Fieldbus in the future

It was a long time coming, but the era of fieldbus has well and truly arrived. Now rapidly becoming the method of choice for communication between process control systems and field instrumentation, the fieldbus concept is finding its greatest success in greenfield sites, particularly in developing countries where new large-scale processing projects are more common.

Because these projects can specify and install fieldbus systems from the start, they can take full advantage of the lower cabling requirements and number of connections that the technology needs, cutting commissioning time through the use of technicians familiar with the protocol and saving on CAPEX (capital expenditure) and OPEX (operating expenditure).

So, what are the prospects for the fieldbus market and how is this valuable technology developing to meet users’ needs?

A recent ARC report revealed that the market is narrowing to two primary choices — namely Foundation Fieldbus H1/HSE and Profibus DP/PA, with both offering the same sort of scope for savings in CAPEX and OPEX.

Profibus finds its niche in industrial I/O applications, for which it was specifically designed. It uses familiar twisted pair field wires, supports multi-drop topology, can communicate with field devices from different manufacturers, and can use I/O function blocks in the field device.

But can either of these two protocols, or some other, develop into the standard fieldbus system?

There are hundreds of different fieldbus protocols, both open standards such as Profibus and proprietary protocols supported by a single manufacturer. Open standards have the major advantage that they are supported by a large number of DCSs (distributed control systems) and PLCs, whereas proprietary systems are limited in the number of devices they can support.

Everyone is looking for a single common standard — the International Electrotechnical Committee (IEC) tried to introduce one, but was unsuccessful. Simpler protocols, such as the ASI-BUS, will no doubt survive — it is very suited to simple tasks such as switching and status information. However, a lot of protocols will inevitably fade away because they are too specific and are supported by too few companies to produce the flexibility and economies of scale that users increasingly demand.

Both Profibus and Foundation Fieldbus (FF) have large installed bases and are unlikely to disappear in the near future. A single standard is still no closer, although Profibus is starting to adopt some of the features of FF. Both will continue to develop and perhaps converge as they go after the same applications.

An added consideration in intrinsically safe applications is the number of fieldbus devices allowed to be present. In these applications, devices and barriers are designed to eliminate the risk of any energy released by an electrical fault being sufficient to trigger ignition.

The type and number of fieldbus devices permitted in an intrinsically safe area varies according to the type of hazardous atmosphere present and whether the user opts for the ‘Entity’ or fieldbus intrinsically safe concept (FISCO) intrinsic safety models. The entity model is based on the assumption that all equipment used in an intrinsically safe environment is individually approved for use in that environment, including the wire used to connect the devices. Although it is recognised worldwide, the model only permits a maximum dc electrical current of 83mA in the wire and a maximum of 18.4V per device.

In contrast, the FISCO model greatly simplifies installation of field-enabled devices in intrinsically safe environments. Proven by practical testing, FISCO enables a greater maximum dc electrical current (256mA gas group IIB), permitting more devices to be installed per segment.

Despite it not being a worldwide standard, FISCO is steadily becoming accepted throughout Europe and is part of the FF specification. The ability to integrate a fieldbus with other systems is crucial when creating control applications and both FF and Profibus provide standard interface files and interoperability strategies to allow system integration.

Yet, customers are looking to manufacturers for further testing, to give them confidence that the products offered will be truly compatible with the major protocols. ABB, which supports both Profibus and FF directly, tests fieldbus devices within its Industrial IT systems (800xA) to provide additional confidence and reduce project testing time.

Similar topologies

Both protocols have similar topologies, with a high speed backbone and links to the slower speed PA and H1 field segments. FF switched its development of the H2 high-speed bus to take advantage of high speed Ethernet, and FF HSE now offers the most flexible solution for fieldbus integration.

Profibus uses RS485 or fibre optic media for its high-speed bus, as this is a well understood technology. Profibus has perhaps the weakest integration method as it allows for cyclical device access via the GSD file, adding a further level of complexity during commissioning. Device commissioning and parameterisation is also relatively poorly supported, often requiring multiple external software tools.

The open standard of Field Device Tool (FDT) and Device Type Manager (DTM) neatly solves this problem, providing total field device support from a single software tool. DTMs contain full information about the devices in the Fieldbus network (see PE Sept 2004, page 33) and make it easier to interrogate ‘Smart’ devices, which contain valuable status and performance information that cannot be accessed via an analogue link.

There is a team working on an FDT standard for FF and also to enhance the Device Descriptors (DD), a newer competitor to Device Type Managers. These enhancements enable device developers to organise the large number of parameters in complex devices logically and also allow the inclusion of images to help users configure the device. They will also allow greater compatibility between FF, HART and Profibus.

HART devices offer a degree of maintenance information and remote access for device configuration and support, similar to the fieldbus protocols. With HART, however, communication with the device is overlaid upon the 4-20mA analogue signal. It is therefore slow, although access to device data is available with the correct tools and components. The ability to pass HART information up to host systems is becoming more popular and asset monitoring applications can be used to maximise HART device availability to a similar degree to that of FF and Profibus devices. The use of HART DTM also provides total device support for all the available features.

The future of fieldbus in general holds out the prospect of more low power devices, and devices that store their own documentation, making commissioning easier. Devices with more sophisticated algorithms and improved safety protocols are also on the horizon. Also, there is scope for devices to become more intelligent in communicating maintenance messages. Another advantage would be more intelligent use of process information when reporting errors.

Wireless access is another prospect, allowing the use of handheld PDAs that would enable engineers to check the status of processes as they walk through the plant. Fibre optic communication would also be useful, preventing the interference with data that can be produced by radio frequency emissions. The capabilities of FF will see more and more devices using high speed Ethernet, as well as more sizeable projects using the protocol. Larger numbers of vendors will lead to greater confidence among users in designing bigger projects, knowing that they are not relying on one vendor to supply all the equipment. PE

Gareth Johnston is Fieldbus Technology Specialist with ABB.