Ethernet POWERLINK - The real-time Ethernet
What is that?
Powerlinks nowadays serve to replace or supplement the field buses which (for example) connect field devices / actuators etc. in an installation with a control unit. As such, the Ethernet Powerlink belongs to the real-time Ethernets. They make it possible for example, to create a computer network of computers at the office level with those in production installations and in doing so, to connect all of the important components. In this example, the computers in the production installation would be called “field devices”. The networking of these computers is called “vertical integration of the automation technology”, and it is very essential in automation technology, where this technology has become indispensable. However, it is problematic for standard Ethernets as they do not transmit their information in real-time, but a delay of approximately 1 ms must be expected.
The Ethernet Powerlink transmits data in the microsecond range. Originally, it was produced by B&R, and nowadays it is developed further by the Ethernet Powerlink Standardization Group.
Currently, Ethernet Powerlink is being offered in two versions.
- Ethernet type 0x3e3f: Produced by B&R, this was previously used as the basis for further development. As such, it was only a kind of intermediate solution.
- Ethernet type 0x88ab: This is the current version, which in comparison with the first version already had been developed further, e.g. CANopen device profiles, Powerlink Safety, electronic data sheets, master poll response.
The advantage of Ethernet Powerlink over other standard Ethernets is the fact that it is guaranteed to transmit the data, and it does this in very short, isochronous cycles using configurable time response. Ethernet Powerlink also can synchronize all network nodes in time (this is done in the microsecond range). Currently, it reaches cycle times below 200μs and a jitter of less than 1μs.
For data exchange with nodes in networks, it is possible to specify a communication protocol similar to CANopen through Ethernet Powerlink. When both elements operate together, they are treated by a so-called Powerlink protocol stack that requires no special hardware. In this way it is possible to realize master and slave nodes with standardized Ethernet components. As such, they are also available for different operating systems. Ethernet Powerlink also uses the device profiles of CANopen.
Ethernet Powerlink is located in the OSI layer model. This model was developed as the design basis for communication protocols in computer networks. It has communication tasks in seven layers that build upon each other (application layer, presentation layer, session layer, transport layer, network layer, data link layer, bit transfer layer). As such, each layer has special requirements.
How does the data transmission function?
Ethernet Powerlink is normally operated with twisted-pair cables and in this case, it is a Fast Ethernet. The readily available 8P8C/RJ-45-, the M12 plug-in connectors and glass fiber cables can be used with this. However, in the case of the latter, additional delays as a result of media converters can be expected.
Wiring in accordance with the Ethernet Powerlink standard (according to the IOANA guidelines) guarantees that planning and installation function correctly. So-called repeating hubs instead of switching hubs should be used in order to minimize the delay and the jitter (if desired).
Naturally, collisions in the network must also be prevented. Deterministic data transmission can only be guaranteed in this way. This is ensured by so-called controlled nodes (CN) which are only permitted to transmit when requested. This controls a managing node (MN).
The start of a cycle forms the start of cycle and/or SoC. Now each node is polled from the managing node with a poll request (PReq), whereby the controlled node is answered by a poll response (PRes). Here, other Powerlink devices can “listen”, as the answers are sent as an Ethernet multicast. The advantage of this is that, this way, the controlled nodes can communicate with each other. Not every device has to be polled. This avoids long cycle times. The asynchronous phase starts after the cyclic phase. It starts with the start of asynchronous (SoA). Here, it is possible that in each case, one controlled node selected by the managing node sends data. Data from normal, i.e. non-deterministic networks and the Powerlink network can be exchanged via special gateways.
The Powerlink Packet
The Powerlink packet consists of the header and the payload. Everything is contained in a normal Ethernet frame, which should have 64 to 1500 bytes. It is important to know that so-called jumbo frames, i.e. those with a size in excess of 1500 bytes, are not permitted in a Powerlink network. Such jumbo frames are not standardized and are excessively large. They are only practical when the protocol overhead can be minimized by them. However, this must first be tested by checking if switches or routers can handle a jumbo frame and if this results in a speed advantage.
A Powerlink header consists of...
- 1 bit reserved
- 7 bit message type
- 8 bit target node number
- 8 bit source code number
Defined message types are SoC, which defines the start of a new cycle, PReq, which polls cyclic data of the CN, PRes, which sends current cyclic data of the CN, SoA, which signals the start of the asynchronous phase and ASend, which sends the asynchronous data.