August 2011

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The Unheralded Network Topology

RS-485 may be the “forgotten” or “taken for granted” “boring” network, but it is the mainstay of many building and industrial automation systems.

“This simple process of focusing on things that are normally taken for granted is a powerful source of creativity...”
 Edward de Bono, Writer, leading authority in field of creative thinking. (b.1933)

Jim Sinopoli
Jim Sinopoli PE, RCDD, LEED AP
Managing Principal,
Smart Buildings LLC

Contributing Editor


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Ethernet networks and information technology infrastructure have steadily penetrated nearly all building systems and networks. Decades ago Ethernet was primarily used to connect desktop PCs to application servers for databases, printers and the Internet.  Over time traditional telephones went the way of Ethernet via Voice-Over-Internet-Protocol (VoIP). Next there were a series of analog building devices and systems taking on some Ethernet and IT technologies: video surveillance cameras, access control card readers, IPTV, Wi-Fi controllers, BAS network controllers and a slew of other specialized devices.  All of the databases, workstations and servers of today’s modern building systems, such as lighting or BMS, are all evolved from IT infrastructure.  Despite its dominance, Ethernet networks are not the sole network topology in building systems, they are probably not the dominant network topology and in some cases not even the right or appropriate network topology for particular building applications.  Unquestionably Ethernet is ubiquitous, popular and visible - but in the guts of most buildings are RS-485 networks. RS-485 may be the “forgotten” or “taken for granted” “boring” network, but it is the mainstay of many building and industrial automation systems.

When you take a look RS-485 applications it’s not so much where do we use RS-485, but rather, where don’t we use RS-485. Much of the building’s electrical equipment will typically be managed and monitored via a RS-485 network, including electrical distribution, switchgear, back-up generators, UPSs, ATSs, vehicle charging stations, etc. It’s probably an RS-485 network that’s connecting and monitoring power or flow meters and providing data on electrical, water and natural gas consumption, and onsite power generation from wind or solar sources. So if you have an energy management application, the energy data is probably being gathered by an RS 485 network. You’ll also see RS-485 used for BAS field devices and sensors, for monitoring and/or managing boilers, CRAC units, elevators, video surveillance cameras, specialized systems using PLCs, lighting control, shading systems, audio systems, and home automation systems. RS-485 is used on factory floors and even for “vehicle buses”, which are essentially networks to monitor and manage such vehicles as aircraft. Many of us simply don’t realize or appreciate the extensive utilization of RS-485.

Ethernet vs. RS-485
Ethernet vs RS-485Ethernet and RS-485 are quite different. Ethernet topology as we know it today is a star or “hub and spoke” configuration; interestingly, early Ethernet used transceivers on a coaxial bus, somewhat like the RS-485 topology. Each Ethernet outlet is typically “home run” back to the IT network switch in the nearest telecom equipment room. RS-485 topology is different; it can be point-to-point or multi-drop with multiple nodes, or a bus topology which is communicating bi-directionally over a single twisted pair. It has a resistor at each end of the bus to reduce electrical noise interference.  It also typically operates in a master-slave configuration, where the master initiates all communication to the nodes.

While star and ring topologies are not consistent with the basic RS-485 standards there are ways to create such topologies for RS-485 by using a RS-485 hub or repeater. For example a device such as a surveillance camera, is connected to the hub in essence creating a small RS-485 network or span, with the hub being a drop point on the larger RS-485 network.

contemporary One of the advantages of a RS-485 network is distance. Where Ethernet is limited to 100 meters between the device connection and the network port RS-485 can handle a bus of 1,200 meters. Data rates on Ethernet networks are much higher than RS-485 networks. Data rates on RS-485 can be as high as 10 Mbps but will vary with distance; the longer the network, the slower the data rate. While there is a significant different between the two networks types regarding data rates, it turns out not to be a major issue because the typical devices being monitored or managed by a RS-485 network simply don’t generate a lot of data.

Ethernet itself is a physical and data standard and uses communication protocols such as TCP (transmission communication protocol) and IP (Internet protocol). RS-485 standards simply address electrical parameters and don’t bother with standardizing communication protocols, speed, format, physical connectors or type of cable. This means even if you have similar devices from different manufacturers on a RS-485 network the compliance with electrical standards of RS-485 does not guarantee interoperability of the devices. However, the RS-485 marketplace has addressed this matter through the adoption of popular communications such as Modbus in buildings and Profibus in industrial automation.

It’s likely that a RS-485 network is interfaced or connected at some point to an Ethernet network, passing data off to a server or higher level network. In these cases an RS-485 to Ethernet gateway is used; some may be simply protocol converters and others may require the “Master Node” to be on the Ethernet side.

WiringOne of the main benefits of RS-485 is wiring. It uses simple bus wiring and the potential length of the cable is long, the result being minimal wiring and costs. With shorter network buses 24 gauge twisted pair can be used; longer buses may require 18 gauge shielded cable. One of the more interesting aspects and a commonality with Ethernet is that RS-485 can use the typical IT cable. This is the Category 6 cable specified in the latest EIA/TIA/ANSI 568 specification. The use of this type of cable has several benefits: it’s widely available, there are plenty of contractors available to install it and it typically costs less than the specialty RS-485 cabling. In addition, the use of standard cable infrastructure for RS-485 networks in new construction allows overall cabling infrastructure to be consolidated with the results being less cable contractors to coordinate, lower unit cost for the cable and less required cable containment (conduit and cable tray). Another option to take advantage of is to use shielded Category 6 cable. It’s a modest increase in costs but shielding will help protect the RS-485 network from any sort of electromagnetic interference from an outside source.

IT companies and professionals entering the building system market would do well to become more informed about RS-485 networks in order to better assess the extent and opportunities and limitations in the penetration of Ethernet infrastructure in building systems. These two network topologies will co-exist for many years to come.

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