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  • Writer's pictureBob Trask

What is EtherCAT?

Explore the industrial Ethernet system's unique functional principle and how it simplifies implementation for end users while expanding the number of available devices

The EtherCAT logo - a globe with communication wrapping around it

Some call EtherCAT ‘The Ethernet fieldbus.’ Simply calling EtherCAT an industrial Ethernet protocol does not do it justice because there has always been and remains much controversy about using Ethernet as a fieldbus. A fieldbus is the mechanism for a controller to talk to the device level: the sensors, the actuators, the servo drives, etc. We are not talking about IIoT or cloud mechanisms. We are talking about mechanics, the devices that make things work.


What matters here is temporal determinism and ease of use. Industrial controllers need to communicate with field devices on a consistent time basis. This just does not work well with an IP-switched network. There are too many delays, latencies, costs and wasted overhead for an IP-based Ethernet protocol to be of much use in modern day control systems.


So how does EtherCAT get around this?


EtherCAT is not an IP-based protocol. It uses standard, unmodified Ethernet frames, and it uses these frames in a special way.


Rather than generating an Ethernet frame for every device in the field, EtherCAT generates one frame. As the system boots up, an EtherCAT controller polls every field device and essentially asks, “Who are you? What are you? What are you reading? What are you writing? OK, when the Ethernet frame comes through the system, this is your pointer to your data.” Nothing else is needed. The device does not have to be addressed specifically.


This does two things. First, the controller only needs to copy a segment of data directly from memory, populate the Ethernet frame, and send it on its way. Second, the controller’s CPU is not tied up with bit-wise memory access and is freed to handle its main job of control rather than dealing with the fieldbus.


We call this implicit addressing. As the frame goes through the system, each EtherCAT field device knows where its data in the frame is (kind of like assigned seats on a train). Explicit addressing is not needed. You can really think of EtherCAT as a shared memory because it reads and writes data directly to controller memory.


Now you have one frame that can talk to all field devices without having to generate a separate frame for each, and you make the most of the ubiquitous Ethernet physical layer, meaning you can use normal, inexpensive, widely available Ethernet components. Nothing about EtherCAT negates the IEEE 802.3 Ethernet physical layer. The EtherCAT Technology Group (ETG) affectionately calls this methodology ‘Ethernet on the fly.’ (Check out this video for more information.)



Topology


Often EtherCAT is touted as a performance bus, which it is, but it is often overlooked that EtherCAT also has a particularly flexible topology thanks to its special functional principle. EtherCAT is a full duplex system without switches. This makes line topology possible, which is always more suited to industrial systems rather than a star-connected, switched-based topology (in theory, switch-based approaches support line topology as well, but in practice cascaded switches are to be avoided). And if one needs to do a drop from the line, that is also possible with EtherCAT – even complex tree topologies are supported.


There is infinite topology flexibility, even including media redundancy or hot connect groups. You can connect devices any way that works best with your system. No matter how the system is connected, it looks like a ring to the controller. This greatly saves on design effort, cost, and simplifies installation. No complicated network configuration is necessary.


EtherCAT network flexible topology
EtherCAT wiring is flexible (EtherCAT Technology Group)

There is only one EtherCAT


A huge advantage of EtherCAT is there is only one revision – version 1. There are functional enhancements, but an EtherCAT device developed today can still replace a device developed in a network deployed 15 years ago. The basic EtherCAT functionality has never changed, and no one expects there ever to be anything but EtherCAT 1. This is because EtherCAT was extremely well thought out from the beginning. We will have an article from the executive director of the ETG describing EtherCAT’s history in a future installment of this series.


Why does EtherCAT use 100 Mbit/s Ethernet?


Because of the efficiency in the way EtherCAT uses Ethernet, one frame for many, there is rarely a reason to even think about a Gigabit network. Gigabit Ethernet is no savior, though it’s often touted as such. 100 Mbit/s has many advantages: greater noise immunity, lower hardware costs, less heat dissipation and a much larger product offering. The PHYs and magnetics are ubiquitous and inexpensive. We are often misled into thinking Gigabit is always better. One could agree when talking about consumer technologies, but for an industrial control system, Gigabit leads to much greater costs and complications, especially on the device end. Do you really need a Gigabit interface to a sensor?


So while EtherCAT with 100 Mbit/s meets virtually all cycle time requirements today and in the future, there are more and more devices and applications that require more bandwidth. Therefore, development has started on EtherCAT G, with which EtherCAT will also support bit rates of 1 Gbit/s and beyond. This will coexist with the thousands of existing 100 Mbit/s EtherCAT devices. EtherCAT G will not make the existing EtherCAT devices obsolete; it will be another extension used for devices like vision systems, high-sampling-rate measurement applications, and extremely advanced, coordinated-motion systems. Above all, it will do this while keeping the vast selection of existing EtherCAT 100 Mbit/s products available – and not only that: The combination of Gigabit backbone and 100 Mbit/s segments for field devices maintains robustness and low cost while providing more bandwidth for those who need it. Vendors will not have to redesign their products for a different physical layer.



Simplicity is king for the user


Life is complicated enough. EtherCAT strives to make things simpler. EtherCAT is even easier to use than any traditional fieldbus system – there are users who switch from Modbus to EtherCAT for the sake of simplicity. And it is far easier to use than any other industrial Ethernet technology: no IP or MAC-Address handling, no switch or router configuration, no topology limitations, no security issues. And users don’t even have to completely understand the underlying design. That is the point. This is even true of distributed clocks, which are used to time synchronize a system on the order of nanoseconds. It is a testament to the work of the ETG that EtherCAT is easy to use. There is an elegant simplicity in using EtherCAT, no matter the controller, no matter the device. There is a well thought out consistency. All EtherCAT components use the same format. This is on purpose and it promotes ease of use when working with any EtherCAT device.


Diagnostics


EtherCAT makes use of several Ethernet 802.3 technologies in a unique way. The most compelling is link lost detection. We have all seen our PC’s Ethernet LED go green after plugging into a network. This is built into Ethernet. The physical link detection is monitored by EtherCAT and if a link is broken or lost the EtherCAT device will close the port, return the frame and inform the controller that a link is missing. Furthermore, each frame is monitored for bit errors by each device, and if there is one, its origin is also located. This allows for pinpoint diagnostics. By far, most EtherCAT network issues are caused by cables and connectors. Having exact error localization is a great benefit and one of the reasons EtherCAT has become so popular in the entertainment industry, which we will discuss in a future article.


The ETherCAT "Ethernet on the fly" principle in action
Ethernet on the fly: EtherCAT packs the Ethernet frame with data of many devices. (EtherCAT Technology Group)

Conformance


All this means nothing if field devices cannot be accessed and configured without a lot of effort. The ETG is keenly focused on conformance and interoperability. It is the obligation of all EtherCAT vendors to strictly conform to the standard. This conformance allows for unparalleled interoperability – a major reason why EtherCAT has become so popular. Facilitating interoperability between controllers and field devices is complex and vastly important. That is the task of the ETG so a user can move forward with confidence that the selected devices will always work.


Every EtherCAT vendor is obliged to use the EtherCAT Conformance Test Tool, which performs a self-test of a field device. In addition, the device can still be sent to an accredited test laboratory, where it receives a certificate after passing the test – of course, also an additional confirmation for users. The EtherCAT Conformance Test Tool is continuously refined. The ETG learned lessons from older fieldbuses, which offered static conformance tools that quickly became dated. Consequently, the vendors wasted much effort with old or alternate revisions of a conformance tool.


To protect the EtherCAT technology, the ETG has issued a set of policies that all members have accepted together with the by-laws. These primarily serve to ensure implementation conformance, thus ensuring the interoperability of the devices. The licensing policy also serves the same goal: to provide access to the technology for all device vendors, while avoiding variations that cause problems in the field of operation. Membership in the ETG is free of charge. After all, the ETG’s motivation is not to collect money, but to protect the technology from falsification on the one hand, as shown, and to promote it and facilitate its further advancement on the other. In this way it is it is possible to pick and choose devices and controllers from different vendors without worrying about whether they will work together. And the success of this policy proves ETG right: the variety of EtherCAT devices and the number of manufacturers supporting EtherCAT are unmatched.


No single vendor has all the answers. Having the ability to choose the devices that are best suited to controlling your system is a huge advantage.


Conclusion


The EtherCAT Technology Group feels the communication network is the single most important technological component of a control system. One of the messages of the ETG is to select this network first, then the controller. EtherCAT has many advantages to the control system integrator with ease of use and for the end customer, unparalleled diagnostics. We find that for many users that fairly compare fieldbus technologies, EtherCAT quickly becomes the favorite. Its unique functional principle is compelling and makes it “the engineer’s choice.”


Want to learn how EtherCAT can deliver unmatched performance and flexibility in your applications? Contact ETG or your local Beckhoff sales engineer today.


 

Bob Trask of EtherCAT Technology Group

Bob Trask, P.E., is the North American Representative for the EtherCAT Technology Group.


A version of this article previously appeared in Control Engineering. It is republished here with permission from ETG.

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