Ethernet Powerlink
Ethernet Powerlink ( official spelling: Ethernet POWERLINK) is a real-time Ethernet to transmit real-time data in the microsecond range. The main purpose is the transmission of process data in automation technology.
Ethernet POWERLINK was originally developed by B & R and is used today by the open end user and vendor group EPSG (Ethernet Powerlink Standardization Group ) developed as an open standard and specified.
- 3.1 Device Profiles
- 3.2 Electronic Data Sheets
Survey
Designed from the outset with consideration for standards compliance, leads Ethernet Powerlink a mixed polling and time slice mechanism for deterministic transmission of data. This will be achieved:
- The guaranteed delivery of time-critical data in very short isochronous cycles with a configurable time behavior
- The time synchronization of all network nodes with very high precision in the sub- microsecond range
- The transfer of the less time-critical data traffic in the reserved asynchronous channel
Current implementations of Ethernet Powerlink achieve cycle times of less than 200 microseconds and a temporal precision (jitter ) of less than 1 microseconds.
Ethernet Powerlink also specifies a system modeled on the CANopen communication protocol for exchanging user data with nodes in the network. Both parts together are dealt with by a Powerlink protocol stack. For this no special hardware is required, making it easy to both master and slave nodes realized with standard Ethernet devices. We have therefore also open-source master and slave stacks for various operating systems.
Data transmission
Since Ethernet Powerlink is located on the layers 2 and 7 in the OSI layer model, it is in principle from physics used ( Layer 1) independent. In practice, however, it will mainly operated with twisted-pair cables as Fast Ethernet ( 100Base -TX). Both the commercial and the industrial 8P8C/RJ-45 M12 connections are allowed. The use of optical fibers is also possible, but with the additional delays are taken into account by the media converter.
For a clean cabling the Ethernet Powerlink standard refers to the IOANA Guide to planning and installation of industrial networks ( IAONAs Industrial Ethernet Planning and Installation Guide ). To minimize delay and jitter, repeating hubs are recommended instead of switching hubs (switches) in the real-time domain, however.
Versions
Ethernet Powerlink currently exists in two versions:
Version 1 (Ethernet type 0x3e3f ) is a proprietary approach of B & R, but it did open early as the basis for further developments.
Version 2 (Ethernet type 0x88ab ), however, is the current Laid-Open standard EPSG, which was extended by various mechanisms (CANopen device profiles, Powerlink Safety, Electronic Data Sheets, Master Poll Response ).
Although both types of protocols have strong similarity, the following information applies only to version 2 because version 1 is considered as a transitional solution. Some participants can work with both protocol versions.
2006, the development of Gigabit Powerlink was announced by the EPSG.
Data format
Each Powerlink packet consists of a header and the actual payload. This package is part of a normal Ethernet frame, which must have a size between 64 bytes and 1500 bytes. Jumbo frames ( > 1500 bytes ) are not allowed in a Powerlink network. As Ethertype for Ethernet Powerlink 0x88AB assigned by the IEEE.
The Powerlink header itself is composed of:
It defines the following message types (Message Type):
Real-time communication
In order for a deterministic data transmission can be guaranteed, collisions must be avoided on the network. For this, the data transmission by a particular subscriber, the Managing Node ( MN), controlled. The individual network participants, the Controlled Nodes (CN) may only transmit when they were decidedly prompted.
A cycle starts with the message Start of Cycle (SoC ). Then, each node individually from MN with a Poll Request ( PReq ) is queried, to which the CN with a Poll Response ( PRes ) responds. Since the responses are sent as Ethernet Multicast, other Powerlink devices can listen. Thus, cross-traffic between the CNs is possible. In order to keep the cycle time is small, each device must not be queried in each cycle ( Multiplexed Stations). The response time of a device ( ) is an important quality characteristic.
After completion of the cyclic phase asynchronous phase begins with the start of Asynchronous (SoA ) package. In this phase, each from a particular MN CN send non- cyclic data. About special gateways can be used in the asynchronous phase data from a normal, non- deterministic network and the Powerlink network exchange.
Object directory
Following the CANopen fieldbus all communication objects and all user objects in an object dictionary (OD ) are summarized in Ethernet Powerlink. The object directory is in the Powerlink device model the link between the application and the communication unit. Each entry in the object directory represents an object and is characterized by a 16 -bit signal index. Per index may in turn contain up to 256 sub-indices. This can be differentiated up to 65536 × 254 Nutzeinträge per device. ( The sub-indices 0 and 255 can not be used freely. ) In profiles, the assignment of communication and device profile objects to a respective index is well defined, and thus it is with the object directory is a single interface between the application and the communication defined externally.
Device Profiles
For a number of device classes device profiles have been defined. This device profiles define the functionality and the structure of the object directory for the respective devices. By the use of devices corresponding to a particular profile, a higher independence is achieved by equipment manufacturers. Ethernet Powerlink used while the device profiles of CANopen. Transformation rules determine which objects are the CANopen device profiles are used in Powerlink devices. Here it is taken into account that at Powerlink the length of the user data ( PDOs) is larger.
Electronic Data Sheets
For the use of Powerlink devices electronic data sheets are necessary. These are called XDD files ( XML Device Description ) is stored corresponding to the standardized XML format according to ISO 15745-4, and describe both the main parameters of the objects in the object dictionary of a device as well as other parameters such as the supported communication services. Configuration tools can read data sheet files and communicate with their help with the device and parameterize it if necessary.
Standardization
Ethernet Powerlink was included in the standards IEC 61784-2, IEC 61158-300, IEC 61158-400, IEC 61158-500 and IEC 61158-600. ( The standard IEC 61784-2 specifies communication profiles, the IEC 61158 standard services and protocols of fieldbuses. )
In the asynchronous phase can send any Ethernet frames. Therefore, inter alia, all IP-based protocols at higher layers, such as TCP, UDP and above, are used in the Ethernet Powerlink network. Specifically, Ethernet Powerlink supports the following standards:
- IEEE 802.3 ( Fast Ethernet)
- IP-based protocols (ICMP, UDP, TCP, ...)
- Standard device profiles: CANopen EN 50325-4 for automation
- IEC 61588 for the real- time domain synchronization (future versions )
Diagnosis
It can use standard diagnostic tools such as Wireshark ( freeware ) or OmniPeek be used ( commercial). For more specific diagnostic tools appropriate to the EPSG website are listed.
Transmission of safety- critical data
For safety- critical applications, Powerlink can be expanded with the additional open openSAFETY protocol (formerly Ethernet Powerlink Safety ). The safety- critical data is split into two subframes at openSAFETY and protected with checksums. The safety function of the network is provided by a separate safety controller. Secure and non-secure participants can co-exist in a network and exchange non-essential data for the safety function.
OpenSAFETY is implemented as a protocol for the Application Layer. As such, it can be implemented on a variety of Industrial Ethernet network topologies. openSAFETY is tested by TÜV Rheinland and TÜV Süd, and for use in safety-critical applications according to IEC 61508 SIL 3 and Category 4 of the Euro standard 954-1 released.
Others
Ethernet Powerlink should not be confused with Power over Ethernet, the power supply through the ( unused ) wire pairs, or with PowerLAN.