Caught in the DACT
In the past many signal transmission technologies have been devised in order to send alarm signals to a monitoring station with the intent of summoning a local fire brigade. Each one reflected the newest and greatest transmission technologies of its time.
The technicians of the day always believed that the means of transmission they were using represented the best signal transport available, and it certainly was in its day.
Past examples include direct connect, McCullough Loop, Derived Channel (active multiplex) and digital signaling using a Digital Alarm Communicator Transmitter (DACT) in conjunction with a Digital Alarm Communicator Receiver (DACR). Of all of these transmission methods, the DACT received the widest acceptance, and it’s still the preferred method of signal transmission among fire technicians today.
As technology advances, so does speed and reliability. Today, not only do we send alarm, supervisory and trouble signals, but we send enhanced data.
The Need for Reliable Signaling
The fire alarm industry has long prided itself on providing reliable signaling through the use of digital communication. Traditionally, the exchange of information using this technology takes place using a DACT in conjunction with a DACR. Today, however, there are combinations of signal transport technologies designed to rush data from the alarm panel to the central or supervising station, and most often this still involves the use of a DACT and a DACR.
Before fire code was an issue, a single phone line was used to transport data from an alarm panel to either a central or supervising station. At some point the industry recognized that bad things can happen when something goes wrong with this single signal path. A telephone cable within the facility itself can suffer damage by way of a raging fire, rendering it useless. Or the outside telco (telephone company) connection can also go sour due to extenuating circumstances beyond the control of either the end user or fire/burglar alarm company.
The conclusion that manufacturers and field engineers came to was that two phone lines are better than one. And it was only a hop, skip and a jump to the added stipulation that two telco carriers were needed to move these signals from premises to a monitoring facility, thus creating the need for a primary and secondary DACR phone number in panel programming.
This led the industry to incorporate these and additional requirements into the body of what we now know as NFPA 72, National Fire Alarm Code (NFAC), published by the National Fire Protection Association (NFPA).
DACT Mechanics and Fire Code
Chapter 8 of NFPA 72, 2007 Edition, goes to great lengths to describe how a DACT is supposed to operate. Section 184.108.40.206, Digital Alarm Communicator Systems, provides the nuts and bolts concerning digital communication in a fire alarm environment. Novice fire alarm technicians would do well to obtain a copy of this document and to read it thoroughly. You need to understand what the code requires in order to pass any final inspection you conduct.
For example, in Section 220.127.116.11.1.5, DACT Transmission Means, an outline is provided with regard to how a DACT interfaces with the outside world, including the use of two receiver numbers – one for primary and the other for secondary. The need to transmit test signals every 24 hours is also contained in this section.
In Section 18.104.22.168.1.1(B), NFAC also warns that we must use a loop-start phone line as opposed to a ground-start where it involves our primary telephone line (telco 1). In past years ground-start lines were commonplace in PBX phone systems as well as public pay telephones. Although it’s unlikely you’ll run across one of these at this point, it is possible.
In the old days to use a ground-start circuit we’d just add a ground-start module. Most of the time this consisted of a simple relay connected to a trigger voltage on the motherboard that activated when the digital dialer went off-hook. The normally open (NO) relay output was used to bring the high side of the phone line in contact with earth ground. This caused the telco’s central office (CO) to go off-hook, creating a traditional dial tone, thus switching line voltage.
By contrast, loop-start lines are designed to go off hook with a change in impedance/resistance. By code, the only time you can violate this rule is when using a ground-start phone line as a secondary signal path (telco 2).
Code is also explicit in how much time it should take to send a typical signal to the central station. Section 22.214.171.124.1.3(B) calls for a maximum of 90 seconds from the time the DACT goes off-hook to successfully transmit data. Once the DACT goes off-hook it dials the number, verifies the integrity of the connection with the DACR, sends the data, receives an acknowledgement of receipt, and then it goes back on-hook.
Most of these requirements are automatically met by manufacturers, but you need to make sure the equipment you use is listed for commercial fire in all cases but residential.
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