What is EtherCAT Open Mode?

EtherCAT has two modes of operation: Direct mode and Open mode. Direct mode is commonly used and is known for its efficient real-time performance, while Open mode allows integration with existing IT infrastructure through standard switching devices, albeit with higher latencies. In this document, we will explore both modes and highlight their key characteristics.

Even though the two EtherCAT modes, Direct mode and Open mode, were already defined in the first EtherCAT specification V1.0 from 2004, nearly all EtherCAT networks today use Direct mode. In fact, most users have never even heard about Open mode.

Currently, the specification is available in the document titled "ETG.1000.3 EtherCAT Specification Part 3 – Data Link Layer service definition"

EtherCAT Direct Mode

In Direct mode, one EtherCAT segment is connected to the MainDevice as shown in Figure 1. The MAC address fields of the Ethernet frames are ignored. For communication, all EtherCAT SubDevices use EtherCAT SubDevice Controllers (ESCs), while the MainDevice uses a standard Ethernet port.

Direct mode is the standard in applications using the EtherCAT Device Protocol (EDP). It does not require switches, as the SubDevices typically have two or more ports that enable daisy chaining and other topologies. In Direct mode, the Ethernet controller in the MainDevice is exclusively used for EtherCAT.

One of the key advantages of Direct mode is the "Processing on the Fly" principle, which, along with the very low delays in the SubDevices, allows for hard real-time measurement and control applications with cycle times of one millisecond and faster. The "Propagation Delay" is highly consistent across the network segment, and is only slightly affected by temperature fluctuations, thanks to hardware-based processing in the SubDevices (handled by the EtherCAT SubDevice controller). The total delay per SubDevice is a maximum of one microsecond.

Figure 1: EtherCAT Segment in Direct mode

EtherCAT Open Mode (EOM)

While Direct mode is typically used for its efficiency and real-time performance, Open mode offers more flexibility by allowing integration with existing IT infrastructure. In the EtherCAT Open Mode (EOM), one or several EtherCAT segments may be connected to a standard (existing) switching device, as shown in Figure 2. This mode is particularly useful when integrating EtherCAT networks into larger, shared IT environments.

To enable usage of EOM, the MainDevice first must support EOM and be configured accordingly in an EtherCAT Configuration tool like EC-Engineer. The first device per segment must provide additional functions to access the EtherCAT segment from the MainDevice, which is why this first SubDevice is called the "Segment Address Device."

  • The MainDevice must support and be configured for EOM.
  • The first device, known as the "Segment Address Device," provides access to the EtherCAT segment from the MainDevice. One benefit of EOM is that all other SubDevices after the Segment Address Device can operate without any modifications, just as they do in EtherCAT Direct Mode.

The first device within an EtherCAT segment has an ISO/IEC 8802-3 MAC address representing the entire segment. This device has a feature called the "EOM port." The EOM port replaces the destination address field with the source address field and the source address field with its own MAC address within the Ethernet frame. This ensures that, if the frame follows the coding rules of EtherCAT, it will be returned to the MainDevice after processing by all SubDevices.

If this type of frame is transported via UDP, the EOM port will handle the source and destination IP addresses and the UDP source and destination port numbers in the same way as the MAC addresses. This ensures that the response frame fully satisfies UDP/IP protocol standards.

Additionally, the EOM port protects the SubDevices within the segment against unauthorized access by MainDevices or generic Ethernet devices.

EtherCAT Open Mode also uses the EtherCAT Device Protocol (EDP) to communicate between the MainDevice and SubDevices via the switched network. Additionally, other protocols, such as TCP/IP, can share the same IT infrastructure.

Using a switched network in EtherCAT Open Mode introduces trade-offs. On the positive side, Open Mode provides flexibility, enabling the integration of EtherCAT with existing IT infrastructure. However, since all Ethernet frames share the same infrastructure and switches typically operate on a "Store and Forward" basis, this results in significantly higher latencies within an EtherCAT segment. These latencies are not consistent and cannot be precisely calculated in advance, which impacts the minimum achievable cycle time. The achievable performance depends heavily on the specific IT network, its configuration, and current network load.

Key Trade-offs Between Flexibility and Latency:

  • Flexibility: Integration with existing IT infrastructure, allowing for mixed operation.
  • Latency: Higher and inconsistent latencies due to shared infrastructure and 'Store and Forward' switching.
  • Cycle Time Impact: Minimum achievable cycle time is affected by network configuration and load.

Figure 2: EtherCAT Segments in Open mode

EtherCAT Open Mode: Communication Types

The communication type for the EOM network segment can be set to "RAW" or to "UDP". Mixed operation among different network segments within the same IT infrastructure is possible.

  • RAW: This communication type is typically chosen for applications requiring minimal processing overhead and the highest possible performance. RAW communication directly utilizes Ethernet frames without additional encapsulation, resulting in lower latency.
  • UDP: This communication type is used when compatibility with IP-based networks is needed. By encapsulating EtherCAT frames within UDP/IP, the network can leverage existing IP routing and infrastructure, providing greater flexibility at the cost of higher latency.

Communication Type: RAW

In the RAW communication type, VLAN tags are utilized. The outgoing frame's Destination MAC address corresponds to the EtherCAT Segment address and is verified by the EOM port of the segment address devices. Once confirmed, the frame is routed to the SubDevices.

The Source MAC address is assigned by the network card in the MainDevice. The EOM port then swaps the Source and Destination addresses, ensuring the processed frame returns to the MainDevice.

Principle frame structure of an EtherCAT frame with VLAN tag

Outgoing frame: MainDevice to SubDevices

Incoming Frame: SubDevices to MainDevice

Communication Type: UDP

In the UDP communication type, an EtherCAT frame is embedded into a UDP/IP frame. In this case, the EtherType for IP (0x0800) is used, and the destination port number is 0x88A4, as assigned by the Internet Assigned Numbers Authority (IANA). The maximum length of such a frame is 1,518 bytes.

The handling of MAC addresses (destination and source) is similar to RAW Communication, with IP addresses set accordingly. In the outgoing frame, the source field contains the address of the MainDevice, and the destination field contains the address of the Segment Address Device (EOM port). For the incoming frame, these addresses are swapped to ensure proper routing back to the MainDevice.

Outgoing frame: MainDevice to SubDevices

Incoming Frame: SubDevices to MainDevice

Example of a Segment Address Device: Beckhoff EK1000 Coupler

The Beckhoff EK1000 EtherCAT device can be used as a Segment Address Device, serving as the interface between EtherCAT Open Mode and standard EtherCAT SubDevices using EtherCAT Direct Mode. Key capabilities of the Beckhoff EK1000 include:

  • Ethernet Port X001: Connects to a switching device, enabling operation in EtherCAT Open Mode by handling MAC and ID addresses.
  • Internal EOM Port: Manages data handling and forwards the EtherCAT frame via the integrated EtherCAT SubDevice Controller (ESC).
  • EtherCAT Port X002: Allows connection of additional EtherCAT SubDevices.
  • Right Side Expansion: Any Beckhoff EtherCAT slices can be connected to the right side of the device.

For more detailed information, visit Beckhoff's EK1000 product webpage: https://www.beckhoff.com/ek1000

Beckhoff®, EtherCAT®, are registered trademarks of Beckhoff Automation GmbH.

 

Summary

In conclusion, EtherCAT provides two modes of operation: Direct mode and Open mode, each serving different purposes based on application requirements. Direct mode is highly efficient, offering minimal latency and real-time performance, which is ideal for control applications that demand quick response times. On the other hand, Open mode provides greater flexibility by allowing integration with standard IT infrastructure, though this comes at the cost of increased and variable latency. Understanding the trade-offs between performance and flexibility is key to selecting the appropriate mode for your application.