned in the system is used for addressing. This is done to reduce the bit depth of the network address, since using a long factory identifier can significantly affect traffic in the line and increase communication overhead. However, the use of assigned addresses carries a potential problem of the appearance of so-called duplicates, i.e. two or more devices on the line with the same network address. In this case, the system may behave unpredictably. The worst case scenario is when, in standby mode, the duplicates respond to a request in the same way and synchronously, and the control panel receives the correct response. If the status of one of the duplicates changes, they will give different responses, different packets will overlap, and the control panel will most likely simply receive nothing. This can cause a very unpleasant situation at a real facility: everything is fine in standby mode, but the «Fire» notification from the detector is not transmitted to the control panel. Therefore, it is extremely important how the communication protocol identifies and copes with this collision. To resolve this problem normally, the protocol must have an automatic mechanism for identifying duplicates in the communication line. The system itself must have a way to show the service personnel devices with the same address. The process of addressing devices is most often assigned to a person, which is usually the cause of errors. However, there are protocols with the ability to automatically address devices. In this case, addresses are specified in the order of the devices in the communication line. In addition to the address, the device can also specify its zone affiliation. A zone can include several addressable devices and is used to logically combine and organize the joint operation of equipment within the zone.
Each network device can contain several elementary logical subdevices. For example, a multi-criteria fire alarm that can simultaneously analyze several factors of fire occurrence: temperature, smoke, CO concentration in the air. Both the generalized state of the alarm and the values of each measured parameter can be transmitted to the control panel. In relatively old communication protocols, such devices usually occupied several addresses on the communication line at once, which significantly reduced the real information capacity of the system and complicated the setup process. After all, as a rule, in this case, addresses overlap and errors appear on the line. For example, a device has address 10 and, being a two-component device, occupies 2 addresses at once. Therefore, the next free address on the line will be 12. If the next device is assigned address 11, the addresses will overlap. Modern protocols use subaddressing technology, i.e. logical subdevices can be organized inside the device, each with its own internal address. Thus, having one external address of the device and using additional internal addressing of subdevices, it is possible to access each element of the device. The ability to obtain diverse information from addressable devices allows implementing a wide range of service functions. For example, a fire alarm can transmit the smoke chamber dustiness value, sensitivity level, etc. to the control panel. At the operation stage, this allows for more efficient organization of system maintenance.
Communication protocols are mostly bidirectional, i.e. it is possible to transmit information not only from the addressable device to the control panel, but also in the opposite direction. The channel for transmission to addressable devices is used to control and configure the equipment. For example, setting the sensitivity threshold for issuing alarm notifications. Moreover, this can be done dynamically, changing the sensitivity of fire detectors based on the situation at the facility: during the day, a lower response threshold is set, and at night, the sensitivity can be increased.
The most important parameters of the communication protocol are the maximum number of addressable devices in the line and the time of delivery of the alarm notification from the addressable device to the control panel. Usually, from 100 to 250 devices are supported with the alarm delivery time of no more than a few seconds. Most protocols are based on the polling mechanism of communication with addressable devices: the control panel sequentially polls all devices on the line, each time receiving the device status. Considering that the number of devices on the line can be more than a hundred, as well as the low exchange rate, the polling period of all devices can be a significant time. Accordingly, the delivery time of the notification in the communication line can be equal to the polling period in the limit. To increase the speed of notification delivery, interruption and group request mechanisms are used. Let us consider one of the possible options for implementing an interruption. In each polling cycle, the control panel provides a special time window during which the device with an urgent message can report it. Having learned about the presence of an extraordinary message, the control panel must find out the address of this device, and in the general case there can be several such devices. By performing one or more address resolution requests, the PPC can gain access to the desired device. After this, the normal device polling procedure is resumed.
An interesting feature of some communication protocols is the ability to directly exchange data between devices, i.e. a kind of sensor network is built. For example, a fire notification from a detector is transmitted directly to a siren, bypassing the control panel. This allows for decentralized logic to be implemented in the system, which increases flexibility and reliability in general. In Russia, however, it is still impossible to do without a control panel, since the decision to issue an alarm notification must be made at the device level.
The market share of addressable systems, including domestic ones, is constantly increasing. This is facilitated by the general development of digital technologies and the reduction in the cost of addressable systems, which are initially noticeably more expensive than traditional ones. And the fact that people are striving to improve the quality of fire protection. To install a system not for a fire inspector, but for themselves, to protect their property and life, after all. Insurance companies should also make their contribution to the formation of the market for professional systems by setting preferential rates for facilities equipped with high-quality equipment.