As CNC devices and servo drive devices in industrial machinery CNC systems are widely used, the demand for related communication interface standards is also becoming increasingly strong. This article introduces the relevant understanding in the formulation of national standards. I. Project Overview and Background This project is based on the arrangement of the "1995 Science and Technology Development Plan of Machinery Industry" (Mechanical Industry Standard Formulation and Revision Part) of the State Bureau of Quality and Technical Supervision and the former Ministry of Machinery Industry. The Beijing Machine Tool Research Institute is the responsible drafting unit for the national standard for real-time serial communication data link between control and drive devices of industrial machinery electrical equipment (project number 97440306 (109)), which adopts the IEC 1491:1995 international standard. The formulation of this standard is currently under review and revision. This standard will be under the jurisdiction of the National Technical Committee on Standardization of Electrical Systems of Industrial Machinery. The CNC system of industrial machinery includes CNC devices and servo drive devices. With the development of servo drive technology, digital servo drive devices have been widely used. Different manufacturers produce different CNC devices and servo drive devices. Due to the lack of a standard communication interface, most data servo systems currently can only be matched with the manufacturer's own CNC system and are not compatible with each other, limiting their wider application. To solve this problem, in 1990, some well-known German CNC and servo system manufacturers, together with the University of Stuttgart, jointly initiated the establishment of the SERCOS Association. They developed and defined SERCOS (Serial Real-Time Communication System), a serial real-time communication system between CNC devices and servo drives. Its purpose was to establish a standard for the interface between CNC and data servo systems in Germany and to develop related products to ensure product interchangeability. Referring to this system, in November 1995, the International Electrotechnical Commission (IEC) issued the international standard CEI/IEC 1491, "Equipment of industrial machines—Serial data link for real-time communication between controls and drives" (abbreviated as SDL system). The SDL system specifies the topology, message format, data content, and data exchange protocol of the serial interface between CNC devices and servo drives using optical fiber as the transmission medium. It provides a unified standard communication interface for design and manufacturing, making interoperability between CNC devices and digital servo devices possible. The main contents of this standard are summarized below. II. Topology and Composition of the SDL System An SDL system is essentially a communication network between a CNC device and a digital servo device. Figure 1 shows a typical SDL system topology. The control unit transmits command signals, system status, and monitoring signals to the digital servo drive through the interface. After receiving the command, the digital servo drive begins operation and sends sensor feedback signals, the status of the digital servo drive, and monitoring signals back to the control unit. SDL can adopt a single-ring or multi-ring structure as needed. Optical fiber is used as the data transmission medium between the CNC device and the digital servo drive. In the ring, one end of the CNC device is called the master station, and the other end of the digital servo drive is called the slave station. The master station connects to the slave stations in the ring. One master station controls only the slave stations within the same ring. A slave station can connect to one or more digital servo drives as needed. If each master station connects to only one slave station, a star topology is formed. Information transformation within the loop occurs between the CNC device and the digital servo drive, not between the digital servo drives themselves; the information flow is unidirectional. Communication between the CNC device and the digital servo drive is real-time. A typical device includes a torque loop, a speed loop, and a position loop. The SDL system should be able to handle all three operating modes. ☆ The digital servo drive only has torque control; ☆ The digital servo drive has speed and torque control; ☆ The digital servo drive has all closed-loop control, including position control. III. Layered Structure of Data Transmission In 1984, the International Organization for Standardization (ISO) proposed the 'Open Systems Interconnection Basic Reference Model' (OSI), which specifies the Open Systems Communication Architecture. It has a seven-layer structure, from lowest to highest: Physical Layer, Data Link Layer, Network Layer, Transport Layer, Session Layer, Presentation Layer, and Application Layer. However, the OSI system and its series of protocol standards are only conceptual and functional architectures, without specifying concrete implementation specifications and details. Although the SDL system has hierarchical functionality, it does not have a complete structure like the OSI. Figure 2 illustrates in detail the structure between the master and slave transmission layers. The highest transmission layer is aperiodic transmission. Above aperiodic transmission is the application layer, which includes program instructions supported by aperiodic transmission, and aperiodic transmission is supported by periodic transmission. IV. Data Content and Classification Data content is classified into periodic and aperiodic transmission data according to the transmission method. Tables 1 and 2 show the content of periodic and aperiodic transmission data. Periodic transmission data is characterized by 'speed' and 'synchronization' to meet the requirements of real-time communication. Aperiodic transmission data has a wider range, but its speed is much slower than periodic data. The transmitted data and sequence are determined during system initialization. Data types are divided into: ☆ Working data: This refers to data processed by the SDL system. This data is assigned an identification number (IDN) by the system. The SDL system is not only a data transmission system but also provides many data formats and instructions for controlling CNC devices, digital servo drives, and machine tools. All these data and instructions form a data block. The IDN gives the data block a name, attributes, unit, minimum and maximum input values, and the data itself. This data is stored through the IDN. The system has a total of 2^16 = 65535 usable identification numbers. 0-32767 are system-defined standard data. Data from 32768 to 65535 is reserved for definition by the manufacturers of the CNC and digital servo drives. ☆ Parameters: To ensure error-free system operation, parameters are used to adjust the digital servo drives and CNC. ☆ System program instructions: Used to activate the functions between the digital servo drives or between the digital servo drives and the CNC. ☆ Instructions and feedback values ​​are usually included as periodic data in messages. Based on their role in the system, data can be further divided into: ☆ Service channel data: Data that travels between the CNC and digital servo drives and passes through the service channel. The service channel is the periodic or non-periodic data transmission specified by specific control words and status words in the MDT, such as the input or display of certain data on the CNC end. ☆ Periodic data: Configurable message data portion in each communication cycle. The design allows for the exchange of periodic data between the CNC and digital servo drives using 2-byte and 4-byte data strings with certain word length combinations. ☆ Initialization data: This data initializes the communication system and defines the operating parameters of the CNC and digital servo drives. V. Data Exchange Protocol The main function of the data exchange protocol is to describe the data format, timing, and error correction in data exchange within the communication system. System data exchange occurs between the master and slave stations. It includes the exchange of operational data and program instructions. All operational data and program instructions are assigned an Identifier Number (IDN). A data block with an IDN contains several units, as shown in Figure 3, representing the data block structure. The SDL system on the service channel distinguishes between periodic and non-periodic data transmission. During periodic data exchange, only the operational data of unit 7 in the data block can be transmitted. All unit transmissions of the data block can only occur through the service channel. Non-periodic data exchange is performed in several steps through a dedicated data field for periodic data exchange. The form and length of data exchange depend on the conditions of the SDL system and the operating mode of the digital servo drive, i.e., position control, speed control, or torque control. Important information, such as status signals from the digital servo drive or control signals from the CNC device to the digital servo drive, is always transmitted periodically. Whether the transmission of other operational data is periodic (e.g., instructions, feedback values) or non-periodic (e.g., limit values) depends on the application. All data exchange between the master station and slave stations or digital servo drives is conducted through defined messages. There are three different message formats: MST, MDT, and AT. ☆ Master Synchronization Message (MST): Used for synchronization, sent by the master station at the beginning of the transmission cycle. ☆ Indicator Message (MDT): Used as an indicator value, sent by the CNC system to the servo drive. ☆ Servo Drive Message (AT): Sends the actual value from the CNC system to the servo drive. The processing of the management section within the messages is automatic. The data field contains specific, definitive information and is processed according to the three different message formats and interface states. The system first initializes. After communication phases CP0, CP1, CP2, and CP3, it reaches the normal operating state CP4. Application of the IEC 1491 International Standard According to relevant data, SDL can operate at 4 Mbit/s; a maximum of 254 digital servo drives can be connected per ring. The programmable period is 0.062ms, 0.125ms, 0.25ms, or any integer multiple of 0.25ms. The topology shown in Figure 1 includes a 32-bit instruction value (e.g., position or speed) and a 16-bit limit instruction value (e.g., torque). The digital servo drive can transmit the actual values ​​of position or speed and torque to the CNC device. The system also allows the transmission of non-periodic data up to 8k bits/s, such as parameters, diagnostics, and text. Such a system can be used to generate predetermined motions, such as a trapezoidal motion with a 2ms period, an increment of 2mm, and an acceleration of 1000mm/s², to generate electronic gear control, and to form electronic cams. GM Drivetrain Div. uses the SERCOS system as part of its Open Modular Architecture (OMAC) controller as part of its network. THOMSON has obtained a production license for a single-chip controller using the SERCOS interface, producing the SERCOS Interface Controller SERCOS410A ASIC chip, which is available for use by other CNC device and servo manufacturers. A SERCOS410A terminal controller is used as the master station (one unit), with several slave stations forming the connection lines. Each terminal uses a microcomputer and NRZI encoding, achieving a transmission speed of 2-4 Mbf/s. From the above, it can be seen that speed and reliability are crucial for open communication between CNC devices and digital servo devices. With technological advancements, the requirements for accuracy and speed in SDL systems are becoming increasingly stringent. Furthermore, due to the complexity of SDL systems, they are not yet widely used in control systems in many countries. However, given their importance to open systems, they will undoubtedly gain more attention in the future.