Buses for business: Automation professionals have a plethora of tools in their toolboxes
July 22, 2010 | By Ian Verhappen
There must always be an economic incentive for development of a new product or protocol, and the same is true for fieldbuses. In this case, the drive is from different vertical segments or industries that use industrial automation and see the benefits of all digital communications.
Figure 1 shows a subset of the fieldbus options available in the market today and the approximate niches in which they fit. The "simpler" buses, which have fast update times and very small (typically on- or off-type) messages, are in the lower right while more complex buses with larger data packet sizes are in the upper left side of the figure.
The bottom horizontal access lists the type of sensors and controllers typically associated with each of the protocol in the diagram, while the vertical axis provides an indication of the type of I/O associated with the bus. The horizontal axis along the top describes the type of bus by colour and protocols below and, as we will see by examining a few of these protocols, each of these buses target a different type of communication and industry.
Starting with a simpler bus, AS-interface (http://as-interface.net/) connects simple sensors and actuators including the power supply over a two-leader bus. AS-Interface is a master/slave protocol and every AS-Interface slave is freely addressable and can get connected to the bus cable in any arbitrary place. This makes modular construction possible with no limits to the structure and hence any network topology can be used (e.g. bus, star, or tree topologies).
Cable and network range 100 metres but this is scaleable by repeater to up to 300 metres. A single AS-i message typically has a four-bit data load. The repetition of a single telegram requires only 150 µs and this time period is already taken into account in the specified cycle time of the network. Because AS-i is primarily an on/off protocol (though it can support analog signals) it is predominantly found in factory automation.
Devicenet and Controlnet are part of the Open Device Vendors Association (ODVA) and this protocol is typically used in factory automation as well to connect motion controllers and PLCs. Devicenet, which is based on the CAN (Controller Area Network – the same network used in automobiles) supports both branched and daisy chain networks. It uses CSMA/CA (Collision Sensing Multiple Access/Collision Avoidance) with an arbitration scheme to prevent secondary collisions if a collision is detected. Devicenet uses a unique five-wire (four conductors plus ground) cable to provide both signal and power. Depending on the data transfer rate (125 – 500 kbps), cable-type networks of up to 500 metres can be installed.
The two field level versions of Profibus (http://www.profibus.com/), DP (Decentralized Peripherals ) and PA (Process Automation), are each targeted to different industries though the two are closely linked because ALL Profibus PA messages are transferred through a gateway to the Profibus DP protocol. Up to 126 I/O devices can be connected to a PROFIBUS DP cable while Profibus PA uses the same physical layer as Foundation Fieldbus H1. Profibus DP uses four-wire RS-485 as the physical layer and, like Devicenet, the cable length depending on the bit rate used (bit rates range between 9.6 kbit/s to 12 Mbit/s) between two repeaters is from 100 to 1200 metres.
Lonworks (http://www.lonmark.org/), developed by Echelon Corporation, is one of the protocols in the BACnet standard for building automation. Building automation is where Lonworks is most commonly used (including the elevator you rode on the way to work today). The most common deployment of this protocol uses twisted pair signal wires that operate at 78 kbit/s using differential Manchester encoding. The Lonmark organization uses profiles (a similar concept is used by Profibus) to provide a basic set of generic functions (open and closed-loop sensors and actuators and a controller) from which a broad set of applications are implemented.
WorldFIP was one of the protocols on which Foundation Fieldbus was based and today has limited use, predominantly in France. Lastly, readers are familiar with Foundation Fieldbus, which is targeted to the process automation market – just like Profibus PA.
As you can see, just like a carpenter has more than a hammer in their toolbox, automation professionals have a range of tools and protocols as well. If you would like to see us cover some of these other buses in future columns, please email me and/or the editor (firstname.lastname@example.org) and we will work to schedule it into the editorial calendar.
Ian Verhappen, P.Eng. is an ISA Fellow, ISA Certified Automation Professional, and a recognized authority on Foundation Fieldbus and industrial communications technologies. Verhappen operates a global consultancy Industrial Automation Networks Inc. specializing in field level industrial communications, process analytics and heavy oil / oil sands automation. Feedback is always welcome via e-mail at email@example.com.
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