In the March 2002 issue, I reported on the activity within the Telecommunications Industry Association (TIA) to develop a cabling system in support of building-automation systems (BAS). At the time of writing that article, the document was out for industry ballot. Based on the vote, the draft document was approved and is to be published as TIA/EIA-862 Building Automation Cabling Standard.
In the March issue, I described the basic requirements and scope of this document and introduced two new industry terms—horizontal connection point (HCP) and coverage area cables. Additionally, I promised to discuss the alternate cabling topologies allowed for coverage area cables in addition to the traditional star topology.
As a refresher, the BAS standard specifies the use of the star topology in the backbone and from the horizontal cross-connect (HC) to the horizontal connection point (HCP) or BAS outlet; but for the coverage-area cabling extending from the HCP or BAS outlet, additional cabling topologies are allowed, depending upon the application requirements. In addition to the star topology, bus and ring topologies are allowed in the coverage area.
Only those of us who have been involved with TIA TR 42 committees can really appreciate the significant departure this represents for the standards body. There have been a number of sacred rules in this committee, none more so than the star topology. However, the committee recognizes the fact that BAS is different from voice, data and video and that these alternate topologies have a place and fulfill a requirement with building automation.
Coverage-area cables are those cables that connect either the BAS outlet or the HCP to the BAS device. These coverage-area cables shall meet the same requirements as the horizontal cable that they connect to and shall comply with TIA/EIA-568-B.2 or B.3, which means they shall be Category 3, Category 5e or fiber.
Remember that the HCP can be implemented as a cross-connection, unlike the consolidation point in TIA/EIA-568-B.1. The BAS standard recognizes the use of functionality of the HCP in essences to transform a star topology into a bridge connection and a chain. In the star topology, one coverage-area cable is connected to the end of each horizontal cabling link (i.e., one-to-one relationship.)
In the bridge connection, up to 10 coverage-area cables can be connected to the same horizontal cable. Bridged connections shall not be cascaded (existing at more than one location on the same channel). Chain connections are implemented when an input and return connection are provided on each of the BAS devices. The chain is provided by connecting the return connection of one BAS device to the input of another BAS device at the HCP. No more than 10 branches should be connected in a chain configuration.
In addition to these three topologies implemented at the HCP, coverage-area cabling can be configured to support multipoint bus and ring topologies. Unlike the previous examples, in which the coverage-area cables were physically installed in star but connected to form a bridge and chain connection, these two topologies allow the coverage-area cables to be physically installed as a bus or ring. Additionally, the multipoint bus can also be implemented as part of a bridge connection. The multipoint bus requires that the bus be terminated with an end-of-line device (i.e., resistor). The multipoint ring is typically used with fault-tolerant circuits and provides physical redundancy.
So while the TIA committee and resulting standard was unwilling to consider alternate cable types for BAS, it was willing to recognize alternate cabling topologies based on the system requirements of BAS such as fire alarm, security and access control and energy management systems. Only time will tell if the TIA/EIA-862 Building Automation Cabling Standard becomes a market-accepted standard and assists the industry in developing a structured cabling approach to building automation. EC
BEAM is director of systems marketing at AMP NETCONNECT Systems. He can be reached at 336.727.5784 or firstname.lastname@example.org.