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Introduction to Real-Time Ethernet II the EXTENSION JULY–AUGUST A Technical Supplement to Control Network Volume 5 Issue 4 © 2004 Contemporary Control Systems, Inc. Introduction to Real-Time Ethernet II By Paula Doyle, a doctoral researcher with the Circuits and Systems Research Centre at the University of Limerick in Ireland INTRODUCTION IEEE 1588 defines two separate types of clocks: In “Real-Time Ethernet I”, we introduced the basic ordinary and boundary. Boundary clocks (BC) are concepts of Ethernet’s capacity to deliver a real-time employed in devices such as hubs or switches—where (RT) communication system. “Real-Time Ethernet II” more than one PTP communication path (port) exists. introduces some of the RT solutions available to Ordinary clocks exist in devices having a single port— e.g., normal network devices. Each BC port can act as industry today*: PROFInet, EtherCAT and ETHERNET Powerlink. It also provides an introduction to a single a master or ordinary clock in its own segment. standard, IEEE 1588 that is growing in popularity PTP is for networks that support multicasting but amongst RT Ethernet developers to provide sub- keep multicasts within a subnet and where each local microsecond synchronization accuracy of distributed clock fulfills exacting requirements. The grandmaster clocks over Ethernet. clock (GMC) is the best clock in the system—with the best inherent stability, accuracy, resolution, etc. * EtherNet/IP is included in the full article available at defined by the standard [2]. The Best Master Clock http://www.ccontrols.com/pdf/volume5n4.pdf Algorithm (BMC), run by every live node, determines IEEE 1588 clock quality. Within each subnet, the BMC determines the master clock; in a single-subnet system the master IEEE 1588 [1] specifies “A protocol to synchronize is the GMC. independent clocks running on separate nodes of a The GMC determines system synchronization; distributed measurement or control system to a high system clocks synchronize their subnet clocks to the accuracy and precision.” IEEE 1588 is, or will be, system. There is only one GMC per system, and only incorporated into EtherNet/IP, ETHERNET Powerlink, one master clock per subnet. EtherCAT and PROFInet—making it a popular standard Synchronization is performed as follows. All for delivering RT over Ethernet. masters periodically broadcast “Sync” messages In IEEE 1588, all network nodes down to the containing an estimate of the time the message will transducer level contain an IEEE 1588 clock, physically leave the master. The precise receipt time of synchronized with all network peers (see Figure 1) using these messages is noted at the slaves. The precise Precision Time Protocol (PTP). At device level, sensors sending time of the message is noted at the can timestamp their data locally and actuators can grandmaster. All precise timing measurements are operate at a precise time, avoiding stack and performed as close to the physical layer as possible—to application delays between transducer and controller. eliminate the delays from the network stack and The accuracy of the system depends on the operating system—while the estimated times are synchronization of local RT clocks. calculated by the IEEE 1588 code at the Application Layer (see Figure 2). Following the Sync message, the M master transmits a related “Follow_Up” message Rules: Node 1588 1 Grandmaster / System GMC containing the precise sending time of the Sync 1 Master / Subnet S Node 1 Slave / Switching Device OC message. A slave uses the transmission and reception Ethernet times to calculate its offset and can initiate 1588 synchronization with the delay measurement, which is S BC Switching Device not periodic and not performed as often as the S M M Node 1588 OC 1588 protocol synchronization. Sync messages do not BC BC S Node S Node Ethernet propagate beyond their originating subnet. OC OC The resolution of the system clock is the resolution Ethernet of the GMC. If required, the GMC can be synchronized S 1588 BC 1588 to an external source such as GPS. BC Switching Device M 1588 IEEE 1588 is a highly precise system for BC M synchronizing distributed nodes for applications such as motion control and robotics. It was designed for Figure 1—IEEE 1588 Configuration multicasting networks but with the popularity of Industrial Ethernet, Annex D was included for an (No part of the Extension may be reproduced without the written consent of Contemporary Controls.) 1 Ethernet implementation of PTP. Although IEEE 1588 can deal with RT and non-RT traffic. It prioritizes does not alter Ethernet or make it more deterministic or RT traffic and provides full-duplex links for all ports. reliable, it does provide a method for other protocols Contemporary switches (even cut-through) add jitter to do so. A highly synchronized system of distributed that would impact on determinism. PROFInet switches nodes—coupled with an application for handling minimize jitter to where it has a negligible effect. The resolution and controlling traffic—could deliver hard, PROFInet communication model allows both RT and deterministic RT over Ethernet. non-RT traffic to co-exist on one network without additional precautions. By 2005, PROFInet-IRT and SRT will incorporate PROFISafe, the PROFIbus safety solution for Estimate Times Application Layer manufacturing and processing industries. Calculated PROFInet, of all the solutions discussed here offers Network Protocol the greatest determinism—and since this is built into Stack & OS ms delay the PROFInet-IRT device, the systems engineer is Precise Times Sync Detector & Timestamp spared from the burden of configuration to guarantee Calculated Creator RT communication. Physical Layer ns delay EtherCAT EtherCAT (Ethernet for Control Automation Figure 2—IEEE 1588 node timing Technology) is the motion-control RT solution from Beckhoff. It can process 1000 I/Os in 30 µs [5], but PROFInet requires full-duplex. It can use copper or fiber optic cables. EtherCAT is based on the master/slave principal PROFInet [3] is a plant-wide fieldbus standard for and can interoperate with normal TCP/IP-based distributed automation systems. It uses object- networks and other Ethernet-based solutions such as orientation and available IT standards (TCP/IP, Ethernet, PROFInet. It also supports any Ethernet topology, XML, COM). PROFInet is also built on IEEE 802.3 and including the bus. is interoperable with TCP/IP—allowing it to be The EtherCAT master processes RT data via implemented on existing Ethernets. It is compatible dedicated hardware and software (Beckhoff currently with PROFIBUS-DP. use their PC-based TwinCAT OS and TwinCAT Y PROFInet V1, has a response time of 10-100 ms. driver). In the future, further variations will be PROFInet-SRT (Soft Real-Time) allowed PROFInet to introduced that will also provide the same guarantees. work with a factory automation cycle time of 5-10 ms, The current master prioritizes EtherCAT frames over achieving RT solely in software. It uses TCP/IP and a normal Ethernet traffic, which is transmitted in gaps. dedicated software channel for RT communications. The master controls traffic by initiating all transmissions. PROFInet-IRT brings a hard-RT element to the The telegrams are standard Ethernet, and the data PROFInet protocols. The three PROFInet protocols field encapsulates the EtherCAT frame (an EtherCAT allow differing degrees of RT. PROFInet for hard RT header and one or more EtherCAT commands). Each is PROFInet-IRT. command contains a header, data and Working Counter PROFInet-IRT (WC) field. Each Ethernet telegram can contain many EtherCAT commands—realizing a higher bandwidth PROFInet IRT (Isochronous RT) was developed for and more efficient use of the large Ethernet data field systems requiring sub-microsecond synchronization, size and header (see Figure 3). The standard Ethernet typically high-performance motion control systems. CRC is used to verify message correctness. The benchmark for such a system is 1 ms cycle time, 1 µs jitter accuracy, and guaranteed determinism [4]— 14 46 - 1500 4 which IRT fulfills. Ethernet Header Data Ethernet CRC Since software introduces jitter above 1 µs, IRT (unlike SRT) is a hardware solution with highly 2 EtherCAT synchronized Ethernet nodes. Using full-duplex EtherCAT Command EtherCAT Command Header switched Fast Ethernet, it divides the communication cycle into a standard TCP/IP open channel and a 10 2 deterministic RT channel. The channel ratio is system- Working Header Data Counter dependent and is chosen by the systems engineer. Figure 3 Each PROFInet-IRT device has a special ASIC EtherCAT Encapsulation (Application-Specific Integrated Circuit) for handling The EtherCAT master fully controls its slaves. Its node synchronization and cycle subdivision and commands only elicit responses; slaves do not initiate incorporates an intelligent 2 or 4 port switch. transmissions. The two EtherCAT communication The PROFInet switch in every node is highly methods used are “Ether Type” or UDP/IP encapsulation. synchronized, contains a schedule of bus access and 2 The “Ether Type” uses the type field (defined in with no special ASICs. EPL can deliver a cycle time of Ethernet II), which is more commonly known as the 200 µs with jitter under 1 µs. Its frame is encapsulated length field in IEEE 802.3. The Ether Type implementation as illustrated in Figure 4. does not use IP, thus limiting EtherCAT traffic to the originating subnet. Encapsulating commands using 14 46 - 1500 4 UDP/IP allows EtherCAT frames to traverse subnets, Ethernet Header Data Ethernet CRC but has drawbacks. The UDP/IP header adds 28 (20: Ethernet II Frame IP, 8: UDP) bytes to the Ethernet frame and under- mines RT performance through its non-deterministic 1 11 Destination Source Service ID Data stack. EtherCAT slaves range from intelligent nodes to Address Address 2-bit I/O modules and are networked via 100Base-TX, Powerlink Frame fiber optic cable or E-bus (depending on distance requirements). E-bus is an EtherCAT physical layer for Figure 4—PowerLink Encapsulation Ethernet offering a LVDS (Low Voltage Differential Signal) scheme.
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