Getting a system's control devices to communicate effectively has gotten easier with the advent of local operating networks
Interoperability, open systems, and a standard protocol are concepts long sought by facilities managers and others in the electrical industry. People often ask why an engineer can't select the best sensing and control devices — and other products — from a range of manufacturers and have them all work together seamlessly. Those in the control industry are also concerned that while centralized control products based on point-to-point wiring and hierarchical logic systems (Fig. 1) have certain advantages, many users desire a flat system architecture (Fig. 2) in which every point does some control processing. Distributing the processing throughout the network can lower the overall installation and life cycle costs, increase reliability by minimizing single points of failure, and allow the network to serve a wide number of applications. Now, one of the leaders in promoting a distributed network that uses an open protocol is moving for wider acceptance.
LonWorks, developed by Echelon, Palo Alto, Calif., consists of both software (the open protocol) called LonTalk and hardware. The main hardware item is a Neuron microchip that includes three 8-bit inline processors, two of which execute the protocol. The third is used for the node's application. Recognized as an American National Standard (ANSI/EIA 709.1), the LonTalk protocol implements all seven layers of the International Standards Organization's Reference Model for Open Systems Interconnection (ISO OSI), which defines the structure for open communications protocols.
The product developers consider the technology a local operating network (LON), which allows all types of control devices, such as sensors and actuators, to communicate with one another through a common communications protocol. Communications transceivers are standardized, as are object models and programming/troubleshooting tools that make it easy to design and set up interoperable LonWorks-based devices.
Consider the nodes on a LON as “objects” that respond to various inputs and produce desired outputs. When the inputs and outputs of these objects are linked, the network can handle various applications. While a particular node may only serve a simple function, the interchange among various nodes allows the network to carry out complex tasks. Thus, factory production equipment, inventory systems, and distribution systems can be linked together with efficiency and flexibility. Similar functions can be carried out in a hospital, office, or residence.
Products that use the Neuron chip in a way that complies with the LonWorks interoperable standards carry an identification mark. The LonTalk protocol ensures interoperability using concepts called network variables and software definitions called standard network variable types (SNVTs, pronounced “snivets”).
In the past, the LonMark Interoperability Association (LIA) monitored the LonMark certification process and ensured interoperability of the more than 500 products from various manufacturers. However, this year LonMark International, a California corporation with 300-plus members, replaces LIA. The new group wants to support its members in their local markets and is developing a global affiliate network. The first localized LonMark groups should emerge in Asia and North America. Similar efforts are underway in Europe. The original association consisted mostly of system integrators and manufacturers, with building owners and facilities executives making up only 2% of the organization. This latter group is expected to grow extensively to improve the balance in the organization.
In addition to certifying individual devices, the LonMark group is also expanding its scope to cover entire building control systems that connect to the Internet. For example, a Web-based networked control system can dramatically reduce HVAC energy use when compared with the use of conventional controls. In particular, the broad acceptance and ever shrinking cost of Ethernet, TCP/IP, and XML communications is continuing in the building controls industry. XML software language, which is widely used in the IT community to optimize Internet data exchange, may offer a way for building engineers to manage operations and share data via Web services over the Internet or across a campus-wide network. For example, a building owner may use time-of-day pricing for the purchase of electrical energy and then define the loads that will operate in the building based on the fluctuating cost of power. Automated meter reading is another application.
This shift by Echelon from certifying only components to certifying systems makes it possible for an engineer to write an industry- or application-independent specification that clearly shows the benefits of an open system on a project of any size. The new LonMark “system definition” includes five categories: system behaviors, devices, connectivity, device interfaces, and network software and tools. Thus, some of the new products from other manufacturers may include network operating systems, network management software, human-machine interfaces, routers, programmable gateways, and Web servers.
Back to basics. As previously noted, the four major elements of LonWorks are the LonTalk protocol, the Neuron chips, the LonWorks transceivers, and network management and applications software. Let's take a closer look at each of these elements to ensure you understand their function in the overall system.
The LonTalk protocol supports these communications media:
- Power line (powered or unpowered)
- Radio frequency
- Coaxial cabling
- Fiber optics
The LonTalk protocol uses a proprietary collision prediction algorithm that permits a channel to carry its maximum capacity, rather than have its throughput degrade due to excess collisions, as happens with Ethernet. In addition, collision detection is optionally supported on certain media, including twisted-pair. This further enhances response time in cases where collisions do occur. At the fastest LonTalk data rate of 1.25 million bits/second, the LonTalk protocol supports more than 500 transactions per second. For applications that must limit the maximum delay incurred by nodes with high-priority messages, the LonTalk protocol offers an optional priority feature. The highest priority node is guaranteed access to the medium as soon as transmission of any message in progress is completed.
How does the Neuron chip function? Whenever a node program writes a new value into one of its output variables, the new value is propagated across the network to all nodes with input network variables connected to that output network variable.
LonWorks transceivers include the following:
- 78-kbps twisted-pair transceiver
This transceiver allows you to build networks with distances to 4,600 feet (worst case) in loop or backbone configurations. It provides transformer isolation between the node and network for high common-mode noise rejection.
- 1.25-Mbps twisted-pair transceiver
This transceiver provides higher communications rates for distances to 430 feet (worst case). It also provides transformer isolation between the node and network for high common-mode noise rejection.
- Power line transceivers
The power line can provide a good solution for network wiring in many applications. It eliminates the need to install additional wiring, significantly reducing installation costs. These transceivers communicate with either a proprietary spread spectrum or a narrow band technology that provides reliable communications for as many as 6,500 feet on a clear line. Electrical power transformers that impede the signal transmission can be bridged with a simple passive circuit.
- 78-kbps twisted-pair, free topology transceiver
This transceiver allows you to build networks of nearly any configuration, including bus, star, loop, and mixed configurations. It features low standby power and transformer isolation between the network and the node. Nodes with this transceiver can communicate as far as 1,600 feet with no repeater and up to 3,200 feet with one repeater.
- Link power twisted-pair transceiver
This transceiver provides the same communications capability as the free topology transceiver but adds distributed power capability to the network. Power is distributed over the network from a central power supply.
- Radio frequency transceiver
A variety of RF transceivers are available for wireless communications in many different environments. Licensed and nonlicensed versions are available in the 400-MHz to 470-MHz and 900-MHz bands.
The software program for designing, installing, operating, and maintaining a LonWorks network, the LonMaker Integration Tool, uses the Microsoft Visio graphic interface. Available in three editions, this program offers features for integrators and maintenance personnel and provides an easy way to evaluate the program before purchasing.
When using the LonMaker program to install a node, it's necessary to specify which network variables are to be connected between nodes. This can be accomplished by highlighting the output network variable on one node and the input network variable on the node or nodes to be connected. The important information in each node is presented graphically through a standard Windows interface. Only network variables of the same SNVT type can be bound together. In other words, a temperature type couldn't be bound to a pressure type.
SNVTs are inserted into the network during the design stage. A list of more than 100 SNVTs for nearly all physical measurement types including the type of variable, such as integer or floating point, are used. For example, a SNVT for continuous level is defined as SNVT_lev_contin.
Facilities managers' search for a way for all of their control devices — regardless of manufacturer — to work together may never completely come to an end, but now that the LonWorks technology can provide interoperability and efficient communications on an open protocol, they have something to fill the void.
Information in this article came from the following manufacturers: Echelon, Motorola Semiconductor, IEC Intelligent Technologies, and Douglas Lighting Controls.
Network Variables. Communications between nodes on a network takes place using the network variables that are defined in each node. The product developer defines the network variables during the creation of the application program as part of the application layer of the protocol. Multiple nodes share network variables. Some nodes may send a network variable while others may receive. By only allowing links between inputs and outputs of the same type, network variables enforce an object-oriented approach to product development. This greatly simplifies the process of developing and managing distributed systems.