When requesting a fieldbus solution, it is important to define which of the eight protocols it is that are required. The inability of the IEC to come to a single standard for Fieldbus has created a lot of confusion in the marketplace.
Recently, several customers have approached Honeywell to provide fieldbus solutions for their plants. "The solutions that had been proposed by our competition in no way complied with Foundation Fieldbus. However, the customers never specified Foundation Fieldbus and could not understand why Honeywell had such a short list of sites where this technology has been installed and is in control. Some suppliers in the industry state that they have hundreds of sites, several in South Africa!" states Zane van der Nest of Honeywell's Industrial Solutions Division. "Are they lying? Not at all!"
Remember that Profibus, Hart, Control-Net, Device Net, World FIP, Foundation Fieldbus, to name a few, have all been included under the IEC umbrella definition of Fieldbus.
True Foundation Fieldbus (FF), however, has the following features which differentiate it: base level control can be executed in the field devices, true FF will have the Link Active Scheduler function, critical to ensure correct cycle time and processing order of control variables. At this stage, Honeywell claims to be the only supplier to offer back-up LAS in its field devices. (This means that if the primary input output processor fails, the next lowest node on the network will take over this function.)
Further, it is certain that eventually all FF suppliers' instruments will have full Foundation Fieldbus accreditation and that these instruments will then be transparently usable on any other FF vendor's system, through the 'Device Descriptor' function. Foundation Fieldbus devices can also be networked on a single bus in a number of ways: daisy chained, multidrop or star connection.
Thus, reputable suppliers should be making their customers aware of at least the following:
* H1 fieldbus is a 32 KHz network, designed to re-use existing field wiring. Customers are mistaken by believing that they can hang 225 devices onto a network. This is the theoretical limit but remember that only 50 messages a second can be transmitted and if the application needs a cycle time of 200 mS, then a maximum of only 10 devices can be used on that network.
* There is a trade-off between the thickness and type of cable used and the number of devices that can be connected. The more expensive individually shielded twisted pair can be extended up to 1900 m - the more devices connected the shorter the possible length, regardless of quality of the cable.
* Other considerations are the existing availability and reliability needs of the plant. Current design does not include a redundant network but a customer's plant usually requires that the control system be at least single fault tolerant.
* Further considerations are whether the plant is intrinsically safe or not. IS networks are limited by the energy of the barriers - usually 800 mW. Current devices need at least 9 V and typically 18-24 mA to operate. This results in a restriction of up to four devices per network. Customers must remember the capacitance and inductance of the loop could cause power requirements to be exceeded. Remember that current design specifies that the bus terminator be no further than 100 m from the barrier. This means in many instances that the barriers will be mounted in the field in Zone 2. In South Africa many customers have simply defined their whole site as Div1 Zone 1 and they will need to rethink their policy and operations.
* Bus design is also important, the use of isolators to create networks is advised, as a failure of one device should not fail the entire network.
* In the worst case, there is at least a 66% reduction in copper and cabling requirements. However, this does not translate into a 66% installation cost saving, as the power supplies and bus design are more expensive. The true benefit is in the accuracy of the digital communication and that high level applications enhance the overall maintainability of a plant, with an increased up time and a reduction in down time by facilitating better planning.
* H2 Foundation Fieldbus looks as if it is never going to see the light of day. Development on high-speed Ethernet is already well under way with several major suppliers working at Foxborro headquarters in the USA.
Digital field communication is here to stay. Honeywell says that customers can avoid a lot of hardship by asking for Foundation Fieldbus - if that is what they expect at the end of the day.
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