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EnDat digital bi-directional interface is tailor-made for fast-response applications

27 May 2009

Absolute encoders offer significant advantages over their incremental cousins and are very much seen as the future direction for encoder development. As well as their ability to always know where they are positioned, with each angular or linear position having a uniquely identifiable coded output, they offer the potential for axis parameters to be stored and read from the encoder when required – depending on the interface used. However, digital drive systems and feedback loops with position encoders for measured value acquisition do require fast data transfer and high signal transmission reliability.

EnDat, the absolute serial data protocol developed by Heidenhain, has for many years set the standard in terms of speed of transmission and flexibility, and features in electronic drives and controls available from manufacturers worldwide. Now the EnDat 2.2 digital bi-directional interface for encoders has been introduced for fast-response applications with direct drives. This new version, which is suitable for safety-related applications, is backed by new software tools that make implementation of the EnDat absolute serial data protocol simpler and easier than ever.

The increased clock frequency reduces the access time to the position value and makes shorter cycle times possible. This, according to Heidenhain, means that EnDat is now the fastest purely serial interface for position encoders based on the RS 485 transmission characteristics. Since the analogue scanning signals are digitised and sub-divided directly at the place of measurement, it is possible to improve significantly the characteristics of servo motors, such as positioning accuracy and speed stability.

An essential component of EnDat communication, in addition to the encoder, is the EnDat Master software, which is available in three levels of complexity. Designed to run on Field-Programmable Gate Array (FPGA) families and to allow integration and customisation at levels that best suit the customer and the specific application, the three versions are EnDat Master Standard; EnDat Master Reduced; and EnDat Master Light.

EnDat Master Standard is usually delivered as an EDIF (Electronic Interface Design Format) net list that cannot be changed by the customer. The encapsulated code block is completely tested by Heidenhain, with implementation of the code block secured through test vectors. With EnDat Master Reduced only the pure EnDat functionality is integrated and the software does not offer further support regarding convenient handling of the code or a micro-controller interface. As is also the case with EnDat Master Light, in which EnDat functionality is integrated only in part, the code has not been tested by Heidenhain and is considered an example VHDL (Very High speed Design Language) code demonstrating the EnDat interface processes.

In operation, the data transmitted is synchronised with the clock signal from the subsequent electronics, with the type of transmission (position values, parameters, diagnostics, etc) selected by mode commands send to the encoder. Thanks to the serial transmission method, which ensures high system reliability through pure digital data transmission, only four signal lines are needed. This contributes to simpler connection and installation, as well as lower wiring costs.

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