Getting on the Right Motion Bus
- Friday, 01 February 2008
POWERLINK (ethernet-powerlink.org) also employs TDMA to assure hard real-time performance, but instead of a distributed TDMA like PROFIBUS it implements a poll-select procedure. The POWERLINK master splits communication cycle split into a synchronous period and an asynchronous period. The master (called a manager) sends a poll request message to each slave (called a controller) and each slave answers with a poll response message. At the end of the synchronous period, the master requests non-time-critical data from the slaves. Standard Ethernet traffic is not allowed on a POWERLINK network so gateways are used to encapsulate standard Ethernet traffic on the network. POWERLINK is stated to provide less than 1 μs clock jitter and a cycle time of 100 axes at 1 ms.
EtherCAT (EtherCAT.org) uses a standard network interface card on the master but uses specialized hardware on each of the slaves and uses the telegram structure of Ethernet but with a different operating mode. Instead of individual messages being passed from master to slave, EtherCAT uses a single message that passes through all slave devices, like a train passing through a number of stations on a route. Each slave device incorporates a special ASIC or FPGA that can read and write data into the packet while the frame passes by the node, similar to loading and unloading a train while it passes through a station. This enables EtherCAT to offer extremely high performance. Like Powerlink, standard Ethernet devices cannot coexist on an EtherCAT network and gateways are used to encapsulate Ethernet packets into EtherCAT frames. EtherCAT is stated to provide less than 1 μs jitter and a cycle time of 100 axes at 100 μs.