The absolute encoder uses natural binary, Gray code, or PRC code to optically convert the physical grating on the code wheel. It transforms the rotational angle of the connected shaft into a corresponding sequence of electrical pulses and outputs it as a digital signal. It offers advantages such as compact size, high precision, digital interface, and absolute positioning. It is widely used in radar systems, turntables, robots, CNC machines, and high-precision servo systems. The data output from an absolute encoder is typically based on synchronous serial communication. The EnDat interface, developed by HEIDENHAIN, is a digital, full-duplex synchronous serial interface designed specifically for encoders. Not only does it transmit position values for both incremental and absolute encoders, but it also allows the transfer or updating of information stored within the encoder or the storage of new data. Since it uses serial transmission, only four signal lines are required. Data is transmitted synchronously under the excitation of the clock signal from the downstream electronic device. The type of data (position value, parameter, diagnostic information, etc.) is determined by the mode command sent by the downstream device to the encoder. **Introduction to the EnDat Interface** 1. **Key Features of the EnDat Interface** - **High Performance & Cost Efficiency**: The universal interface supports all incremental and absolute encoders, offering low power consumption, compact design, fast system setup, and the ability to float the zero point based on offset values. - **Enhanced Signal Quality**: Encoder-specific optimizations improve system accuracy and provide better contour precision for CNC machines. - **Practicality**: Automatic system configuration, improved reliability through digital signals, monitoring and diagnostics for enhanced safety, and redundancy code verification for reliable signal transmission. - **Improved Security**: Separate location and error bits, along with data checksums and responses, ensure secure data transmission. - **Advanced Compatibility**: Supports high resolution, short control cycles, up to 16 MHz clock speed, and is suitable for direct drive technology.  *Figure 1: EnDat interface encoder data acquisition schematic* 2. **Performance Enhancements in EnDat 2.2** - **Simultaneous Transmission**: Position values and additional information can be transmitted at the same time, with the type of additional data selected via address codes. - **Data Storage**: Includes manufacturer parameters, OEM parameters, operational settings, and status information, aiding in system configuration. - **Full Digital Transmission**: Internal 14-bit subdivision improves signal quality and reliability, enabling higher resolution. - **Monitoring & Diagnostics**: Alerts include light source failure, insufficient signal amplitude, incorrect position calculation, voltage issues, and excessive current draw. Warnings are triggered when extreme values are approached. - **Wider Voltage Range & Speed**: Operates between 3.6V to 14V and supports up to 16 MHz transmission speed. 3. **Timing & OEM Data Storage** During each frame of synchronous data transmission, a data packet is sent. The cycle begins with the first falling edge of the clock, and the measured value is stored. After two clock pulses, the downstream device sends the "encoder position value" command (with or without additional info). Once the absolute position is calculated (tcal), the encoder starts transmitting data from the start bit. Error bits F1 and F2 (exclusive to EnDat 2.2) are used for fault monitoring, and their generation is independent, indicating potential encoder failures. The exact cause is stored in the "operational" memory area for later retrieval.  *Figure 2: Position value transmission without additional information* If additional information is included, it follows immediately after the position value, also ending with a CRC. The content is determined by the selected memory address and is transmitted during subsequent sampling periods until a new memory area is chosen. At the end of the data word, the clock signal must go high. After 10–30 µs or 1.25–3.75 µs (programmable recovery time tm in EnDat 2.2), the data line returns to low, allowing a new data transfer to begin.  *Figure 3: Position value transmission with additional information* Encoders also provide different memory areas for parameters that can be read or written by the manufacturer, OEM, or user. Some areas may be write-protected. Different encoder series support various OEM memory areas and address ranges, so the downstream electronics must be programmed using relative addresses rather than absolute ones. **Circuit Design for Follow-Up Electronic Devices** Users can design their own interface circuits according to the EnDat protocol and electrical specifications. Alternatively, HEIDENHAIN provides specific data processing chips to simplify the design. When using these chips, users need only configure the FPGA registers and send instructions in the chip's acceptable format to retrieve the desired data. By following the RS-485 standard for differential signaling, bidirectional communication of position values and parameters can occur between the encoder and downstream devices, driven by a synchronous clock issued by the latter. **FPGA + Software Macros** MAZet, a partner of HEZEHAN, offers EnDat software macros compatible with Xilinx Virtex/Spartan and Altera Acex/Cyclone series. Custom soft cores can be provided based on customer needs. These soft cores implement all EnDat interface functions, enabling 8-bit or 16-bit data transmission via a 6-bit address line and 16-bit data line. Below are the module diagram and circuit design for the FPGA.  *Figure 4: FPGA module diagram*  *Figure 5: Encoder and subsequent circuit connection module diagram* **Conclusion** HEIDENHAIN’s EnDat interface has seen widespread use across industries and has now been upgraded to a new level. The EnDat 2.2 interface now operates at 16 MHz, meeting the demands of high-performance applications, especially in the electronics industry. Increasing the clock frequency from 8 MHz to 16 MHz significantly reduces the time needed to read position data and shortens the control loop cycle. Simple and cost-effective system designs offer convenience, powerful functionality, versatility, and forward-looking security concepts that guide the continuous advancement of coding control technology.

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