Position-measuring devices which provide an absolute position value are being used increasingly in automation technology. Certain disadvantages of what are termed incremental position-measuring devices are thereby eliminated such as, for example, the necessity of carrying out a reference sequence after switching on in order to find a reference position which is used as reference point for the further position measuring by counting graduation marks.
Primarily serial data interfaces are used for transmitting the absolute position values, since they make do with only a few data-transmission lines, and nevertheless, have high data-transmission rates. Particularly advantageous are what are called synchronous serial interfaces, which have one unidirectional or bidirectional data line and one clock line. Data packets are transmitted via the data line in synchronism with a clock signal on the clock line.
European Patent No. 0 660 209 describes a synchronous serial interface having a bidirectional data line and a unidirectional clock line. In this case, a bidirectional transfer of data—from the sequential electronics to the position-measuring device and from the position-measuring device to the sequential electronics—is possible. The data is transmitted in synchronism with a clock signal on the clock line. This principle forms the basis for an interface known under the name “EnDat.”
German Patent No. 197 01 310 describes a device for transmitting data between a sensing element in the form of a position-measuring system, and a processing unit. By transmitting a reference signal on one of the signal-transmission lines, via which data is transmitted between the sensing element and the processing unit, it is possible to switch the position-measuring system to various operating modes.
Due to the progressive miniaturization in electronic engineering, it is becoming possible to integrate more and more functions into position-measuring devices. Therefore, meanwhile, in addition to the position values, frequently additional information such as the speed, thus, the change in position over time, as well as status information which allows conclusions about the operating state of the position-measuring device, is generated. Microprocessors are used to control complex operational sequences or to perform complicated calculations. That is why, in addition to pure position-request commands, modern data interfaces also have further commands in order to request additional information, or in order to write to or read out memory areas in the position-measuring device.
Standardized interfaces offer the advantage that measuring devices which are equipped with such an interface may be connected directly to sequential electronics, e.g., a machine-tool control. So long as both devices observe the interface protocol underlying the interface, further adaptation is not necessary. On the other hand, it thereby becomes difficult, however, to expand the interface, for instance, in order to provide new commands, or to change existing commands in order to adapt them to special circumstances and possibly an expanded functional scope of the position-measuring devices.