Abstract The IEEE 1451.2 protocol is a networked smart sensor interface standard. The IEEE 1451.2 protocol specifies that a smart sensor consists of two parts: a network adapter and a smart sensor interface module. The sensor- independent interface is the interface between the smart sensor interface module and the network adapter, enabling the network adapter to control the smart sensor interface module and facilitate communication between the two. This paper introduces the design of a sensor- independent interface between a network adapter and a smart sensor interface module that conforms to the IEEE 1451.2 protocol, as well as the field test results. Keywords IEEE 1451.2, TII hot-swappable, UCC3918, smart sensor Introduction In the 1980s and 1990s, smart sensors based on various fieldbus technologies developed rapidly. Due to the large number of fieldbus types, smart sensor interfaces became increasingly complex. In the late 1990s, IEEE successively launched the IEEE 1451 protocol family, proposing a unified sensor interface and a self-describing model for sensors , solving problems related to the compatibility, interchangeability, and interoperability of intelligent sensors . This protocol has been used in many fields such as pressure monitoring, oil level monitoring, and greenhouse environmental monitoring. IEEE 1451.2 (transducer to microprocessor communication protocols and transducer electronic data sheet formats) is a digital point-to-point wired transmission standard in the IEEE 1451 protocol family. As long as the network adapter (NCAP) and smart sensor module (STIM) comply with the IEEE 1451.2 standard, smart sensor interface modules from different manufacturers can be mutually compatible regardless of the network standard used in the measurement and control network, thus facilitating their integration into existing measurement and control networks. Therefore, sensor- independent interfaces conforming to the IEEE 1451.2 protocol are a crucial component of such measurement and control networks. This paper, based on an introduction to the IEEE 1451.2 protocol, details the design scheme of the Transducer Independent Interface ( TII ) circuit in a power system sensor network implementing synchronous phasor measurement. 1. Introduction to the IEEE 1451.2 Sensor Interface Specification The IEEE 1451 protocol family defines a series of standard smart sensor interfaces . The IEEE 1451.2 protocol proposes a wired transmission interface scheme from a digital point-to-point smart interface module to a network adapter. The IEEE 1451.2 protocol ensures reliable data transmission by defining the TII communication protocol, timing and electrical specifications. The sensor independent interface is a 10-wire interface , which can be divided into 4 groups according to function, as listed in Table 1. [align=center]Table 1 Sensor Independent Interface Signal List[1] [img=377,194]http://www.icembed.com/UploadFiles/2008916135859734.jpg[/img][/align] The communication protocol specifies the sampling trigger mechanism and two data transmission methods: byte read/write and frame read/write. IEEE 1451.2 specifies that the smart sensor interface module must be plug-and-play, which is implemented in software through the sensor electronic data form, and the hardware requires the interface to have hot-swappable capability. 2 TII Interface Circuit Design Based on the above standards, the hardware requirements of the TII interface have two functions: one is to realize data transmission based on the existing microprocessor bus; the other is to have surge current control function to support hot-swappable. 2.1 TII Implementation Based on SPI and GPIO SPI (Serial Peripheral Interface) is a four-wire synchronous serial interface widely used for data transmission between microprocessors and low-speed peripheral devices such as EEPROM, Flash, real-time clocks, A/D converters, digital signal processors, and digital signal decoders. SPI has two operating modes: master and slave. One master device can connect to multiple slave devices. Data transmission is controlled by the SPI clock signal SPCK of the master device, transmitting bits in a high-order-first, low-order-last order. The transmission speed of SPI is entirely controlled by the SPCK of the master device. By setting the SPCK frequency, it can adapt to various smart sensor interface modules with different operating frequencies. The module's SPI interface transmission rate reaches up to 1.5 Mbps, far exceeding the protocol's recommended 6 kbps, enabling the SPI-based TII interface technology to meet higher data transmission rate requirements. Figure 1 shows the TII interface circuit diagram. The left side is the smart sensor interface module (STIM), and the right side is the network adapter (NCAP) supporting hot-swapping functionality. Among them, GPIO is the general-purpose input/output pin of the microprocessor, and SN74ALVC164245 is a bidirectional 5-3.3 V level conversion chip. In the power system sensor network designed in the author's laboratory, the above two modules use the AT89S53 and AT91SAM9261 chips respectively. The data transmission and power wiring design scheme between the two is also shown in the figure. [align=center][img=458,266]http://www.icembed.com/UploadFiles/2008916135859580.jpg[/img] Figure 1 TII interface circuit diagram[/align] Relative to the different working modes of the sensor , the TII interface also has multiple transmission modes. The following only takes the sensor mode as an example to introduce its working process: When the network adapter requests the smart sensor interface module to perform a certain task, it first writes the channel address and command to the smart sensor interface module, then triggers the action with the NTRIG signal, and reads the data from the smart sensor interface module after waiting for a data establishment time. When the network adapter needs to write data to or read data from the smart sensor interface module , it first sends the NIOE signal, i.e., pulls SPI_SS low. Since the NIOE signal line is connected to both the SPI_SS and NIOE_S pins, the NIOE signal also enables the AT89S53's SPI. When the AT89S53 detects a valid NIOE signal via the NIOE_S pin, it promptly drives the NACK signal based on the smart sensor interface module's status, responding to the network adapter's read/write request. When the network adapter receives the NACK signal, it begins sending or reading data. The IEEE 1451.2 protocol requires the NIOE signal to be valid throughout data transmission. Therefore, during data transmission, when the STIM reads or writes data from the SPI shift register, it checks the validity of the NIOE signal to determine the data's validity and whether transmission is in progress. After writing the channel command and channel address to the STIM, NCAP triggers the action required by the command via the NTRIG signal. The power system synchronous phasor measurement requires a sampling time accuracy of up to 1 μs[2]. In order to ensure the time accuracy of the action execution, the NTRIG signal is simultaneously connected to multiple sensors or actuators in the STIM. As shown in Figure 2, there are multiple sensor channels in a smart sensor interface module, and each channel collects one signal. When the network application adapter opens a sensor or actuator channel, the AT89S53 enables the enable signal of the corresponding sensor or actuator. The output of the AND operation between this enable signal and the NTRIG signal enables the corresponding sensor or actuator. In this way, the NTRIG signal can accurately trigger the correct channel action. [align=center][img=243,149]http://www.icembed.com/UploadFiles/200891613590621.jpg[/img] Figure 2 Sensor Trigger Circuit Diagram[/align] 2.2 Hot-Swap Control Circuit Based on UCC3918 To facilitate the addition, removal, and replacement of sensor modules in the measurement and control network, the IEEE 1451.2 protocol smart sensor interface module has plug-and-play capability. This necessitates considering the impact of instantaneous current during the hot-swapping process in the design of the sensor's independent interface circuit. When the smart sensor interface module is inserted into the network adapter, the network adapter is already in a stable operating state, all capacitors are fully charged, and the smart sensor interface module is uncharged, with no charge in its capacitors. Therefore, when the smart sensor interface module comes into contact with the network adapter, a large instantaneous current is generated due to the charging of the capacitors on the smart sensor interface module. Similarly, when a charged smart sensor interface module is unplugged from the network application adapter, a low-resistance path is formed between the charged smart sensor interface module and the network adapter due to the discharge of the bypass capacitor, which will also lead to a large instantaneous current[3]. In severe cases, the large instantaneous current during hot-plugging will cause the power supply voltage to drop momentarily, resulting in system reset, or even damage to connectors, electronic components and circuit board wiring. In order to ensure the safe and reliable operation of the system, it is necessary to suppress excessive instantaneous current. For this reason, the UCC3918 chip was used in the design of the interface circuit. The UCC3918 low-resistance hot-swap power controller is a hot-swap controller produced by TI. The UCC3918 has a working voltage of 3 to 6 V, a low on-resistance of 0.06 Ω, and a maximum limiting current of 5 A. With only a few peripheral devices, the UCC3918 can provide complete power management, hot-swap current limiting and circuit breaker functions. The basic working principle of the UCC3918 chip is as follows: When the output current is lower than the maximum allowable current value IMAX, the UCC3918 operates in a low-impedance on-state. When the output current exceeds the maximum allowable current or the fault current threshold, the circuit remains on; simultaneously, the fault timer charges the capacitor CT. Once the capacitor CT voltage reaches a preset threshold, the current output will be turned off for 30 times the charging time. When the output current drops below the maximum allowable current value, the UCC3918 returns from the switching state to the low-resistance on-state. The UCC3918 also provides fast overcurrent protection; when the current rapidly exceeds the fault current threshold, the fast overcurrent protection will turn off the current output. Under extreme conditions such as circuit short circuits, this function provides effective protection for the device. The application design scheme of the UCC3918 is shown in Figure 3. By reasonably selecting the values ​​of the two resistors and two capacitors, the purpose of effectively suppressing instantaneous current can be achieved. [align=center][img=341,226]http://www.icembed.com/UploadFiles/200891613590360.jpg[/img] Figure 3 Hot-plug control circuit diagram based on UCC3918[/align] Wherein, RIFAULT is set according to formula (1): Wherein, ITRIP is the fault current threshold value. RIMAX is set according to formula (2): R [img=112,36]http://www.icembed.com/UploadFiles/200891613590797.jpg[/img] Wherein, IMAX is the maximum load current. When setting the current threshold value of TII , IMAX is set to 1.2 to 1.5 times the normal load current of the intelligent sensor interface module, the fault current IFAULT is set to 4 times the normal load current of the intelligent sensor interface module, and CT is taken as one load capacitance. In order to verify the effectiveness of the above design, an experiment was conducted on the TII interface , and the results are shown in Table 2. One set of experimental conditions involved no hot-swap control circuit, while the other set involved using the UCC3918 hot-swap controller. The normal operating current of the intelligent sensor interface module as the load was 650 mA. The TII interface with hot-swap capability had a maximum instantaneous current of 2.0 A, approximately three times the normal operating current. Without a hot-swap control circuit, the instantaneous current would be nearly five times the normal current. This could cause a sudden voltage drop in the system power supply or damage to the device. [align=center]Table 2 Maximum Instantaneous Current Comparison Table[img=340,59]http://www.icembed.com/UploadFiles/200891613590112.jpg[/img][/align] Figure 4 is a comparison of the current waveforms during hot-swap. The top waveform shows the current waveform with the hot-swap control circuit activated, and the bottom waveform shows the current waveform without the hot-swap control circuit activated. [align=center][img=345,253]http://www.icembed.com/UploadFiles/200891613590784.jpg[/img] Figure 4 Comparison of Hot-Swap Current Waveforms[/align] Conclusion This paper introduces the design and implementation of the independent interface part of a smart sensor based on the IEEE 1451.2 protocol, and verifies the effectiveness of the hot-swap control function through experiments. The designed interface has been applied in power system sensor networks. References [1] IEEE Standard for a Smart Transducer Interface for Sensors and Actuators——Transducer to Microprocessor Communication Protocols and Transducer Electronic Data Sheet (TEDS) Formats, 1997. [2] Xie Xiaorong, Li Hongjun, Wu Jingtao, et al. Feasibility analysis of applying synchronous phasor technology to transient stability control of power system [J]. Power System Technology, 2004, 28(1). [3] Hong Jiaping. Principle and application of integrated hot-swappable controller MAX4370 [J]. Microcomputer Applications [J], 2005(26). [4] Ni Chunhua, Li Boquan. Design and verification of network sensor interface [J]. Sensor Technology, 2005, 24(9). [5] Hang Heping. Design of digital intelligent pressure transmitter based on IEEE 1451.2 standard [J]. Instrumentation Technology and Sensors , 2002(11).