Abstract: The IEEE 1451 standard consists of IEEE 1451.1, 1451.2, P1451.3, and P1451.4. It defines a standardized, universal interface for connecting sensors to networks, establishing a framework for networked smart sensors, enabling sensor manufacturers to support multiple networks. However, the IEEE 1451 standard faces some difficulties in application. This paper briefly introduces the content and development process of the IEEE 1451 standard, analyzes some reasons limiting its application, discusses the relationships between IEEE 1451.x standards, and provides an example of a networked smart sensor based on the IEEE 1451.1 standard for a robotic gripper. I. Introduction To solve the problem of connecting sensors to various networks, a group of insightful individuals, led by Kang Lee, began constructing a universal interface standard for intelligent sensors in 1993. In September 1993, the 9th Technical Committee of the IEEE (Sensors, Measurement and Instrumentation Technology Association) decided to develop a protocol for a smart sensor communication interface. In March 1994, the National Institute of Standards and Technology (NIST) and the IEEE jointly organized a workshop on developing a smart sensor interface and a general standard for smart sensor connectivity networks. Four more meetings were held subsequently to discuss the details of this standard, resulting in the IEEE 1451 Sensor/Actuator Smart Transmitter Interface standard. In April 1995, two dedicated technical committees were established: Working Group P1451.1 and Working Group P1451.2. Working Group P1451.1 was primarily responsible for defining the common target model for smart transmitters and the corresponding interface; Working Group P1451.2 primarily defined TEDS and digital interface standards, including the communication interface protocol and pin definitions between STIM and NACP. A draft standard and demonstration system were released in May 1995. After several years of effort, IEEE members voted to approve the IEEE 1451.2 and IEEE 1451.1 standards in 1997 and 1999 respectively. Two new working groups were also established to further extend the 1451.2 standard, namely IEEE P1451.3 and IEEE P1451.4. IEEE, NIST, and major companies such as Boeing and HP actively supported IEEE 1451 and demonstrated sensor systems based on the IEEE 1451 standard at international sensor conferences. II. Introduction to the IEEE 1451 Standard for Networked Smart Sensors 1. IEEE 1451.2 Standard The IEEE 1451.2 standard specifies a digital interface connecting sensors to microprocessors, describes the Transducer Electronic Data Sheet (TEDS) and its data format, and provides a 10-wire standard interface TII connecting STIM and NCAP, enabling manufacturers to apply a single sensor to multiple networks, giving the sensor "plug-and-play" compatibility. This standard does not specify how signal conditioning, signal conversion, or TEDS should be applied; these are left to individual sensor manufacturers to implement independently, in order to maintain their competitiveness in terms of performance, quality, characteristics, and price. 2. IEEE 1451.1 Standard: IEEE 1451.1 defines a network-independent information model, enabling sensor interfaces to connect to NCAP. It uses an object-oriented model definition provided to smart sensors and their components. Figure 3 shows a schematic diagram of the 1451.1 standard implementation model. This model consists of a set of object classes with specific attributes, actions, and behaviors, providing a clear and complete description of the sensor. This model also provides a hardware-independent abstract description of the sensor interface. The standard uses a standard application programming interface (API) to map the model to network protocols. Simultaneously, this standard optionally supports communication methods for all interface models, such as those provided by other IEEE 1451 standards, such as STIM, TBIM (Transducer Bus Interface Module), and hybrid-mode sensors. The IEEE 1451.1 standard offers numerous advantages for field devices and applications, such as rich communication models, support for client/server and publish/subscribe models, robust models that simplify the development of distributed measurement and control system software and reduce system complexity, modular structure that allows for easy customization of systems of any size, fieldbus independence, and transparency of the bus and field devices to the application, among others. The functional framework of the IEEE 1415.2 sensor interface standard is built around object-oriented system technology, the core of which is the concept of classes. A class describes the common characteristics of functional modules, which are called instances or objects. The concept of basic classes is extended by additional specifications for use in IEEE 1451.1. These specifications include publish sets (events generated by the class), subscribe sets (times corresponding to the class), state machines (a large-scale standard set of state transition rules), and a set of data type definitions (part of the features necessary for interoperability). IEEE 1451.1 Implementation Model 3, IEEE P1451.3 Standard. IEEE P1451.3 proposes to define a standard physical interface specification for connecting multiple physically distributed sensors in a multi-point configuration. This is essential, for example, in some cases where harsh environments make it impossible to physically embed TEDS (Transmitter Bus Interface) into the sensor. The IEEE P1451.3 standard proposes to implement the Transmitter Bus Interface Model (TBIM) in a "mini-bus" manner. This mini-bus is small and inexpensive enough to be easily embedded into the sensor, allowing for a maximum amount of data conversion through a simple control logic interface. The physical connection representation of IEEE P1451.3 is shown in Table 1. IEEE P1451.3 allows transmitter manufacturers to produce transmitters with a high performance-to-price ratio and internal system operability. This standard allows for the design and production of simple devices with relatively low sampling rates and suitable timing requirements, while also being compatible with devices with bandwidths up to several megabits and timing requirements as small as nanoseconds. In other words, devices with different frequency bands can coexist peacefully on the same bus. Figure 4 shows the physical connection representation of IEEE P1451.3. In Figure 4, a single transmission line serves both as the power supply for the transmitter and as the communication link between the bus controller and the Transmitter Bus Interface Model (TBIM). This bus can have one bus controller and multiple TBIMs. The Network Adapter (NCAP) contains the bus controller and the network interface supporting many different terminals, NCAPs, and transmitter buses. If the transmitter bus is internal to the network, the bus controller must be in the NCAP; otherwise, it should be located in the host or other device. A Transmitter Bus Interface Model (TBIM) can have one or more different transmitters. All TBIMs contain five communication functions, as shown in Table 1. These communication functions will utilize at least two communication channels over a single physical transmission medium. The communication channels will share this physical medium with the power supply that powers the transmitter. For high-power transmitters, sharing via the communication cable may be insufficient; in such cases, an external power supply may be provided to drive the transmitter. The simplest system contains only a bus management communication channel, which is used as all communication channels. The bus communication channel is set to a fixed frequency, or at least a low frequency, to ensure that every bus controller can use it. For the simplest system, TBIM communication functions, synchronization functions, trigger functions, and data transfer functions all share the same communication channel. Several TEDSs are defined in IEEE P1451.3. They can be classified in several ways. Some TEDSs are machine-readable and used to allow the bus controller to determine device characteristics; others are text-based and describe how the device operates. Three types of machine-readable TEDS are required for system operation; the others are optional. For situations where memory capacity is particularly small or special environments do not allow TEDS to be stored in the TBIM, the TEDS can be placed on a remote server; such remote TEDS are called virtual TEDS in IEEE P1451.3. The three required TEDSs are communication TEDS, model-wide TEDS, and transmitter-specific TEDS. Communication TEDS defines the communication capabilities of the TBIM. Each TBIM has only one communication TEDS. The overall model TEDS defines the overall characteristics of the TBIM. There is only one overall model TEDS in each TBIM. Transmitter-specific TEDS describes the characteristics of each transmitter. In the TBIM, each transmitter has one transmitter-specific TEDS. Typically, these TEDS are small, only a few hundred bytes in size. However, the memory requirements of the TBIM depend on the number of transmitters in the TBIM. In addition, the IEEE P1451.3 working group is considering several optional TEDS. All TEDS allowed in IEEE 1451.2 may be included. The most commonly used of these optional TEDS is the calibration TEDS. This TEDS provides the necessary constants to convert raw sensor data to engineering units or to the format required by the actuator. Several other TEDS are under consideration, such as the transfer function TEDS, which can be used to describe the characteristics of the transmitter for different input frequencies; and the digital filtering TEDS, which defines the coefficients for setting internal data filtering to obtain the desired frequency response, etc. Figure 4 shows the relationship between IEEE P1451.4 hybrid-mode transmitters and interfaces. The NIST and IEEE working groups have partially completed a unified approach for connecting sensors and actuators to communication networks, control, and test systems. IEEE 1451.1, IEEE 1451.2, and IEEE P1451.3 standards primarily target digitally readable sensors and actuators with network processing capabilities. The IEEE P1451.4 standard focuses on proposing a hybrid-mode intelligent transmitter communication protocol based on existing analog transmitter connection methods. It also specifies the TEDS format for intelligent analog transmitter interfaces to legitimate systems. This proposed interface standard will be compatible with the IEEE 1451.X networked transmitter interface standard. The IEEE P1451.4 interface proposal defines a standard that allows analog sensors (such as piezoelectric sensors and deformation gauges) to communicate in digital information mode (or hybrid mode), with the aim of enabling sensors to self-identify and self-configure. This standard also recommends that communication of digital TEDS data be shared with analog signals from sensors using a minimal number of wires—far fewer than the 10 wires required by the IEEE 1451.2 standard. An IEEE 1451.4 transmitter includes a Transmitter Electronic Data Sheet (TEDS) and a Mixed-Mode Interface (MMI). Figure 5 shows the relationship between a mixed-mode transmitter (sensor and actuator) and its interface in IEEE P1451.4. As a member of the IEEE 1451 standard, IEEE P1451.4 defines a mixed-mode transmitter interface standard, where analog transmitters will have digital output capabilities for control and self-describing purposes. It establishes a standard that allows mixed-mode transmitters with analog outputs to communicate digitally with IEEE 1451-compliant objects. Every IEEE P1451.4-compliant mixed-mode transmitter will consist of at least one transmitter, a Transmitter Electronic Data Sheet (TEDS), and interface logic for controlling and transmitting data into different existing analog interfaces. See Figure 6. The transmitter's TEDS is small but defines enough information to allow a higher-level 1451 object to supplement it. This standard, proposed by the IEEE P1451.4 working group, allows analog transmitters to communicate digitally with an IEEE 1451 object. This standard will define the communication protocol and interface, as well as the transmitter TEDS format. The TEDS here will be based on the IEEE 1451.2 TEDS. However, the standard does not specify transmitter design, signal conditioning, or specific uses of the TEDS. For self-identification, self-description, and setup, it is necessary to develop a standard that allows analog transmitters to communicate digitally. Due to the lack of a unified standard, some transmitter manufacturers have introduced different implementations, but these have acceptance limitations and are not widely recognized. An independent, open-defined standard will reduce conflicts and contradictions among potential users, transmitter and system manufacturers, and system integrators, and make products with IEEE 1451.4 interfaces and protocols compatible. This will undoubtedly accelerate the emergence and acceptance of this technology. The TEDS of IEEE P1451.4 is a subset of the TEDS defined in the IEEE 1451.2 standard, with the aim of minimizing the size of the TEDS memory. Key design elements of the IEEE P1451.4 TEDS include: user-friendly information, plug-and-play functionality, support for all transmitter types, openness to meet individual needs, and compatibility with IEEE 1451.2. The IEEE P1451.4 TEDS will include the following: (1) Identification parameters, such as manufacturer, module code, serial number, version number, and data code; (2) Device parameters, such as sensor type, sensitivity, transmission bandwidth, units, and accuracy; (3) Calibration parameters, such as the last calibration date and calibration engine coefficient; (4) Application parameters, such as channel identification, channel grouping, sensor location, and orientation. Through efforts, the IEEE P1451.4 working group will establish a standard that allows mixed-mode transmitters with analog outputs to communicate digitally with advanced IEEE 1451 objects. One possible implementation is shown in Figure 7. III. Discussion on the Application of the IEEE 1451 Standard Since the IEEE and NIST organizations developed the IEEE 1451 standard for networked intelligent sensor interfaces, several large American companies have actively participated in its development and have demonstrated and experimented with networked intelligent sensors at numerous international sensor expos. For details, please refer to reference [10]. Some companies have also launched development tools for networked intelligent sensor systems. Several years have passed, but few products based on the IEEE 1451 standard have been released. In early 2000, Hewlett-Packard launched the BFOOT11501, 66501, and 66502 series of NCAP chips supporting the IEEE 1451 standard, but by the end of 2000, it announced that production had ceased. This means that the market has not yet fully accepted and widely adopted this new networked intelligent sensor standard. This is the case abroad, while in China, some review articles on the IEEE 1451 sensor interface standard have appeared in journals, but there are no reports on application research results of networked smart sensors based on the IEEE 1451 standard, and no major companies have announced support for the IEEE 1451 standard. Of course, it takes time for a new standard to truly gain market acceptance. However, we believe that in addition to problems with the standard itself, there are other factors contributing to the current situation. The author has been following and studying this standard, and this article presents several considerations for the practical application of the IEEE 1451 standard for your reference. 1. The support of fieldbus manufacturers and sensor manufacturers is indispensable. Since the concept of fieldbus was proposed in the 1980s, due to the lack of a unified international standard at the beginning, some foreign companies, especially some large companies in the United States and Japan, joined together to develop their own bus standards and launched their own products. Currently, the more popular fieldbuses on the market include CAN (Control Area Network), LONWORKS (Local Operation Network), PROFIBUS (Process Fieldbus), HART (Addressable Remote Sensor Data Communication), and FF (Foundation Fieldbus), etc. Due to the monopolistic position of these companies in their respective industry sectors, they have achieved great success in various application areas, resulting in the current coexistence of multiple fieldbuses in the market. Because these companies have different strengths and target users, each bus has its own advantages in its respective field. From a commercial perspective, major fieldbus technology manufacturers are unwilling to abandon their existing products and markets, and are unwilling to seek unification in the short term. As the IEEE 1451 standard was designed to address this interoperability issue between fieldbuses, it naturally struggles to gain traction given the continued independent efforts and reluctance of major fieldbus technology manufacturers to promote its use. 2. Clarifying the Relationship Between IEEE 1451.X Standards: Since the promulgation of the IEEE 1451.2 and IEEE 1451.1 standards, many people have been confused by the division of networked intelligent sensor systems into two modules: STIM and NCAP. Both modules require microprocessors, meaning two separate systems are needed as development tools for STIM and NCAP, undoubtedly increasing the difficulty of promoting and using the standard. A British university once developed a networked intelligent sensor system using Analog Devices' AduC812 as a STIM and Hewlett-Packard's BFOOT66501 as an NCAP (Network Detector). This system was used for weather monitoring and remotely monitored weather changes via an embedded web server, achieving success. However, with the discontinuation of the BFOOT series chips supporting the 1451 standard, no NCAP chip supporting or compatible with the 1451 standard has emerged since. A key factor is the 10-wire TII standard interface defined by IEEE 1451.2. It is precisely because of this unified hardware interface that sensor modules can be plug-and-play. Another misconception is that IEEE 1451.X standards must be used together, which increases the difficulty of standard adoption and practical application. At last year's Networked Intelligent Sensors Working Group meeting, experts who developed the 1451 standard specifically addressed this issue of standard interpretation, updating some aspects of the standard. The key points were: how to use the IEEE 1451 standard and the relationship between IEEE 1451.X. The development of the IEEE 1451.X standard allows them to work together to form a networked intelligent sensor system, but they can also be used individually. The IEEE 1451.1 standard can be used independently of other 1451.X (1451.2, P1451.3, and P1451.4) hardware interface standards. 1451.X can also be used independently without 1451.1, but it must have a similar software architecture to 1451.1, providing physical parameter data, application functions, and communication capabilities to connect 1451.X devices to the network and implement the functions of 1451.1. 3. Networked Intelligent Sensor System Based on IEEE 1451.1 Standard: The IEEE 1451 standard defines the hardware and software interface standard for sensors or actuators, providing standardized communication interfaces and hardware/software definitions. This allows different field networks to interconnect and interoperate through the interface standard defined by IEEE 1451, solving compatibility issues between different networks. It enables sensor manufacturers, system integrators, and end users to support multiple networks and transmitter families at low cost, and reduces overall system power consumption by simplifying wiring. However, how to make the IEEE 1451 standard as practical as possible remains somewhat unclear in China. Based on a new understanding of the relationships between IEEE 1451.X, the authors believe there are multiple possible implementation schemes. We developed an Internet-oriented networked intelligent robot gripper sensor system based on IEEE 1451.1. Considering the characteristics of networked intelligent sensors, we selected the embedded network module NetBox as the NCAP development platform. Its microprocessor is Intel's high-performance, 32-bit embedded microprocessor 386EX. The NetBox module features multiple communication interfaces, including a direct-connection Ethernet 10BASE-T interface, a standard RS232C interface, an expandable RS422/RS485 interface, and a comprehensive and flexible simplified bus interface. It can directly connect to most AD, DA, DIO, timer, and dual-port RAM devices without requiring any interface logic circuits. We developed a data acquisition circuit to collect digital and analog signals from the robot's gripper's multiple sensors into the system. Utilizing the CGI (Common Gateway Interface) principle, the NetBox acts as an NCAP (Network Adapter) and connects to the Internet via an embedded web server, allowing users to remotely access the system through a browser. The system employs a neural network approach to fuse data from multiple sensors in the robot gripper, obtaining its current working state and providing a basis for decision-making regarding the robot's control, safe operation, and movement. Figure 8 shows the overall system block diagram, which consists of sensors, a data acquisition circuit, and the NetBox network module. In the system design, we adopted some advanced technologies from the IEEE 1451.1 standard. The system has the following characteristics: (1) Electronic Data Sheet (TEDS) technology, which can fully describe the type, behavior, performance attributes and related parameters of the sensor, such as the name of the sensor manufacturer, the type and serial number of the sensor, etc., so that the sensor has self-description and self-identification capabilities; (2) Calibration engine technology, which enables the sensor to automatically correct errors and self-compensate; (3) Support for TCP/IP protocol, so that the system can be plugged and used on any INTERNET terminal node with a certain IP address. IV. Future Prospects The IEEE 1451 sensor represents the development direction of the next generation of sensor technology. The introduction of the networked intelligent sensor interface standard IEEE 1451 will help solve the phenomenon of multiple field networks coexisting in the current market. It is believed that with the successive formulation, promulgation, and implementation of the IEEE P1451.3 and IEEE P1451.4 standards, networked smart sensor technology based on IEEE 1451 is no longer confined to the demonstration or laboratory stages. An increasing number of low-cost, networked smart sensors/actuators are entering the market, and are having, and will continue to have, a more widespread impact on human life. Networked smart sensors will bring revolutionary changes to fields such as industrial measurement and control, smart buildings, telemedicine, environmental and hydrological monitoring, agricultural informatization, aerospace, and national defense. Their broad application prospects and enormous social, economic, and environmental benefits will soon be revealed.