Columnbus
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 ( This e-mail address is being protected from spambots. You need JavaScript enabled to view it ) 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 This e-mail address is being protected from spambots. You need JavaScript enabled to view it .
One of the reasons that many people hold back on implementing an industrial wireless system is security concerns. Obviously, you don't want to install a wireless system if it can be a "back door" into your control system. Luckily, the engineers who are developing the standards and products intended for industrial uses understand these concerns. Consequently, a number of security features have been included in the industrial wireless standards to alleviate them. I've mentioned in past columns that the OSI model ensures each layer of the communications protocol is partially independent of all others. Once again, with industrial wireless, this good planning comes into play since the majority of the industrial Ethernet protocols all use 802.15.4 radios for the lower layers of the protocol but then differentiate themselves at the higher 'application and user layers' with unique features. How industrial systems differ The 802.15.4 radio replaces the twisted pair wire associated with HART. And just as each of the fieldbus protocols have a migration path to an Ethernet-enabled version (802.3), 802.15.4 could also be replaced with 6LoWPAN or low power version of 802.11in the future with minimal impact to the protocol itself. Bandwidth sharing at 2.4 GHz beyond ISA100.11a, WiHART (HART 7.1), and ZigBee stipulates that an IEEE 802.15.4 radio be used.  In addition, while the 802.15.4 radio could be operated at a number of different frequencies, only the 2450 (+/-) MHz frequency range is available licence-free worldwide.  Because of this, it's the range most commonly used in the licence free Industry Scientific Medical (ISM) bands. Unfortunately, many other items such as cellular phones, wireless home phones, and various other items including remote control cars and toys use this frequency as well. Added protection Message encryption is one technique used to maintain data integrity and prevent deliberate or inadvertent interception of the data between two nodes on a network. The process automation wireless protocols include industry standard 128-bit AES encryption, unique encryption keys for each message, and an access point that provides rotating encryption keys when new devices attempt or request permission to join the network (see sidebar for other industrial wireless standard features). Techniques such as Direct Sequence Spread Spectrum (DSSS) technology, also known as coding diversity, and adjustable transmission power, or power diversity, also help WirelessHART provide reliable communication even in the midst of other wireless networks. WirelessHART also uses time-synchronized communication (time diversity) as a means to minimize the potential for collisions through the use of "blacking out" channels being used by other devices and networks. All WirelessHART device-to-device communication is done in a pre-scheduled time window, which enables collision-free messaging. In addition, each message has a defined priority to ensure appropriate Quality of Service (QoS) delivery. Fixed time slots also enable the network manager to create and manage the network for any application without user intervention. Keeping it safe The ISA-100.11a standard committee is in the process of revising their document to incorporate a 'use case' that was not adequately addressed in the first revision. The expectation is that a revised document will be approved in 2011 and resubmitted to ANSI for approval at that time. After approval as an ANSI standard it will then be possible to submit the document to IEC for consideration as an international standard. WirelessHART is presently in the IEC approval process as an international standard. Despite the best efforts of the specification developers, all the above capabilities are only as good as what you choose to implement. Including such basic items as managing your signal and antenna gain (receiving strength) at the perimeter of your property, changing default passwords and using defensive indepth security practices will help you protect your wireless system from intruders. When it comes to security, you're only as good as your weakest link. In many cases, the old saying rings true: We have seen the enemy and he is us. Features incorporated into industrial wireless standards • Data integrity and device authentication are two of the three pillars of cybersecurity. The third being authority, or does the device have sufficient security privileges to make the change being requested. • Channel hopping makes it more difficult for a device that is not part of the network to know at which frequency the next transmission will take place. • Multiple levels of security keys for access by different individuals with different responsibilities. This reinforces the concept of authority, the third pillar of security mentioned above. • Adjustable transmit power levels allow the user to manage the signal 'spillage' beyond the boundary of the plant environment. If the radio signals do not go beyond the edge of a facility it will become much more difficult for someone to either "steal" information or capture enough data packets to be able to decipher the data package format so that it can be compromised. • Security servers, similar to RADIUS servers in the office environment, allows the wireless network manager to record every attempt to join the network. By keeping track of all the attempts, the details of failed access attempts can provide an indication of how vigorously someone is attempting to compromise your network. 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 This e-mail address is being protected from spambots. You need JavaScript enabled to view it .
The FDI (field device integration) Project will have a big impact on the future look and feel of digital field sensors, especially after the recent announcement that host suppliers ABB, Emerson, Endress+Hauser, Honeywell, Invensys, Siemens and Yokogawa have joined the FDT Group, Fieldbus Foundation, HART Communications Foundation, OPC Foundation and PROFIBUS in pushing not only the development of this new standard but also incorporating it in their products. So what is FDI? In simple terms, when complete, it will replace all EDDL (IEC 61804-3)-based languages: HART, Foundation Fieldbus and PROFIBUS-PA. Obviously, this is a significant portion of the process automation market. While EDDL is a common text-based description of a device, the text description is normally converted to a "binary DD" through a tokenizer before being shipped with the device. Unfortunately, the format of the binary DD is different for each process Fieldbus even though they originate from the common EDDL language. The above manufacturing company members of FDI have made it a high priority to harmonize the binary DD through secondary standards and tools so the result will be a single binary format file regardless of the protocol of the device. The EDDL file for each protocol will be processed through a tokenizer, much like it is done today; this also ensures backward compatibility, as we would not want to have to replace all our existing devices as a result of this change. Because each protocol is not exactly the same, but rather closer to 90-percent similar, it will be necessary to create an FDI Developer environment for each of the three EDDL-based protocols to assist them in defining how to "map" the various parameters of each protocol to the appropriate FDI parameters. The resulting binary file from the tokenizer is then passed to a "packager," where it will be converted to an FDI file. Note that, at their discretion, the device manufacturer will be able to define a user-interface plug-in that is integrated into the FDI file by the "packager" to create the single common file. Foundation Fieldbus device manufacturers have this discretion today as well by creating "extended" function blocks that contain information beyond what has been fully defined by the Fieldbus Foundation. What is important to end users will be the interoperability of these devices, and that will be insured through the appropriately coloured green "test tool" box, which will provide the necessary check mark from the appropriate organization that the devices are not only compliant with FDI but also backward compatible. This is important when a new device needs to be added to an existing network and everything will have to continue to work together seamlessly. Lastly, when the device is connected and communicating on the network, the process needs to be "reversed," with the DCS/host converting the FDI information into a format useable by the internal system databases. This is not different than is done today, where each system needs to "interpret" the information from the field to the appropriate database register within the host. Note that the user-interface plug-in, which will be used to provide improved access to the maintenance, diagnostic and related parameters in the field devices, will use the knowledge gained from the use of FDT technology and combine that with the open interoperable communications capabilities of OPC UA to provide a platform independent solution to the rich data set contained in a modern digital field device. The working groups hope to have the standards developed by the third quarter of 2010, which means products should start being available in 2011. Do not let this change your plans for using any of these protocols because backward compatibly will be a key to acceptance and a must for the developers/testers. The good news and incentive for this work with all the device and host manufacturers is that they will now only have to develop and support one, rather than three, device description standard, which reduces development and support costs. For end users, this means reduced system complexity and better access to richer data features for all devices on the network. _ Ian Verhappen, P.Eng., is an ISA Fellow, ISA Certified Automation Professional and a recognized authority on Foundation Fieldbus and industrial communications technologies. Feedback is always welcome at This e-mail address is being protected from spambots. You need JavaScript enabled to view it .
How will things get done 10 years from now?” This question has a two-pronged response: how will technology evolve, and how will it be maintained? It’s time to gaze into the crystal ball and forecast the future on both fronts. Before we start, I need to paraphrase Bill Gates: “We tend to over predict what will be accomplished in the next two years and under predict what we will have done in 10 years.” With that in mind, how will industrial automation technology evolve over the next 10 years, and what will be cutting edge in 2020? We will continue to have multiple protocols to serve the needs of niche industries. Ethernet (wired and wireless) will be used more widely than today and PoE (Power over Ethernet) will also be common so devices can be connected via a single cable. Wireless based on 802.15.4 will continue to gain traction with more devices connected, broader coverage and higher speed, but will still predominantly be used for monitoring-only applications. Cyber security will be part of system design and will consider more than external attacks, becoming more robust and user friendly. Operations will realize the benefits of life-cycle asset management and invest heavily in this area. The IEC standard on 3D displays will be adopted and enable the development of the first 3D immersion control rooms, process simulators and engineering design software. The above could be way off since the timelines are different for industrial settings than in the office or other short life-cycle environments. It is not too often someone will replace all their “perfectly adequate” field devices for new digital infrastructure, so a typical life cycle in industrial control is closer to 25 years versus fewer than five years for office computer systems. The above is a forecast of what may be possible, which does not equate to being widely used, just like we experienced with Fieldbus deployment more than five years after the technology was available. Wireless will gain traction around 2013 to 2015; by then, the press will be promoting the “next thing,” which will likely start being delivered around 2020. The second, more important factor to consider is how all this technology will be designed, installed and maintained — by people. As we know, the first half of the coming decade will see the baby boomers retiring in large numbers, and the result could be a worker shortage. Fortunately, technology will provide part of the solution, and to some extent, the recession will help as well since many will either need to or want to continue to work because they either need to or simply “want to stay busy.” Most of my friends enjoy what they do and, therefore, semi-retire and consult two to four days per week on projects that interest them. With fewer people around, and experience not as available as it may have been in the past, how will companies use technology to get the knowledge where it is needed when it is needed? n Remote support: These semi-retirees can be based anywhere. Technology makes it easy to send the information to the expert, or allow the expert remote access to the system under investigation rather than have the expert travel for days to get to the facility with the problem. The economic benefit for the customer is that the expert can now be “cost” and time shared across multiple clients. The expert also has the potential to make more money as they can help multiple customers in one day rather than just one when they needed to physically be there. n Smarter devices: These are able to not only report they have a defect but also the likely cause and what needs to be done to repair it. The fear is that people will rely on this information too much and forget how to do it by the time something happens. Therefore, just like operators have simulators to practice for abnormal situations, maintenance people will have similar tools to keep their skills fresh and sharp. n Virtual environment: When a technician needs to go to the field, they will have the option of a heads-up display projected on their safety glasses of repair procedure and associated manuals to walk them through the repair. n Software intelligence: Asset management systems and control systems will be tightly integrated, and the result will be maintenance optimization routines similar to what is done to optimize production across more than one unit operation today. This software will look for trends and patterns across multiple sensors and networks that indicate a potential problem for perhaps a device or piece of equipment not monitored directly long before it will affect operations so that it can be planned for the next convenient plant outage/service window. The technology to accomplish the above can be done today, though what is missing is the economic incentive and, to some extent, standards to define interfaces between the various systems so that each installation is not customized. In summary, the future will likely see better use of available experts who will be able to access the complete plant infrastructure through digital communications with fewer local staff to manage the day-to-day running of the facility. Ian Verhappen, P.Eng., is an ISA Fellow, ISA Certified Automation Professional and a recognized authority on Foundation Fieldbus and industrial communications technologies. Feedback is always welcome via e-mail at This e-mail address is being protected from spambots. You need JavaScript enabled to view it .
I’ve recently returned from the ARC Forum, which took place Feb. 8 to 11 in Orlando, Fla., where about 500 industry pundits gathered to discuss the future of our industry and what trends are likely to have the biggest impact on the automation industry. This was my first time at the event, and though the information exchanged was interesting, I did not find the majority of the presentations to be forward looking — there were exceptions, and they made the trip worthwhile, but most of the material consisted of how today’s technology was being used to solve typical plant challenges such as energy management, cyber security, lifecycle management, wireless and DCS migration, coincidentally all areas in which ARC conducts industry research reports. One of the benefits of this coincidence is that each session as started by a summary of ARC’s research by their analysts on that particular topic. In some cases, with four tracks running in parallel, it would have been nice to be in two places at once. The press announcements were on Monday afternoon (Feb. 8) and early Tuesday morning (Feb. 9) before the full conference and sessions started. The common theme at this year’s event for all the press releases was ‘Energy Management’ and ‘Open Standards as a basis for Enterprise Integration’ mindful of the associated oversight and responsibility. • Bentley Systems announced a pair of acquisitions to enable them to move more into the enterprise integration space • Yokogawa promoted their entry into the Service Solutions business (no surprise as all their competitors are here and it is the largest revenue growth area for integrated automation equipment suppliers) • Invensys released version 2.0 of their Infusion Enterprise Control System, a very interesting flexible object based architecture where for all intents and purposes anything can connect to anything and the system manages the potential associated exponential growth in connections. One thing that may be driving the need for all this connectivity with the enterprise is a recent SEC ruling that companies will now need to report ‘energy opportunities’ as part of their annual filing. The conference started with a couple interesting presentations from U.S. Homeland Security and Proctor & Gamble. Interesting titbit from the P&G talk: “There is an inverse relationship between cost per use (i.e. how many we make per day) and the complexity of the machine required to make the product,” and Pampers machines were used as the example. The opening session was followed by a disappointing panel discussion — a low-impact way to start an event like this since the most vocal participants on the panel used the venue as a means to promote their company. I was not alone in leaving early. Mike Anderson of DOW, during his presentation, shared that managing unplanned events typically provides a two-time return on investment. There were several interesting items pulled from the in plant mobility session with the presentations by Boeing and Chevron. Steve Venema shared his work on Boeing’s ‘Virtual Enclave’ and how they connect a number of pieces of equipment on the same Layer 2 network. The key components of what makes this work have been incorporated into IETF RFC’s 5201 through 5207 as well as the Open HIP organisation. Chevron, who has been using roving interfaces for five-plus years now, has identified that it’s saving $3- to $5 million per year from their investment, predominantly in pump vibration data analysis. The two key ‘take-aways’ from Chevron’s Raj Patel and Eric Rearwin was: 1)    “greater than 50 percent of refinery assets are not instrumented,’ and 2)    “any data collected must be actionable; if you are not going to analyse what you collect, do not collect it.’ The most exciting presentations of the conference were saved for the end of the day Wednesday (Feb 10). Chevron’s Kevyn Renner’s presentation on RAVE (Real Asset Virtualization Environment) was captivating as he shared how Web 2.0 technologies can and are being used to bring people together virtually to analyse and solve problems by using the best people in the company to meet in the virtual world to discuss and solve problems. This was followed by a Dennis Inverso of Dupont sharing the methods that they use to save millions of dollars per year with their ‘Value Accelerator’ program. There are great tools here that are likely applicable to projects of any size. The ARC Forum also often invites other groups to co-host their meeting with them to encourage the cross-fertilization of ideas between different components of the automation industry. One such group resulted in Thursday morning’s (Feb. 11) Performance Based Outsourcing Program where University of Tennessee faculty member Kate Vitasek shared the results of their study on outsourcing (also the launch of her new book the week before) on a method to obtain win-win outcomes in outsourcing. The basic ‘rules’ for vested outsourcing can be found at www.vestedoutsourcing.com/consulting/tools — and with all the outsourcing being done today, the results of this research are worth investigating. Ian Verhappen, P.Eng., is an ISA Fellow, ISA Certified Automation Professional and a recognized authority on Foundation Fieldbus and industrial communications technologies. Feedback is always welcome via e-mail at This e-mail address is being protected from spambots. You need JavaScript enabled to view it .
One of the key differentiators of Foundation Fieldbus is that with its PID Function Block, it supports control in the field and, as a result, single loop integrity. This means that with Foundation Fieldbus, it is possible to maintain control at the last set point without a host system.Implementing control in the field enables certain applications to be more efficient than with conventional instruments or control in the host. But, like all things, this also means some items need to be considered during the design process. Let’s start with those enablers first.
As we all know, if we do not have a reliable control system with availability as close as possible to 100 per cent, we quickly lose faith in the system and circumvent this with "jumpers," loops in manual and the like that defeat the purpose. The same is true for a fieldbus system. Heck, even "finicky" analyzers have a minimum acceptable availability of 95 per cent.A key part of any control system is the infrastructure that carries the signals from the field devices to the controllers, because without signals there, will not be data on which to control nor a means to manipulate the final control elements (valves, VFD, VSD, etc.) to adjust the process operating conditions. It therefore amazes me that for the sake of a few dollars, the same people who ask for "redundant everything" to maintain high reliability do not take basic precautions during the design, specification and installation of their fieldbus systems to prevent a single fault from adversely affecting the entire network.
One of the challenges that Foundation Fieldbus has faced to date is the integration of motors and drives as end devices. This has been a significant shortcoming, since variable speed and variable frequency drives are being used more and more often in projects because, in many cases, they are more energy efficient than control valves and offer equal or better flow control. If the user chooses a variable drive, he then must choose some other protocol such as DeviceNet or Profibus DP, resulting in a system for which the user must support multiple protocols. This is not necessarily a bad thing and we are all aware that we must always use the best tool for the task at hand – within reason.
There is lots of talk and coverage of industrial wireless technology in the trade press. Once again there are multiple camps at work; WirelessHART, OneWireless, the two industry standards camps and, of course, the ISA100 standard that is also under development, which both camps say they will fully support once it gets adopted. One good thing about the ISA100 standard is that the group developing it is planning to support the ability for it to effectively "tunnel" other standard protocols through or over the ISA100 protocol. This tunneling means that once ISA100 is available, each of the other fieldbus protocols should be able to operate with little or no change to the user layer of the protocol itself. That means when you implement Fieldbus, for example, over ISA100, all the functions and capabilities will remain the same as if you were running over wire. So other than these "standards," what are the other considerations associated with installing a wireless system?
There has been much talk over the years about the mounting of Fieldbus power conditioners and the need for redundancy in these components of the system.The dilemma engineers face is the need to meet the area classification for the devices in the field while still providing not only sufficient power to have the maximum number of devices on a single segment but also to have the longest possible total cable length while still meeting the requirement of a minimum of 9-volts at each device.The now-traditional way of managing a reliable bulk power supply is to mount the power supply and power conditioner in the control room environment where conditions are well controlled and the uninterruptible power supply (UPS) is close at hand.For intrinsically safe installations, there are two options available to end users; the IS (intrinsically safe)/FISCO (Fieldbus intrinsically safe concept)/FNICO (Fieldbus non-incendive Concept) completely live-workable solution, or the high energy trunk solution that uses Fieldbus Barriers in the field junction box to reduce the trunk voltage to acceptable intrinsic safe levels on each of the spurs. However, because the Fieldbus Barrier has both IS equivalent and non-IS energy sources in the same enclosure, you must take special precautions during installation and maintenance. The most important of these is isolating/separating the two types of cables.Until recently, each of these Fieldbus barriers supported four spurs and you could install up to four Fieldbus barriers on a single Fieldbus segment. The reason I say recently is because there are now eight spur versions of the Fieldbus barrier on the market.Despite the fact the Fieldbus specifications allow a maximum of 32 devices per segment, part of the reason for the four barrier limitation on the high energy trunk solution is that several host systems support a maximum of 64 devices per H1 interface card. Since each card has four ports, this works out to an average of 16 devices per port.Depending on the gas group, FISCO and FNICO are able to provide the sufficient current to power 16 devices on a single segment. FISCO and FNICO power supplies work with the much simpler field segment blocks that are similar to traditional terminal blocks in their installation on the field enclosure.However, almost all FISCO and FNICO power conditioners also serve as repeaters, so one possible way to overcome any length restriction is to install another FISCO/FNICO power conditioner at the appropriate distance along the home run cable to boost the signal back to its original strength. (The Fieldbus specification permits the use of a maximum of four repeaters on a single segment and an associated maximum cable length of 9,600 metres.)If, as is normally the case, the bulk power supply is installed in the control room, and bearing in mind that the maximum voltage level for a FISCO or FNICO power conditioner is restricted by the requirements imposed by remaining below the safety factor and ignition curve limitations, the result of the lower voltage may be a restriction on the trunk or home run cable length to the range of 500-600 metres. This distance is normally not a problem in most installations, as the interface room is within this distance from the field devices.The challenges, therefore, become obtaining a reliable bulk power supply in the field so that if required, the power conditioner/repeater will operate with the same level of reliability as the balance of the system and having a non-intrinsically safe energy source in a field enclosure. This second challenge is the same one the Fieldbus barrier solution faces.One option is to have a field-based 24 VDC supply. Another is to run a two-pair cable to the field junction box; one pair with the H1 signal and the other with a 24 VDC supply. Of course, if you’re not using the 24 VDC supply (which you can now connect to the interface room UPS) for anything other than power, you can, if needed, leave the control room at a much higher voltage to supply 24 VDC at the field end of the cable.Fieldbus is called so for a reason. It is a bus-based network designed for use in the field. What is required to make it work is that we as engineers, designers and maintenance workers must continually look for creative ways to use field enclosures for an "inside the box" solution.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 This e-mail address is being protected from spambots. You need JavaScript enabled to view it , or visit his website at www.ICE-Pros.com.
Cable is a part of the control system we often take for granted. Nevertheless, it is one of the most critical components of any system. Not only is it normally the medium by which we transmit the signals and information we use for control, as the Physical Layer of the Open System Interconnect (OSI) model, it is also the one on which all the other layers build. Just a like it is impossible to build a plant without pipes, a facility without a Physical Layer cannot exist. Therefore, cable (or its equivalent for those fiber and wireless enthusiasts in the audience) is truly the component that binds all our control systems together. The Fieldbus Foundation has recently released a specification describing the requirements for cable to be used to connect the various components in a system. What is most interesting about the specification is what is not tested. The parameters not tested include: signal propagation velocity, wire insulation colours (though they do mention as an option that it should be brown for positive and blue for negative), cable jacket colours and characteristics such as temperature pull and cable wire size/diameter. Of interest to all of us here in the northern climes is that the specification calls for a temperature range of -30 to +90 degrees Celsius, not great for Northern Alberta, where the specification is normally for -50 degrees Celsius. All of you need to be aware of that as you specify your cable. Besides specifying the key cable properties themselves, the specification also standardizes how connectors are to be mated to the cable for M12 and 7/8"-16 UN-2A Thd connectors: Other key parameters covered in the specification include:• Characteristic impedance (Zo) – 100 +/- 20 Ohms• Attenuation – 3db/km at 39 kHz• Wire – both 18 AWG for trunks and 22 AWG for spurs gauge wire is included.• Shield construction – Each pair must be individually shielded using metalized polyester tape as the preferred choice though other equivalent options are allowed.• Wire-to-wire capacitance – a minimum of 12 picoFarads/meter based on a minimum 30 m cable length.• Wire twists per meter – a fairly typical 18 to 22 twists per meter.• Jacket resistance – 1 MegaOhm/330 meters (1000 feet) minimum resistance between the cable shield and the metal structure the cable may be in or on. As an end user you should note that the characteristic impedance can vary by +/–20 per cent from what is considered the norm of 100 Ohms – and, as we all know, resistance can change considerably with temperature, so be sure you know how Zo changes as a function of temperature and at what temperature the cable’s characteristic impedance is being given. Otherwise if you purchase a cable with an impedance of 80 Ohms, still within the specification, you may have an "out of specification" cable at operating temperatures. If you, as an end user, think the above may be a bit too "wishy-washy," I encourage you to become a member of the Fieldbus Foundation and as such you will be able to cast a ballot to confirm, deny or, as a minimum, influence and change this or any FF standard. The end result of this specification is that customers will now be able to specify that the cable to be used for their FF project be compliant with FF-844 and have the associated FF "check mark." Of course an offshoot of this is that adventurous end users now have a clear picture of what characteristics are needed to use any other type of cable for their design – provided it meets all the identified criteria. I am sure that some people will go this route, but the question remains – are you willing to risk your entire project on infrastructure that "might" be okay?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 This e-mail address is being protected from spambots. You need JavaScript enabled to view it or visit his website at www.ICE-Pros.com.
All of us take standards for granted. Yet without them, much of what we do today would not be possible, including the use of the Internet, telephones, e-mail and the electricity on which we run it all.In Canada, the Standards Council of Canada oversees all the standards used within the country and represents us on the global stage. The council has offices in Ottawa but, like most standards-setting bodies, it relies on experts from various fields to provide the knowledge on which the standards are based.
Businesses are created for the purpose of generating profits. As such, all investments need to have a positive return to be justified. The incentive or driver for that justification is different at each stage of a project's life cycle.
The recent trend in Foundation Fieldbus power supplies and conditioners has been to develop units that have higher voltage and higher current capacities than needed. This is ironic, since the exact opposite trend is happening to the field devices at the other end of the cable. These devices require less power, lower current, and they are able to operate closer to the fieldbus-specified minimum of nine volts. Let's take a look at what might be considered a reasonable current load for a fieldbus system. As a rule of thumb, use a conservative value of 20 mA for each device when designing a fieldbus system. Most installations do not use more than 12 devices on a single network; not because of physical constraints, but rather to manage the risk of losing so many signals in the event of a single point of failure.
One of the difficulties faced by fieldbus technologies is supporting signal types that they were not originally intended to support. For example, although Foundation Fieldbus supports discrete signals with its discrete input and output blocks, there are few devices that actually make effective use of this capability. Several output devices, such as valves, support the on/off feature, but in the case of limit switches and similar "contact" devices, the present H1 direct options require a field-mounted device that reads the incoming contact or coil status.This is similar to a nano-PLC needing a local power supply voltage to wet the terminals, as well as those of the discrete device being measured/controlled. The device is not truly standalone because it requires a local power supply in addition to the communication cable.
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