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INTRODUCTION

In "Real-Time Ethernet I," Module 401, we introduced the basic concepts of Ethernet's capacity to deliver a real-time communication system. "Real-Time Ethernet II," Module 402, introduces some of the real-time solutions available to industry today: EtherNet/IP, PROFInet, EtherCAT and ETHERNET Powerlink. This module also provides an introduction to a single standard, IEEE 1588, that is growing in popularity amongst real-time Ethernet developers to provide sub-microsecond synchronization accuracy of distributed clocks over Ethernet. .


What is Ethernet/IP?

EtherNet/IP (EIP, where the IP stands for Industrial Protocol) is an open application-layer protocol developed and maintained by CI (ControlNet International), ODVA (Open DeviceNet Vendors Association) and IEA (Industrial Ethernet Association). It is built on the existing IEEE 802.3 physical/data layers and TCP/UDP/IP—giving it optimal interoperability with most information-level networks. EIP offers a real-time (RT) solution when using strict guidelines, but is not deterministic. It uses the open object-oriented CIP (Control and Information Protocol) as its application layer—the same layers 5–7 as both DeviceNet and ControlNet—yielding full interoperability with them (Figure 1).




Figure 1 — Ethernet/IP Stack

CIP [1] is a flexible and scalable automation protocol well-suited to distributed systems with properties such as: object-orientation, Electronic Data Sheets and device profiles. EIP with CIP is not an RT protocol. To achieve RT for EIP, CIPSync (a high-speed CIP synchronization solution) is employed. CIPSync is based on IEEE 1588. Using 100 Mbps Switched Ethernet, CIPSync can deliver synchronization accuracy of better than 500 nanoseconds between devices [2] although jitter introduced by the protocol stack will still be an issue.

EIP uses both TCP and UDP with IP for communication. When a connection-oriented exchange is preferred, e.g. at initialization, TCP is used (Explicit Messaging). Explicit Messaging contains protocol information and service information but does not have strict timing requirements; therefore, it is sufficient to use the slower, yet guaranteed TCP protocol. For RT traffic, EIP uses the unicast and multicast capabilities of UDP to implement the producer/consumer model of communication— popular with control applications. Implicit messages contain no commands, only data. The meaning of this data is configured at initialization, reducing run-time processing in the nodes. [PD1] Typical traffic on an EIP network is cyclic (although CIP also specifies polled, change-of-state and strobed traffic). Network collisions are avoided by switches, and EIP is generally implemented in a star topology.

Unlike other RT solutions, EIP uses UDP/IP for RT communication, adding jitter and non-determinism. If this jitter is quantifiable and does not infract on the system model, the system can still be RT but will be unsuitable for fast and hard RT systems like motion control. Specific advice relating to implementing an RT version of EIP is offered in [3], and although EIP with CIP is not an RT protocol, the currently achievable end-to-end response time in an EIP control system of eight producers and one consumer was determined to be 7 ms—when implemented with the recommendations in the following paragraph.

One recommendation in [3] introduces VLANs and locates all devices sharing time-critical data in the same VLAN so RT multicast EIP traffic will not need to exit the VLAN. Another recommendation is to use the routing functionality of Layer 3 switches and set the TTL of IP multicasts to 1. This keeps RT EIP traffic in its subnet and keeps normal traffic out. Hence, according to [3], it is possible to achieve RT performance between EIP devices (using CIP) if the following guidelines are met:

  1. Devices sharing RT information must co-exist in the same subnet.
  2. The EIP segment must be isolated from the main network multicast traffic.

EIP use of CIP with commercial off-the-shelf components, including TCP/UDP/IP, for all devices will benefit certain customers while an EIP solution using CIPSync will be more beneficial to those requiring sub-microsecond synchronization accuracy.

EIP can cater for many RT systems where the device count is limited, device synchronization is in the order of microseconds and determinism is not required.


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