January 17, 2007 by Ian Verhappen
Fieldbus systems like Foundation Fieldbus H1 and Profibus PA communicate by altering the voltage on the line between Â± 0.75 and one volt (Â± 9 mA), and by measuring the transition across the “zero point” or reference voltage around which they are varying. The minimum voltage for H1 and PA is nine volts, and because the voltage must alternate at a rate of 31.25 kbps, a power conditioner must be used.
The traditional bulk power supply (BPS) for a control system not only converts voltage from normally redundant AC mains to 24 VDC, it also contains circuitry to maintain a linear or constant voltage output over a varied range of demands as dictated by the current load/power rating of the unit. The key word here is linear, because a traditional BPS will work to counteract the voltage variation on the network, which is exactly the opposite of what we are trying to accomplish.
Power conditioners, at their simplest, are nothing more than inductive loads of five millihenry and a 50-ohm resistor on one side of the power supply.
To help end-users in their selection of power conditioners, the Fieldbus Foundation (FF) developed a standard in 2004. This document, FF-831 Fieldbus Power Supply Test Specification, describes more rigorous testing procedures than dictated in the IEC 61158 standard on which it and the physical layer are based. Power conditioners passing this test are able to use the FF “check mark” to indicate their compliance with the FF-831 standard. This is important if you are a Profibus PA user because PA and H1 are based on the same physical layer, and you can take advantage of this testing when purchasing power conditioners.
Fieldbus power conditioners do more than simply introduce an inductance to the circuit as a means to enable the transmission of the fieldbus signal; they often integrate other features into the unit such as energy management or limitations to meet the needs of Intrinsic Safety, FISCO and FNICO, as well as the ability to incorporate a terminator for the network.
Terminators are required on either end of the fieldbus network to match the system impedance and prevent potential “echoes” due to signal reflections or other forms of degradation. Using only one terminator or more than two terminators on a segment will result in signal degradation.
Of course, because a fieldbus network is a parallel wiring system with the terminators within typically 100 metres of either end, it is possible to install the power conditioner anywhere within the network. However, because it is typically mounted in the interface room, where the host system or distributed control system reside with the UPS and reliable power at one end of the network, the terminator is activated in the power conditioner via the use of a dip switch.
Lastly, the complexity of industrial networks is continuing to increase. Therefore, the need to monitor and maintain the system is now considered in design, installation and operation. Fortunately, more tools are becoming available to provide this capability, and in a future column I will describe some of the critical parameters that need to be monitored to insure continued system reliability.
Ian Verhappen is an ISA Fellow, ISA certified automation professional, adjunct professor at Tri-State University and director of industrial networks at MTL Instruments, a global firm specializing in fieldbus and industrial networking technologies. E-mail him at Ian.Verhappen@ICE-Pros.com, or visit his website at www.ICE-Pros.com.
- ODVA announced that the first edition of the CompoNet Specification has been approved by its technical review board and is scheduled to be published by ODVA in the next publication cycle of the CIP Networks Library and associated network specifications. Complementary to the entire family of CIP Networks – DeviceNet, ControlNet and EtherNet/IP – CompoNet is designed to meet the requirements of applications using large numbers of simple sensors and actuators by providing high-speed communications with configuration tools, and combining this with efficient construction, simple setup and high availability.
- CLPA (CC-Link Partners Association) recently released its CC-Link safety specification, which describes technological details of this open field network conforming to the international safety standards IEC61508 SIL3 and EN954-1 Category 4.
- The FDT Group has formed a new technical project group called “Device Information Model for OPC UA.” The scope of the project is the specification of a device information model for OPC UA. This will allow OPC UA client applications to access device data and its description through the respective device type manager in a common way.
- ARC has prepared a white paper on ProfiSAFE for network safety. The document can be downloaded at www.profibus.com/celummdb/doc/RPA_GERMANY/News/arc_white_paper_profisafe.pdf.