The Principles of Industrial Ethernet

HBK is a member of PI North America and the EtherCAT Technology Group – two non-commercial, member-supported, non-profit organizations that support and promote industrial Ethernet within industry.

This article explores the background of automation networks before addressing a main theme – the main principles of industrial Ethernet.

Covering a variety of industrial Ethernet buses prior to evaluating HBK's range of equipment designed to transmit signals from various equipment onto an industrial Ethernet bus.

The History of Automation

Automation is essentially guided by three main factors.

  • Product quality: An automated process can eliminate the possibility of human error, enhancing product quality, narrowing rejection bands and limiting the number of discarded parts at the end of a process.
  • Process speed: An automated process may be quicker than a manually operated process due to the fact it typically takes less time for a machine to complete a task than a human operative. Automated processes also provide the benefit of having the capacity to run continuously with the least possible breaks required.
  • Cost factors: The capacity to manufacture a higher quality product with fewer rejected parts, for a greater amount of time is extremely likely to boost potential profits.

Early automated systems primarily depended on pneumatics and steam. These systems were bulky, large and demanded vast amounts of raw materials to operate. Steam-based systems exploit considerable thermal energy amounts, which was generally considered dangerous, inefficient and wasteful.

Using steam or compressed air in the vicinity of moving parts will always necessitate the use of oil, which ultimately results in wear of the parts, as a consequence, routine maintenance and replacement will be necessary.

The first automated systems were usually developed for specific tasks, which meant it was not so simple to adapt or modify the system if a process evolved.

The 1950s witnessed a shift towards the 4-20 milliamp control signal. These devices could be applied with considerably lower costs involved and did not demand a significant raw material overhead that was prevalent with older systems. They were also less susceptible to noise or loss unlike their earlier voltage-based counterparts.

However, 4-20 milliamp control signals tended to burn out with use as they required relays. Therefore, routine maintenance was essential for such systems. Furthermore, issues were difficult to diagnose and maintenance was challenging as these systems relied upon complex ladder logic.

During the late 1960s, the programmable logic controller (PLC) emerged. PLCs aimed to streamline the complex analog ladder logic into digital systems that were simple to understand and diagnose. When PLCs were introduced initially, most of these were integrated as centralized structures.

A centralized structure would generally integrate a single PLC into a control cabinet or control room somewhere on the manufacturing floor, with the IO from machines fed back to the PLC.

Effector signals would then be transferred from the PLC back to machines. This configuration demanded a significant amount of raw material by way of copper wiring, and it was necessary to run each signal back and forth between the PLC and the machine.

As systems expanded outwards and developed into more complex systems, there was a movement towards decentralized structures. These decentralized structures would presumably utilize a primary PLC in a control room or control cabinet, but they would also make use of a series of PLCs distributed around a facility.

These distributed PLCs would gather all the localized IO from a single machine and transmit it to the local PLC. This significantly reduced the amount of wiring necessary. In cases where it was not necessary to monitor these signals in the control room, or if required elsewhere on the floor, the remote IO from a distributed PLC could be communicated via a single wire back to the primary PLC.

Communication between distributed PLCs and the centralized PLC was generally performed utilizing fieldbus communication, such as DeviceNet, ControlNet, Profibus, or CAN. However, these fieldbuses had their drawbacks.

While nodes on a CAN bus had the capacity to transmit data to the controller at any point or timed interval, it was inconceivable to determine if or when this data would arrive at the controller as these networks did not possess determinism.

CAN utilized message priority fields whereby important messages would be transmitted faster than less important messages, but this presented random delays to other signals that may be required in case of fault or failure.

Industries began to demand predictable performance and faster processing, but devices such as CAN did not have the capacity to meet this demand. However, industrial Ethernet eventually met the demands of industry.

Conceptualizing Networks

Many professionals or users may be familiar with a conventional industrial Ethernet switch. It utilizes a standard RJ 45 jack, and while in some ways industrial Ethernet may appear to be similar to standard Ethernet, it has been tailor-made for the industrial setting.

Before evaluating particular industrial Ethernet technologies, it is crucial to establish a basic understanding of networks. The open systems interconnection (OSI) model is a seven-layer model that characterizes networks.

Layers one to four are generally descriptive of the conventional network structure, while layers five to seven are typically concealed behind applications. End users are not likely to interact with these layers directly.

OSI Layer 1

The physical layer of the OSI model is Layer 1. This is the layer that transmits or the signal, physically defining the connector’s shape. For instance, the shape of an RJ 45 jack or the shape of a USB Type A versus a USB Type C. Layer 1 also establishes communication voltages and timing.

Regarding wireless communication protocols such as Bluetooth or Wi-Fi, Layer 1 determines which frequencies are used.

OSI Layer 2

Layer 2 of the ISO model is the data link layer. This details how data passes from node to node across a network. A term commonly inextricably linked this layer is the MAC address (media access control address).

Each network adapter has a distinct MAC address, and MAC addresses are how the information sources and destinations are recognized within a network. At this point in the OSI model, a piece of data is known as a frame.

OSI Layer 3

Layer 3 of the OSI model is the network layer. This handles the communication between contrasting networks. These networks are not innately different, and they are not the same network.

The term generally associated with this layer is the internet protocol (IP) address. IP addresses can help determine which network or sub-network a device is on. Routers are labeled so because their responsibility is to route data across these networks.

OSI Layer 4

Layer 4 of the OSI model is the transport layer. This determines how the transmission of the data takes place. One of the principal functions of this layer is to establish how data is segmented or divided before it is sent via the network.

Two of the terms used to identify this layer are Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).

TCP is a slower communication technique,  but it provides the key advantage that the integrity of data is secure. Everything necessary for the receiver of the information to order the information appropriately and ensure it has been correctly received is integrated within these frames.

UDP is occasionally referred to as the 'fire and forget' mechanism as the client will transmit information to the server with no concern for whether it arrives or not. This enables accelerated communication over UDP, but there is no guarantee that the information will be received correctly or reach its destination.

OSI Layers 5, 6 and 7

OSI layers 5, 6 and 7 are generally not discussed as they are typically hidden behind the application.

Layer 5 is a session layer that manages the connection between server and client. Layer 6 is the presentation layer that controls and translates syntaxes where various protocols are used and layer 7 is the application layer - the layer that the user directly interacts with.

Industrial Ethernet Technology

When evaluating industrial Ethernet, it is initially recommended to explore the terms 'industrial' and 'Ethernet' separately.

Traditional Ethernet

Ethernet is an IEEE standard - 802.3 - which in principle defines layers 1 and 2 of the OSI model (the physical layer and the data link layer), though a majority of users acknowledge the practical applications of this standard – for instance, plugging in an Ethernet cable when Wi-Fi is inaccessible.

However, when evaluating communication over Ethernet, it is crucial to look at each single layer of the OSI model - not just 1 and 2.

For instance, a user browsing the internet via a web browser may click on a link that creates user data. This user data is transmitted to the application layer, which adds the information it needs to generate a message. Data is then transferred to the transport layer, where information regarding which ports are to be used is incorporated into the TCP header.

Enormous sets of information will be broken into segments before passing down these segments to the internet layer. Then an IP header with IP addresses for the source and destination are added afterwards.

This produces a datagram that is delivered to the Ethernet (network access) layer, where the destination for MAC addresses and sources are incorporated into the header and error, which ensures information is added into the trailer. This process is known as encapsulation.

The encapsulation process must occur every single time a device on a network produces data. It needs to pass it down each of these layers, and the server must unpack all of this information in reverse order once it is received at the destination. This is a protracted process that is not always good for control purposes.

Ethernet is the cornerstone of the internet and is a flexible, powerful technology; but is comparatively impeded by the encapsulation processes and what is known as a 'store and forward' mechanism.

As a packet is transmitted along an Ethernet network, a router must have the capacity to see the complete packet before it can determine the destination. This also presents delays that are not useful for control applications.

The Ethernet standard includes a range of collision detection mechanisms, but when it comes to stopping the collision of data frames from colliding, that would corrupt them, it introduces random delays, which reminds us of the reason that original fieldbuses were not desirable for communication purposes.

Defining Industrial Ethernet

Industrial Ethernet can generally be defined as any fieldbus that integrates the IEEE 802.3 standard. Despite this all-embracing definition, there are some obvious market leaders throughout the field.

The three largest fieldbuses are:

  • EtherCAT – initially brought to market by the Beckhoff corporation and now supported by the EtherCAT Technologies Group.
  • Profinet - brought to market by the Siemens corporation and now supported by PI North America (in the USA).
  • Ethernet/IP – initially brought to market by the Allen-Bradley Corporation and now part of Rockwell Automation: Supported by the ODVA.

EtherCAT

EtherCAT is a real-time and deterministic bus - two key terms when contemplating industrial Ethernet. The notion of real-time is related to cycle time (or pulses), with real-time cycles generally taking less than 10 milliseconds.

EtherCAT can reach cycle times down to the sub-millisecond range, with several controlled demonstrations generating cycle times as low as two and a half microseconds.

Determinism is associated with changes in the space between pulses. To be categorized as deterministic, industrial Ethernet buses should demonstrate an extremely small amount of change between these gaps. This change is sometimes referred to as 'jitter' in networking terms.

EtherCAT devices usually contain a certain kind of a switched Ethernet interface, which indicates that they naturally support ring and line topologies.

Thus users can daisy-chain devices together and feed them back to the PLC with a single Ethernet cable. This eradicates the need for network switches while also being appropriate for EtherCAT's powerful data communication protocol.

EtherCAT uses an 'on the fly' approach, sometimes known as the data train model. This approach means that data is processed by a node as soon as it realizes that that information is for that node. This completely contradicts standard Ethernet's 'store and forward' mechanism, which must observe the entire frame before acting in response.

Since EtherCAT networks are usually packaged into line or ring topologies, these can facilitate one frame per cycle in contrast to convention Ethernet, which can only accommodate one frame per cycle for each device.

This significantly reduces the bandwidth requirements for the network while enabling identical packaging of the information to a standard Ethernet frame.

However, the way this information is addressed is different. EtherCAT frame includes additional segments that help differentiate between the type of traffic that is being transmitted.

EtherCAT features a real-time data stream that facilitates direct streaming of data from the network access layer up to the application layer, bypassing Ethernet encapsulation steps entirely. Bypassing layers means decreases in delays, enabling exceptionally fast cycle times in EtherCAT.

Profinet

The OSI model for Profinet closely resembles that of EtherCAT. The basic Ethernet portion is reserved for non-time-critical tasks, including configuration, diagnostics, or relaying data to sophisticated IT systems.

A real-time data stream enables direct forwarding of the data from the network access layer to the application layer, sidestepping the encapsulation process.

Profinet differentiates between these types of traffic differently compared to EtherCAT. Profinet utilizes a field known as the 'Ether type', which is contained within a traditional Ethernet header, meaning that standard network infrastructure can be utilized to construct more complex topologies.

Most Profinet-enabled devices contain some sort of switched Ethernet interface. Lines and rings are inherently supported and it may be possible to use regular, unmanaged Ethernet switches to generate stars, lines or drops.

Applying these topologies with EtherCAT would necessitate the use of specialized hardware that maintains the EtherCAT protocol.

Profinet varieties exist across a spectrum, each of which are identified by equipment class gradings - A, B or C. Real-time data streams with cycle times as low as 250 microseconds can be achieved with the latest versions of Profinet, though 1 to 10 milliseconds is common for entire lead times.

Profinet also provides an unsynchronized real-time (IRT) data stream whereby bandwidth is specifically assigned to these tasks and cycle times and can achieve times around 31 microseconds.

Profinet also includes a range of smart networking features to achieve these exceptionally low cycle times. These include:

  • Fast forwarding, whereby as soon as data is identified, it is immediately processed by a node
  • Dynamic frame packing, whereby the Profinet controller recognizes the topology of a network and can distinguish portions that may be in line topologies. If it pinpoints this directly, it can pack information for adjacent nodes into a single frame, limiting the overall number of frames. As information is removed by those nodes in the line, frames are repacked into smaller sizes, facilitating faster transmission times as they travel across the line.
  • Segmentation, whereby Ethernet TCP/IP messages segmented by the transport layer observes real-time frames are introduced in between TCP/IP segments. This means there is no waiting for common Ethernet traffic to transmit before real-time information is received.

Utilizing the IRT channel of Profinet eradicates the need for segmentation because bandwidth is specifically reserved and not shared with standard Ethernet messages.

EtherNet/IP

EtherNet/IP is the closest thing to Ethernet because it does not include a real-time channel. Note, the 'IP' in EtherNet/IP refers to 'industrial protocol,' which is a reference to the common industrial protocol (CIP) - the application layer that EtherNet/IP is based on.

This is the same application layer applied by DeviceNet, ControlNet, and other ODVA supported fieldbuses. This accounts for the common acceptance of EtherNet/IP across the industrial Ethernet community.

EtherNet/IP applies two forms of messaging in place of a real-time data stream; enabling explicit and implicit messaging.

Explicit messaging is called explicit because all of the information a client must communicate with a server is clearly specified within the packet. Advance configuration is not necessary, which is handy when configuring a device on a network using explicit messaging.

It works over TCP, so there is an assurance that these parameters are being applied to the device. However, explicit messaging is slower, so it is retained for the configuration portion of messages

Conversely, implicit messaging requires a certain amount of setup beforehand, but explicitly stating the information in the frame is no longer necessary.

As a result, smaller frames are produced, allowing faster reception of the information when applied in combination with a UDP data streaming protocol. Implicit messaging is typically applied for remote IO feedback to a PLC.

Emerging Trends in Industrial Ethernet

Network standards are ever-changing, and new standards are constantly in development. One of the current industry buzzwords is Time-Sensitive Networking (TSN), which uses distributed clocks within a network. TSN vastly improves determinism and limits variation between data cycles.

None of the common protocols (EtherNet/IP, EtherCAT, or Profinet) presently support TSN, but their websites detail the current efforts being put into implementing this into their standards.

Network bandwidth is continually increasing, and while as little as 10 years ago, 100-megabit communication was the norm, now personal computers can ship with 1-gigabit networking capabilities, which can be expanded to 2.5, 5 or 10-gigabit networking.

This will potentially lead to larger, more interconnected systems – an ongoing trend in smart and connected industry applications.

Industrial Ethernet Solutions from HBK

HBK provides a wide variety of systems and solutions developed for transporting users' equipment signals onto industrial Ethernet buses.

Users of the QuantumX and SomatXR lines – the company's universal data acquisition modules – may already be familiar with the CX27C industrial Ethernet gateway module. This device converts data between standard quantum communication into an industrial Ethernet bus, presenting fast update rates of up to 4.8 kilohertz.

HBK's PMX multi-channel industrial amplifier system can be utilized to gain access to industrial Ethernet networks through a range various industrial Ethernet gateway cards that fit into the PMX. These cards are on hand for EtherCAT, Profinet and EtherNet/IP.

ClipX devices, which are single channel modules that can interface with industrial Ethernet via EtherNet/IP, EtherCAT and Profinet can also be acquired. PMX provides update rates of up to 9.6 kilohertz, with the ClipX supplying update rates around 4 kilohertz.

As many as six ClipX devices can be connected over an internal data bus, and all of these can be interfaced via a single industrial Ethernet gateway.

HBK provides solutions for the direct measurement of torque which can be issued to the industrial Ethernet bus. Torque interface module (TIM) devices, namely the TIM-EC and TIM-PN, are consistent with EtherCAT and Profinet, respectively.

The TIM-EC instruments facilitate direct integration of measurement from a T40B through the proprietary TMC communication bus. Users of HBK's various torque transducers, including the T12 or T21, can also utilize the frequency output on these instruments to link to the TIM-EC, broadcasting measurements from those devices onto the industrial Ethernet bus.

HBK's Genesis HighSpeed line presents an optional interface the EtherCAT which can be acquired on tethered V revision modules. However, such connectivity is only available on the tethered mainframes, but users will receive update rates of up to 1 kilohertz.

Summary

Industrial Ethernet is propelling innovation, automation and quality across a range of industries. This trend is set to continue as technology continues to boost bandwidth capacity increases and calls for a smarter, more connected industry in order to continue to become more common and mainstream.

HBK's wide variety of industrial Ethernet devices, gateways, and compatible hardware are the optimal choice for any business wishing to introduce, refresh or scale-up industrial Ethernet connectivity across its facilities.

This information has been sourced, reviewed and adapted from materials provided by Hottinger Baldwin Messtechnik GmbH (HBM).

For more information on this source, please visit Hottinger Baldwin Messtechnik GmbH (HBM).

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