An overview of

DEVICE LEVEL RING (DLR)

FOR ETHERNET/IP NETWORKS

Overview of the Device Level Ring Architecture

Control engineers discovered the one big disadvantage of linearly segmented topology: when a device failed, all the downstream devices became inaccessible. Any device or cable failure – especially one near the switch – could disable nearly the entire segment. In high availability systems, these failures could be catastrophic.

The Device Level Ring protocol for EtherNet/IP solves this problem. It provides fast network fault detection, detecting and bypassing device or cable failures in as little as ~400-800 microseconds when using beacon-based mode and several milliseconds when using announce-based mode.

DLR Devices

A DLR network consists of 3 primary devices with specialized capabilities:

• Ring Supervisor: A device that manages the ring by recovering from faults and collecting diagnostic information. In DLR mode, ring supervisors send beacon frames from their primary port. A beacon frame arriving at the secondary port validates the ring integrity. In non-DLR mode (failure mode), ring supervisors send messages on both the primary and secondary Ethernet ports.
• Beacon-based Ring Node: A DLR-enabled ring node that monitors the connection with neighboring nodes and reports failures to the ring supervisor. By processing high-speed “heartbeat” beacons, it provides the sub-millisecond recovery required for high-speed motion control and safety applications.
• Announce-based Ring Nodes: DLR-capable devices that support the protocol but lack high-speed beacon hardware. They rely on slower “Announce” messages and link-status detection. With recovery times of several milliseconds, they are best suited for general I/O, discrete manufacturing and non-motion applications.

While not recommended by the ODVA, non-DLR devices (i.e., devices with dual Ethernet ports but no DLR capability) can be inserted into a DLR network. Non-DLR devices cannot report ring failures and create blind spots that make it difficult to pinpoint fault locations. A ring tap can be used to provide full ring capabilities to a non-DLR device. Failure location in a device level ring is more difficult with increasing numbers of non-DLR devices.

Non-compliant devices that fail to meet specific Ethernet implementation guidelines are strictly prohibited because they can have unpredictable and adverse effects on fault detection and ring restoration.

DLR Operation

In a ring topology network configuration, each device gets connected to two others, with the last device connected back to the first, thus creating a circular (i.e., ring) communication path. The ring supervisor logically blocks standard data from passing between its two internal ports while leaving them open for diagnostics.

At regular intervals, the supervisor sends high-speed beacons out of both ports simultaneously, sending frames in opposite directions around the ring. If a beacon fails to reach the supervisor’s receiving port, it identifies a break and immediately unblocks the redundant port, effectively transitioning the network from a ring into two linear segments originating from the supervisor.

In normal DLR operation, the ring supervisor continually sends beacon frames through the ring to verify ring integrity. Beacon frames arriving at the secondary port validate ring integrity. Cable or device failure are detected when the ring supervisor fails to receive beacon frames on its secondary port.

DLR Failure Detection and Recovery

One of the advantages of ring networks is the ability to detect the location of a failure. Ring nodes sensing the loss of a next-door neighbor report the failure to the primary port of the ring supervisor. That process works best if both neighbors have DLR capabilities.

When a failure is detected, a ring supervisor switches into non-DLR operating mode, treating the ring as two linear segments – one operating from its primary port and one operating from its secondary port. While in linear operation, the ring supervisor continues to send beacon frames from its primary port. Once the failure is repaired, the beacon frames again begin to arrive at the secondary port, and the supervisor restores DLR active status to the ring.

In the diagram (right), a failure at ‘A’ would be reported to the ring supervisor by DLR Node Y. A break at ‘B’ though would be reported by the Tap at DLR Node Z and could only be located as someplace between node Z and node Y.

Limitations of DLR

DLR technology is single fault tolerant. The network fails on multiple simultaneous faults in the ring. Another disadvantage of DLR is additional complexity. The DLR object must be configured at each ring node.

Device Level Ring and EtherNet/IP

It is possible to argue that Device Level Ring is irrelevant in discussing the operation of an EtherNet/IP network. Device Level Ring is a protocol designed to handle data transfer in a physical architecture that describes a topology and organization for a network. It says nothing about how that network is used. EtherNet/IP, on the other hand, is about messaging and data organization. It says nothing about how its messages move and over what physical network.

Where EtherNet/IP and Device Level Ring come together is at the embedded Ethernet switch in the EtherNet/IP device. The Ethernet switch must process the messages it receives. Application layer EtherNet/IP messages are processed as they normally would in a star topology network. DLR protocol messages are processed per the Device Level Ring specification. These messages maintain the ring and are never received by the EtherNet/IP device application layer.

If you consult the ODVA specification, there are several Ethernet embedded switches that are qualified for EtherNet/IP operation.

• Common Services: The Device Level Ring Object supports several common services including Get Attribute All and Get/Set Single Attribute.
• Object Specific Services: The Device Level Ring Object supports four object-specific services:
  • Verify Fault Location: Causes a device functioning as a ring supervisor to issue the Locate Fault ring protocol message.
  • Clear Rapid Faults: Causes a device functioning as a ring supervisor to return to normal operation after a fault/restore cycle.
  • Restart Sign On: Causes a device functioning as a ring supervisor to refresh the Device Level Ring participants list.
  • Clear Gateway Partial Fault: Causes a device functioning as a redundant gateway to return to normal operation.

The Industrial Context of Device Level Ring

To understand the practical application of DLR, it is helpful to contrast the types of networks and topologies commonly found in a plant. Even though much of the infrastructure (i.e., cables, switches, routers, etc.) appears to be identical, there is a practical difference to how each network operates.

Types of Factory Networks

The following table describes types of networks in industrial settings.

Note: The original version of this table was developed by ARC Advisory Group, a leading research and advisory firm focusing on technologies and best practices in automation technology.

Information Networks (IT)

Philosophically, information technology (IT) or enterprise networks operate more like a utility. The IT department provides resources and services to an ever-changing set of customers with constantly changing service requirements. Using a centralized management philosophy, IT protects corporate assets and users from each other and the outside world. This dynamic environment is managed using a very structured infrastructure of Microsoft Windows computers, Cisco switches and routers, firewalls and other standard networking hardware.

In the IT world, ring topology is anathema. It’s unreliable, slow and disruptive when new nodes are added. However, the qualities that make a ring topology deficient for corporate IT are actually advantageous for the factory floor and Device Level Ring implementation.

Control and I/O Networks (OT)

The philosophy of operational technology (OT) networks is to build, deploy and test a control network until it meets the reliability, performance and quality goals, then never touch anything again. OT accomplishes this using special topologies, operating mechanisms, protocols and systems.

Factory floor networks across a plant are not only quite different from each other but are an integral component of a specific manufacturing machine or production process. These networks are quite static and composed of a very diverse infrastructure of controllers, actuators and sensors developed by a broad base of vendors.

Control devices come with unlimited varieties of capabilities, operating parameters, communications capabilities and functionality. In this more static yet more diverse world, control engineers build custom networks that must meet specific operating requirements dictated by the product being manufactured.

Advantages of the Device Level Ring Protocol for OT

Device Level Ring is the specific implementation designed to meet these industrial goals by providing several key advantages:

• Network Continuity: Device Level Ring technology is designed such that the network can continue to operate with a single node fault. A single fault will not destroy the entire network as the loss of a single node does in an IT system.
• Deterministic Communication Speed: With fewer messages on the ring (sometimes just one), ring systems can operate slower than other architectures – but that is generally not an issue as machinery systems are usually limited by their underlying mechanical and physical interfaces.
• Rapid Fault Detection and Recovery: Device on the ring can interrogate and monitor the ring status and act when disruptions occur to the ring. Devices can look for alternate paths to reach other process devices. IT systems seldom need this kind of feature.
• Minimal Disruption: Disruption from new nodes is another difference but a moot point for the factory floor. Unlike IT, factory floor systems are very rarely added or removed.

Evolution of Network Topologies

To fully appreciate why the Device Level Ring protocol was introduced, it is important to understand how network topologies evolved.

There are many ways to connect Ethernet nodes together into a network. Early in the life of Ethernet, the Ethernet Bus topology was all that existed. Every Ethernet device was connected by physically tapping into the bus and connecting a device to that tap.

Today, there is a wide variety of physical topologies that can be used including linear, ring, bus, mesh and star.

Star Topology

Star topology is the most prevalent topology used in industrial control systems. In a control system connected using star topology, all network devices are individually connected to a central switch, which is then connected to a higher-level switch, a manufacturing router or a plant-wide router.

One significant advantage of a star topology is that a cable or device failure is not catastrophic. Star networks are robust and extensible. Node failure does not affect the entire network, and it’s nearly effortless to add nodes to a star network.

Disadvantages include the limitations imposed by the number of available switch ports, switch cost, the additional cabling required for each node and the trust needed in the switch reliability as it is the single point of failure in a star network.

Linear Segments

The development of a network switch on an integrated circuit (IC) not only radically reduced the costs of network switches and led to the development of a specialized three-port IC that could be deployed in end devices and enable daisy-chained Ethernet networks. Using this technology, Ethernet resembled the RS-485 Modbus networks of earlier generations.

In linear networks, all devices use an embedded three-port switch with two ports for network connections and one port for an internal connection to the device. Linear topology is very prevalent in manufacturing applications containing a large number of devices as the per switch port cost of a device is radically smaller than other EtherNet/IP network topologies. The advantages of linear topology are lower-cost deployment and the ease of adding additional device nodes.

The big disadvantage to linear networks is that any cable or device failure makes all downstream devices inaccessible. For that reason, many control engineers avoid connecting critical control equipment using linear networks.

Another, less obvious problem is linear segmenting tends to hide performance problems from the control engineer. If the autonegotiation sequences between one pair of devices in a linear segment results in half-duplex or lower baud rate operation, message traffic downstream of that portion of the segment will operate less efficiently. This problem is hard to detect but not difficult to prevent. All devices in a linear segment should be configured for full-duplex with the highest possible baud rate for the network and autonegotiation disabled.

Hybrid Topology

Some manufacturers prefer to use a hybrid topology, a combination of the other. In hybrid type networks, two or more of the other topologies are implemented in different parts of a manufacturing system to take advantage of the different features of each topology. While this provides the control system designer with the various advantages of the different technologies, it adds to the complexity of the system and makes it less maintainable.

Ring Topology

Ring topology is very prevalent in manufacturing applications where network and process availability are key requirements. In ring type networks, devices are daisy-chained to each other with the last device connected back to the first device and messages sometimes traveling in either direction around the ring.

Usually, some sort of a ring master manages traffic on the network, preventing messages from circling endlessly and managing device and network failures. Devices lacking the dual Ethernet port required to participate in a ring can use a tap device to participate in a ring network. The significant advantage of a ring topology is that control systems can continue operation after a cable or even device failure. The major disadvantage to rings is significant additional network complexity and the extra cost of ring enabled end-devices and tap devices.