`Ultra high speed` Ethernet reaches down to I/Os

The German industrial computing specialist Beckhoff has developed a real-time, industrial implementation of Ethernet which, it says, can be used right down to individual I/O points.

The EtherCAT (Ethernet for Control Automation Technology) system, launched at Hannover, will service 1,000 digital I/O points in 30 microseconds.

It can also communicate with 100 servo axes in just 100 microseconds. During this time, all axes are provided with set values and control data and report their actual position and status. The axes can be synchronised with a deviation of less than 1 microsecond.

The system uses special chips in each I/O terminal, but the other components are standard Ethernet cards, cable, and software. The I/O terminals can be connected in simple linear fashion, the way Ethernet was originally used, without worrying about collisions affecting the determinism.

The I/O system can be installed using either twisted-pair cables or optical fibres. Users who want to incorporate other Ethernet devices into a network, can add branches using electronic switches.

Unlike other recent real-time Ethernet implementations, such as Powerlink and Profinet V3.0, which require special connections in the controller, EtherCAT uses standard Ethernet cards costing £50 or less.

The EtherCAT technology does not use high-level TCP or UDP protocols. It relies on sending short "telegrams" through the system. The Ethernet packet is no longer received, then interpreted and copied as process data at every connection.

A newly developed FMMU (fieldbus memory management unit) in each I/O terminal reads the data addressed to it, while the telegram continues through the device. Similarly, input data is inserted while the telegram passes through. The telegrams are delayed by only a few nanoseconds.

The Ethernet terminals are contained in Beckhoff`s modular bus terminal housings. But unlike the standard bus terminals for Profibus or Interbus, there is no internal sub-bus; the Ethernet protocol remains intact right up to the individual Ethernet terminal. In the PC, Beckhoff`s TwinCAT network card operates as a fieldbus card.

Because the Ethernet functionality of the operating system is maintained, all operating system-compatible protocols can be used in parallel on the same physical network. This not only includes standard IT protocols such as TCP/IP, HTTP, FTP and SOAP, but also Ethernet protocols such as Modbus TCP, ProfiNet or Ethernet/IP.

Beckhoff says that all protocol processing takes place in hardware and is thus independent of CPU performance and software implementation. The claimed update time of 1,000 digital I/O in 30 microseconds includes terminal cycle time.

"The extremely high performance of the EtherCAT technology enables control concepts that could not be realised with classic fieldbus systems," suggests Beckhoff spokesman, Frank Metzner. "For example, the Ethernet system can now deal not only with velocity control, but also with the current control of distributed drives. The tremendous bandwidth enables status information to be transferred with each data item.

"With EtherCAT, a communication technology is available that matches the superior computing capacity of modern industrial PCs," he adds. "The bus system is no longer the `bottleneck` of the control concept. Distributed I/Os are recorded faster than is possible with most local I/O interfaces."

Metzner says there is no restriction on the type of Ethernet device that can be connected in the EtherCAT strand via a hub terminal. The protocol tolerates other Ethernet-based services and protocols on the same physical network, usually with a minimum loss of performance. The system uses special buffers to delay non-real-time Ethernet data until there is a gap in the real-time transmission.

The first EtherCAT products are scheduled to be available at the end of 2003.