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Ensuring Rigorous Isolation in Fieldbus Designs
Understanding Fieldbus interface isolation technologies and their associated tradeoffs is the first
step in making sure your design is as tough as the environment it works in.
by Vincent Ching and Foo Chwan Jye, Avago Technologies
Fieldbus (or field bus) is a diverse collection of industrial computer network protocols and
network standards used for real-time distributed control. Unified and harmonized under the IEC
61158 standard, Fieldbus provides designers with a well-stocked arsenal of technologies for
connecting, controlling and managing automated equipment in manufacturing plants and other
challenging environments.
A complex automated industrial system, such as the manufacturing line shown in Figure 1, is
often built on an organized hierarchy of controller systems. In this hierarchy there is usually a
human machine interface (HMI) at the top, where an operator can monitor or operate the system.
This is typically linked to a middle layer of programmable logic controllers (PLCs) by a nontime-critical communications system such as Ethernet. At the bottom of the control hierarchy is
the Fieldbus that links the PLCs to the components that do the assembly line work, such as
sensors, actuators, electric motors, console lights, switches, valves and contacts.
Common Fieldbus Types
The IEC 61158 Fieldbus standard defines eight different protocol sets, also known as “Types.”
These Types are used as the foundation for a larger number of application-specific Fieldbus
standards and their associated physical, data link and application layer specifications. The most
commonly used standards for industrial automation applications include Profibus, DeviceNet and
CAN Bus, which are briefly described below.
Profibus-DP (Decentralized Peripherals) has been adopted by over 200 suppliers that offer
compatible devices. It is accepted and used globally for plant and process automation. It is an
open standard (based on EN 50170) digital communications interface, or Fieldbus, designed
specifically for use in industrial applications. It can be implemented using either copper-based
(EIA/RS-485) or fiber-based transceivers; its optical variant supports data rates of up to 12
MBaud, making it the fastest Fieldbus available today.
Profibus-DP can link plant supervisory systems, process automation systems and peripheral
devices such as feeders, mixers and pneumatic conveying systems. It supports communication
distances in excess of 2000m.
DeviceNet is a low-cost industrial network for connecting industrial devices such as limit
switches, photoelectric cells, valve manifolds, motor starters, drives and operator displays to
PLC processors and PCs. It implements the Common Industrial Protocol (CIP) over the CAN
bus. This network technology delivers moderately high data rates and helps eliminate expensive
hard-wiring while providing device-level diagnostics. DeviceNet’s data rates and communication
distances are shown in Table 1.
The Controller Area Network (CAN) bus is a rugged serial digital bus designed for harsh
industrial environments. Its flexibility and robust characteristics have helped it gain widespread
acceptance in many applications and markets including factory automation, building automation,
aircraft and aerospace as well as in cars, trucks and buses. CAN’s physical layer is a differential
serial bus based on the EIA/RS-485 standard with non-return to zero (NRZ) coding. When
combined with CAN’s higher-layer error detection/correction and fault confinement techniques,
it enables highly reliable transmission of commands and data. As with other electrical Fieldbus
variants, its reach varies according to the data rate it is required to support, ranging from a
maximum of 40m at 1 Mbit/s to a maximum of 6 Km at 10 Kbit/s. A summary of the capabilities
and characteristics of the common Fieldbus technologies is presented in Table 2.
Three Big Challenges
The three primary challenges faced by anyone designing electrically based Fieldbus interfaces
are noise (EMI/RFI) immunity, high voltage isolation and bus latency. As we shall see, all three
issues are related in some way to the isolation components used in Fieldbus designs to ensure the
bus controller and the electronics behind it are electrically protected against EMI/RFI-induced
transients and high-voltage incursions.
Due to the lower cost and simpler installation requirements of copper-based electrical interfaces,
many Fieldbus networks employ them rather than the fiber-based alternatives available for CAN
and some other Fieldbus standards. While usually slower and able to support shorter network
spans than their optical counterparts, electrical transceivers offer lower wiring costs and direct
compatibility with the large installed base of older, “legacy” systems already in use at many
industrial facilities.
Unfortunately, stray electromagnetic energies cause electromagnetic interference (EMI) and
radio frequency interference (RFI) on the copper Fieldbus cabling. RFI is usually a narrow-band
phenomenon and can originate from a wide range of sources such as mobile phones, power lines,
transformers, medical equipment, electromechanical switches, some motors, and many other
unintentional emitters found in the industrial environment. EMI wreaks havoc upon an
unprotected network in two ways. Radiated EMI travels through the air and conducted EMI
travels along a conductive path. Both flavors of EMI can interfere with the proper operation of
the equipment, contaminate the bus signal beyond recognition and, in some cases, physically
damage electronic components.
The discipline of EMI/RFI mitigation is usually achieved by a combination of removing or
reducing any sources of interference and good design practices, which include selecting devices
that are less susceptible to EMI, optimizing the layout to minimize coupling effects, and using
proper shielding practices.
In addition, Fieldbus standards require the use of isolators at every network transceiver to
prevent EMI, RFI, or stray voltages from working their way from the bus cabling to your
equipment, or from escaping from your equipment onto the bus. All commercially available
isolator solutions are based on CMOS or bipolar integrated circuits and use capacitive, inductive
or optical isolation techniques. Optical isolators, or optocouplers, are the most commonly
employed isolation technology in Fieldbus applications. They have provided electrical safety in
industrial applications for decades and come in a variety of form factors, voltage ratings and
speed grades to fit nearly any application (Figure 2).
The Shocking Truth
In addition to protecting against EMI/RFI effects, designers must consider equipment and
component safety, especially when high voltages (above 48 VDC or 110 VAC) are involved.
This requires protecting the equipment and anything connected to it against the unfortunate
ability of Fieldbus’ copper-based UTP cabling to act as a conductor for the high voltages often
found in industrial systems.
The Fieldbus transceiver in an industrial automation system is usually surrounded by motor
starters, servo drives, programmable logic controllers and power converters. And in many
factories, instruments and automation control boxes are co-located with high-powered industrial
systems that may contain power supplies and control elements that operate in the multi-kilovolt
range.
All these factors make it imperative that the isolators connected to a copper Fieldbus protect
against whatever potentially lethal AC or DC voltages might unexpectedly emerge from your, or
someone else’s equipment during normal operation, an equipment malfunction or other disaster.
Whatever technology they use, isolator components must have insulation characteristics that help
to separate circuits that present a danger of electrocution from other circuits, as well as to
separate certain parts of the equipment with which a user may come into contact or that connects
to other equipment.
Additionally, the circuit must be safe both during normal usage and under fault conditions. The
IEC 60747-5-5 safety standard defines two levels of insulation with a clear distinction for safety.
Basic insulation is by definition considered to fail or become shorted under a single fault
condition. In contrast, the “reinforced insulation” rating can only be approved and applied to
“failsafe operation-qualified” components that deliver both basic protection from electric shock
and also support “failsafe operation” designs that enable safe user access during an equipment
failure.
IEC 60747-5-5 was specifically written for optical isolators only, but devices using other
isolation technologies, such as magnetic or capacitive isolation barriers, have obtained “basic
insulation” certifications to this optocoupler safety standard. As noted, this basic insulation may
not provide failsafe operation performance, so the non-optically isolated devices cannot be
considered failsafe and therefore should not be accessible to a user.
Designers must also be aware of the isolator’s contribution to a Fieldbus network’s overall
system propagation delay. System delay consists of the combined delays of the CAN transceiver,
CAN controller and the optocoupler. It is critical that the sum of the component propagation
delays not exceed the specified value to ensure the device is capable of working with the
maximum cable distance at the corresponding bit rate. The CAN standard calls for a maximum
delay time of 40 ns for the isolator. This indirectly translates to a speed of 25 MBd (1/40 ns).
Comparing Optical and Magnetic Isolation Technologies
Over the years, manufacturers have added new functionalities, increased insulation, higher
isolation voltages and smaller packages to their optocoupler product portfolios.
Nevertheless, the relatively high complexity of optocoupler structures and performance issues in
earlier generations created a market opportunity for other types of isolators based on magnetic
and capacitive technology. As a result, several magnetically coupled isolator products have
emerged on the market that claim to deliver better integration, size and speed. While the
manufacturers can substantiate some of these claims, fundamental questions about their
reliability and robustness have remained unanswered. And, as we shall see shortly, each isolation
technology behaves differently in the presence of high voltages and strong electromagnetic
fields.
Some of the claims about magnetic isolators were recently evaluated in a series of tests that were
designed to determine how long a particular isolation device can successfully insulate one side of
its isolation barrier from high voltages on the other side. The test was structured to assess a
device’s reliability in terms of two parameters: high-voltage performance and insulation
integrity.
The high-voltage life test was a destructive test where high voltage was applied to magnetic
isolator devices from a well-respected manufacturer and optocouplers from Avago. In
accordance with the devices’ data sheets, 2.5 kV was applied constantly to the magnetic isolators
and a group of comparable optocouplers were subjected to a higher voltage (3.75 kV). The
magnetic isolators were destroyed between 8.5 hours to 10.5 hours of continuous test, while the
optocouplers survived a minimum duration of 168 hours at the higher 3.75 kV voltage.
Distance Through Insulation (DTI)
Internal clearance, or distance through insulation (DTI), is another important metric for electrical
protection. When applied to optocouplers DTI is defined as the direct distance between the
photo-emitter and photodetector inside the optocoupler’s cavity. Figure 3 illustrates how DTI is
measured in the wide-body and standard optocoupler packages. Some equipment standards
require a minimum of 0.4 mm DTI for reinforced levels. Examples of such equipment standards
are: IEC 60651 (Medical) and IEC 61010 (Measurement and Control). No exemption is given for
these two equipment standards.
Current suppliers of non-optical based isolators do not offer products that meet the minimum
DTI specification of 0.4 mm. Their current product portfolios have typical DTI specifications of
about 0.018 mm; this reason alone makes designers think twice about choosing a magnetic or
capacitive isolator for a medical or a test and measurement application.
While the optocoupler’s LED/photodiode combination is considered immune against EMI due to
the optical coupling path, the magnetic isolators do have limitations with respect to EMI because
of their microstructure and magnetic coupling. Magnetic coupler failure can occur at DC (0 Hz)
as well as at various frequencies and different field strength levels.
The results of these studies indicate that optical isolation provides superior EMI performance and
can withstand much higher electromagnetic fields than other isolator technologies currently on
the market. This supports the assertion that optocouplers are the best choice for demanding
Fieldbus applications.
Optocouplers in Action – A Practical Example of a High Speed Fieldbus Design
Figure 4 illustrates how optoisolators are implemented in a typical Fieldbus transceiver circuit.
As the multiple part numbers beside the opto-isolators indicate, there is a wide range of
optocouplers available to meet any speed, signaling/operating voltage and level of protection
required by the various Fieldbus standards.
Designing a safe and robust Fieldbus-based industrial system can be challenging, but the process
can be made easier by paying attention to four key issues. The first is to observe safety standards
for isolation devices. Look for optocouplers that meet the IEC 60747-5-5 standard with
“Reinforced Insulation” to ensure failsafe operation. Next, carefully consider the reliability of the
isolator’s high voltage insulation to minimize the potential for component breakdown when the
system is exposed to a high voltage surge. It is also essential to use design, shielding and
isolation to mitigate the effects of EMI and RFI. And finally, one must ensure that the
transceivers’ isolators meet or exceed the mandatory distance through insulation (DTI)
requirement: A greater than 0.4 mm requirement for both medical and test and measurement
equipment has excluded magnetic and capacitive isolators from use.
Avago Technologies, San Jose, CA. (408) 435-7400. [www.avagotech.com].