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High-Speed Optocouplers in Industrial Communication Networks
Today’s industrial networks require accurate data rates for communication in real time and
resistance to electrical noise. A new dual-channel, bi-directional 25 Mbit/s optocoupler meets the
requirements of today’s industrial networks while providing a high level of isolation in a small
form factor.
by Chwan Jye Foo, Avago Technologies
Industrial communication networks present unique challenges and require optimized solutions to
operate effectively. In today’s factories, many different programmable elements must be
precisely controlled so that they will operate in harmony. The communication channel between
industrial equipment, controllers, sensors, actuators and other components operates in real time,
and must be immune to electrical noise as well as providing safe electrical isolation. Also,
industrial communication networks need to provide increasing throughput or production output
as industrial control systems become more complex.
Fieldbus (or fieldbus) refers to a family of industrial computer network protocols used for realtime distributed control of instruments. As shown in Figure 1, an automated industrial system
such as a manufacturing assembly line usually uses an organized hierarchy of controller systems
to function. The top hierarchy is a human machine interface (HMI) where an operator can
operate and program the industrial system. This is typically linked to a middle layer of
programmable logic controllers (PLCs) or Input/Ouput (I/O) boxes by a non-time-critical
communications system such as Ethernet, which by its nature can tolerate dropped packets of
data.
At the bottom of the control hierarchy is the fieldbus that links the PLCs to the “assembly line”
components, such as sensors, actuators, electric motors, switches and valves. These
communication links need to be fast so that latency to the sensors and actuators is minimized and
system response time is fast, plus they must be real-time links that cannot tolerate interference.
In such an industrial environment, high voltages, magnetic fields and noise are commonly
present and are caused by motors, power switching and other sources, presenting a challenge to
deliver both high speed and resistance to excessive electrical noise so that the real-time nature of
the PLC system is preserved. Overall reliability and protection are critical to avoid production
downtime.
Trends and Challenges
Industrial Ethernet networks (e.g., Profinet, EtherCAT) were introduced in the early 2000s and
they gained acceptance at the supervisory control and basic control levels in the automated
industrial system hierarchy. According to IHS, the number of industrial Ethernet processautomation nodes is forecast to double in size with 14% compound annual growth rate (CAGR)
through 2016. Along with this growth in industrial Ethernet, fieldbus networks such as Profibus,
DeviceNet and Interbus, are still dominant at the bottom hierarchy of the instrumentation and
remote I/O level communicating in real-time deterministic protocols.
This trend presents several challenges for industrial automation and machinery communications.
One is to integrate the industrial devices communicating among different fieldbus platform
technologies with the new industrial Ethernet networks. Another challenge is to create an
efficient data collection process with more reliable data transfer. A third challenge is to enhance
the security of industrial networks to resist external attacks. These challenges place more
demands on equipment functionality and better network performance.
To meet the challenge of different needs of integration and make industrial devices compatible
with different existing and new networks, the automation industry is looking for different form
factors in embedded communication solutions to suit their own configurations. There are
requests for off-the-shelf communication modules, a communication mounting “brick” that can
attach to network connectors, or even a communication chip that is mounted on a printed-circuit
board with other hardware to be designed by the user. A semiconductor chip of small slim and
low profile, and which can be mounted easily and suit the different form factors of embedded
network equipment, is needed to address these challenges.
With the use of industrial Ethernet and fieldbus networks, there are many different network
protocols or “languages” to translate and communicate between devices. Higher speed reliable
data transfer is essential in creating efficient data collection and processing between industrial
devices communicating through these networks. Good electrical isolation immunity is important
to minimize electrical noise propagating across network buses and to ensure signal integrity. As
highlighted earlier, increased safety through electrical insulation capability between factory
devices helps ensure protection.
The industrial Ethernet networks provide more functionality and easier access to industrial
applications, but one area of concern is the security of a network with links to external
communication or the Internet. It is important to prevent any unauthorized access to the
industrial devices or to the control level of the entire automation system. Also, an upcoming
functional requirement for new industrial Ethernet network devices is to provide the new Internet
Protocol version 6 (IPv6) support in view of the current Internet Protocol version 4 (IPv4) public
address pool depletion.
New devices still need to support legacy protocols as many of the machinery and “assembly line”
components have been in production for many years and still use legacy networks. These place
more demands on the new modules to house more advanced chips with secure safety and
backward-compatibility communications features. There is a desire to produce modules that can
support both legacy fieldbus protocols as well as newer industrial Ethernet protocols. This makes
it necessary to pack more functionality into a given size module. Another direction the
equipment makers are taking is to offer very thin form factor modules that support a subset of
protocols, but which take up less space in the rack. In these multi-protocol modules, isolation
components are needed to address the legacy fieldbus protocols.
To address these two differing design requirements, it is advantageous to have a slim design
module where the back side of the PCB can mount low profile semiconductor components. This
allows layout design flexibility with more chips able to mount on the same area size of the PCB,
and saves floor space while offering compact embedded communication solutions. Such trends
can be seen as leading equipment makers market their communication or I/O modules as being
compact and having mounting rail width of 12 mm, 24 mm or 48 mm. Figure 2 shows the back
side of a PCB in an I/O module. The components such as isolation products mounted on the PCB
back side should not be taller than the housing of the back plane connector (<2 mm).
Using high-speed optocouplers such as the ACSL-7210 addresses the demanding isolation and
protection needs of today’s industrial systems. This solution offers 67% faster data rates than
competing solutions while providing 50% higher isolation capability and the thinnest packaging
available.
Figure 3 shows the typical application diagram for the 25MBd dual-channel bi-directional
ACSL-7210 and the 10MBd ultra-low-power single-channel ACPL-M61L providing isolation in
Profibus (RS485) fieldbus communication. The ACSL-7210 isolates the transmitting and
receiving data channels, for example between a microcontroller or digital signal processor (DSP)
of a factory “assembly-line” device and a fieldbus transceiver. Another optocoupler, the ACPLM61L, isolates the transmit enable signal (Figure 3).
Meeting Industrial Communication Requirements
The ACSL-7210 is a dual-channel bi-directional 25 MBd high-speed digital optocoupler
optimized for full duplex industrial communication applications such as Profibus fieldbus and
Serial Peripheral Interface (SPI). The ACSL-7210 utilizes Avago proprietary and patented IC
and packaging technologies to achieve 3,750 VRMS signal isolation in a low-profile small-outline
(SO-8) package while supporting high-speed full-duplex data communications with data rates of
maximum 40ns propagation delay (25 MBd).
Because it has buffered input data channels, the ACSL-7210 does not have a direct LED-driven
configuration as seen in a typical optocoupler. The user need not consider the calculation of LED
forward current and forward voltage to turn on/off the LED. The ACSL-7210 only requires an
isolated power supply at the input side to transmit isolated digital signals, so it can retrofit
existing isolated network modules with minimal software and schematic changes (Figure 4). The
main design consideration will be the layout to fully utilize the low-profile height of the ACSL7210. It can be placed on the back side of the PCB layout and free up the existing front side area
for more chips and passive components to enable better network performance.
The packaging process of stacking the LED die directly on a silicon IC substrate enables higher
integration in monolithic IC packaging and a very low profile. Figure 5 shows a cross-sectional
view of one of the two channels in the ACSL-7210. The Input logic signal controls the CMOS
LED driver buffer IC, which supplies current to the LED. The photodetector IC comes with two
transparent layers: SiO2 passivation or insulation, and light-transmissive polyimide on top. The
LED attaches to the photodetector IC with a transparent connecting layer. Standard die attach
process is used to make all the placements.
Unlike the conventional standard LED that emits light on the same side as the metal contacts,
Avago developed a back emission LED that emits light from the reverse side of the LED. This
allows LED to stack on top of the detector IC (Figure 6).
This packaging technology provides the advantage of high integration, with ACSL-7210 being a
dual-channel bi-directional optocoupler suitable for Profibus isolated data communication
applications. Another advantage is the low profile package at ~1.6 mm tall. This allows the
ACSL-7210 to be mounted on the back side of the PCB board where the height is usually
specified at less than 2.0 mm to maximize the use of board space. This leads to slim, compact
housing designs in the digital PLCs or I/O boxes.
Avago Technologies, San Jose, CA. (408) 435-7400. www.avagotech.com