S. Ditting, HIMA, Brühl, Germany
Ethernet-APL (advanced physical layer) brings the benefits of digital
communication to harsh environments of the process industry at the field level.
The universal use of Ethernet-APL makes it possible to consolidate an
infrastructure for safety-related and non-safety-related communication. Regardless,
complete separation is preserved and maximum availability and safety are
ensured. The digitization through the use of Ethernet-APL ensures full
information transparency from a sensor to the cloud. This information is
available for evaluation across all automation levels throughout the entire
plant lifecycle. This article describes the features, benefits and challenges
of the new technology and draws comparisons to preceding ones.
Time to market is crucial to the success of plant operators. For new
plants, the focus is on the planning, engineering and construction phases. For
existing plants, however, the primary concern is efficient change management,
as plants today must respond flexibly to market demands. Moreover, maintenance
and repair account for a significant proportion of a plant’s operating costs
and can frequently exceed the acquisition costs. This is why it is important to
consider all components and their interactions throughout the entire lifecycle
of operating a process plants for cost effectiveness.
The pursuit of increased flexibility is as old as automation itself. A
major step in this direction was decentralization through remote I/O. Here,
data of various I/O types can be converged or distributed onsite. The
connection to higher-order levels for processing is established via digital
networks. This approach has improved over the years, resulting in remote I/O
concepts [e.g., for operating functions, safe automation, flexibly configurable
I/O, accessing the explosive atmospheric zone classification (Ex-Zone)].
Unfortunately, these concepts are often oversized and merely exchange process
values at the field level.
The Highway Addressable Remote Transducer (HART) takes a different
approach. The communication protocol exploits the existing analog signals and additionally
communicates modulated data to the devices directly connected to the field
level. This approach, however, still has drawbacks, such as the reduced
accuracy of the analog values, a still-considerable wiring effort and missing
properties for functional safety.
A good combination of the two concepts, fully digital communication with
a direct network connection of the sensors and actuators to the field level,
would be the ideal solution. Ethernet-APL is designed precisely for this
purpose and enables end-to-end digital communication down to the field level of
The tasks of Ethernet-APL. Modern
communication systems are structured in several layers (FIG. 1). Each
layer provides different capabilities and can be replaced by other technologies
operating on the same layer.
lowest layer is the so-called “physical layer.” Like “Fast Ethernet,” “wireless
local-area network (WLAN)” or “fiber optics,” APL describes the physical
transmission of data. Each of these connections is designed for a specific area
of application. Ethernet-APL combines some vital features for process
automation (FIG. 2),
The International Organization for
Standardization/Open Systems Interconnection (ISO/OSI) layer interchangeability
allows for easy conversion from Fast Ethernet to Ethernet-APL. Pepperl + Fuchs proves
the point with an Ethernet-APL switch that is used as a media converter from
Fast Ethernet to Ethernet-APL.
respect to the physical layer, it makes no difference which higher-order layers
are involved in the communication and which data is exchanged. Well-known
higher-order layers are Internet Protocol (IP) and transmission control
protocol/user datagram protocol (TCP/UDP). These higher-order protocols are
also interchangeable if they are on the same layer. It is only in the
application layer that data is given meaning. A protocol of a higher layer
widely used in automation is Modbus, which can use TCP or UDP as well as Fast
Ethernet or Ethernet-APL. Modbus is a simple example. For universal automation
solutions, however, more modern and universal protocols with a wider range of
applications are more interesting.
example of a widely used, well-proven and open industrial protocol is PROFINET.
In addition to many other benefits, it also provides PROFIsafe, an open
protocol for functional safety that establishes a “black channel” between a
host and a device, ensuring detection of potential errors in intermediate
communication layers. Consequently, thanks to the interchangeability of the
underlying layers, PROFINET and PROFIsafe can be transmitted via both Fast
Ethernet and Ethernet-APL.
Ethernet-APL is stepping up to replace existing 4 mA–20 mA solutions as well as remote I/O.
This is a major promise and requires manufacturers and users to rethink their
applications. As with all technology or paradigm shifts, digitization should
not be an end in itself. It should create true added value in real plants.
Test setup at
BASF. BASF in Ludwigshafen, Germany, has set up a fully functional network
with Ethernet-APL components to gain practical experience and insights. The
test setup also includes a prototype with Ethernet-APL, PROFINET and PROFIsafe.
This is the first fully functional SIL 3 communication via Ethernet-APL
worldwide. The concrete connection must be viewed on two levels:
COMPARISON TO EXISTING
following sections describe the individual benefits of combining Ethernet-APL
with PROFINET and PROFIsafe as compared to previous technologies (4 mA–20 mA, HART, remote I/O, fieldbuses).
mA–20 mA wiring. The
4 mA–20 mA interface
is presently dominant in process automation, primarily for the transmission of
safety-relevant data. With regard to the connection technology, Ethernet-APL offers
an advantage over 4 mA–20
mA technology in that a connection can be established easily over almost any distance
into the field using a single cable. Where increased availability is required,
a ring can be used. As such, the wiring effort with Ethernet-APL is
significantly lower than with 4 mA–20 mA. Furthermore, there is no need for a marshaling level
and, for systems in Ex-Zones, an Ethernet-APL switch replaces Ex isolators,
thus saving additional space and cost within the control cabinet.
mA–20 mA accuracy. One
downside of a 4 mA–20
mA interface is that signals from the sensor to the safety-related controller
must be repeatedly converted between analog and digital, which means that the
transmitted analog value is prone to interference. This inaccuracy cannot be
compensated for by a high-resolution analog-to-digital conversion within the
controller. For this reason, safety margins must be planned.
modern transmitters process the internally available data in digital form
anyway. With digital communication, data can be made available unaltered and
with higher accuracy. So, safety margins can be reduced and the plant can be
operated closer to its limit. Depending on the process, this can result, for
example, in increased output, lower power consumption, or improved quality of
the goods to be produced.
4 mA–20 mA information content. The
information content of 4 mA–20
mA is rather low and the value must always be interpreted in the processing
unit. Furthermore, the valid range for process values is between 4 mA and
20 mA. As a result, a value above 20 mA is not a valid process value
and indicates an error state. In turn, this means that either a process value
or an error will be displayed.
digital transmission, multiple values (e.g., process value, health status of
the field device) can be transmitted. If the field device detects the need for
maintenance, this can be communicated in the digital data record. The plant can
continue to operate until preventive maintenance is performed, thus ensuring
increased plant availability.
4 mA–20 mA with HART. HART modulates digital data
onto the analog signal, allowing for additional information such as the health
status of a field device to be transmitted in addition to the process value.
This transmission via HART is very slow, does not provide much data, and
affects the accuracy of the process value. Also, the additional data cannot be
used for safety-related applications.
with a digital connection with Ethernet-APL and PROFINET, large amounts of data
can be made available very quickly. With PROFIsafe, the information can be used
directly for safety engineering. For example, in the case of differential
pressure measurements, even additional process values (both pressures) can be
evaluated from a safety viewpoint. Even the units of the measured values can be
transmitted, making it unnecessary to interpret the data in the safety-related
controller because this information is directly provided from the field.
configuration and startup are possible with both the HART and the
Ethernet-APL/PROFINET solution. Thanks to its faster transmission capability,
Ethernet-APL offers much more extensive options.
Remote I/O with or without
flexible I/O. Compared to the 4 mA–20 mA solution, remote
I/O with or without flexible I/O have the advantage of less wiring and thus
usually are more flexible with easier planning. In the end, however, they only
move the I/O into the field and are still 4 mA–20 mA interfaces. Therefore, the lower
information content and the inaccuracy of the process values (almost)
correspond to the direct 4 mA–20
mA solution. Depending on the design, the hardware overhead and space
requirements are also significantly lower with Ethernet-APL than with remote
Fieldbus systems. Fieldbus
systems such as PROFIBUS represent a first approach to establishing digital
communication at the field level. Generally, serial connections are used here.
Fieldbus has the drawback of being very slow (factor 300) and quite error-prone
compared to Ethernet-APL (nothing is more annoying than a forgotten terminating
Ethernet-APL: Future capability
and challenges. Modern approaches to
process automation benefit from the use of Ethernet-APL to extend
high-performance networks down to the field level. Concepts such as Namur Open
Architecture (NOA), modular type packages (MTPs), modern asset management
systems (AMSs) or even visionary approaches such as “control in the field” can
only realize their potential if performance and information content from the
field increase significantly.
course, new technologies bring new challenges. For minimum complexity and
maximum cost efficiency, all field devices in a plant must be consistently
integrated via Ethernet-APL. Consequently, the entire sensor and actuator
portfolio should be available at the field level for operation and safety
a single digital input or output does not yet make sense from an economic point
of view. In this respect, remote I/O do have their merits. They can be
integrated in the same network, even if other protocols are used on higher
layers. The principle should be: Networking where possible, wiring where necessary.
security aspect should also be considered from the outset. Ethernet-APL is
merely a “physical layer,” so the security concepts developed both in the
standard (IEC 62443) and by user organizations (e.g., PI) can be applied.
Added value with Ethernet-APL. Ethernet-APL
qualifies for both new (greenfield) and existing plants (brownfield). The
benefits described here relate to technical characteristics. Depending on the
application, however, these benefits can be extended even further.
may vary from one plant to the next—different field devices are suitable for different
applications. The openness of the interfaces combined with the resulting
compatibility and interoperability make it possible to use the best field
devices in their respective class (best-of-breed). The definition of “best”
varies from plant to plant. For example, “best” for one plant may be high
accuracy, while for another it may mean high speed or low costs.
openness of Ethernet-APL combined with PROFINET/PROFIsafe also improves the
spare parts situation, as alternative devices or successor products can be used
more smoothly. The data already in digital form can be perfectly forwarded to
higher layers via OPC UA.
Independent open integration. The
author’s company’s flexible safety platforma offers a complete
portfolio of solutions that meet the highest functional safety and availability
requirements, enabling both centralized and highly efficient decentralized
solutions (FIG. 3).
In decentralized solutions, safety-related communication between the safety
controllers and to the remote I/O occurs via the company’s proprietary protocolb.
safety-related systems support all common communication options, including OPC
UA and PROFINET. This means that the safety platforma can be
flexibly combined with the leading distributed control systems (DCSs). Ethernet-APL
is now the first to enable the efficient, digital connection of safe field
devices. The author’s company calls this concept “Independent Open Integration,”
enabling plant operators to implement their individualized solutions. Ethernet-APL
now offers even more options for communications and an excellent digital
highway down to the field level to implement customized solutions with
best-of-breed products for end-to-end safety. HP
DITTING is a Product Manager at HIMA with more
than 4 yr of practical experience as an Application Engineer and 2 yr as a
trainer before moving into product management. His responsibilities include the
compact SIL3 controllers HIMatrix and industrial communication. In this context,
he actively began his work in the field of automation security in 2006. Besides
several publications in in professional journals and congress papers, he actively
works on different security committees like DKE/UK931.1 and VDMA. Ditting
graduated with a degree in electrical engineering (specializing in automation)
at the BA Stuttgart.