Introduction In the 1980s and 1990s, intelligent sensors based on various fieldbus technologies developed rapidly. Due to the large number of fieldbus types, intelligent 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 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 forms) is a digital point-to-point wired transmission standard in the IEEE 1451 protocol family. As long as the network adapter (NCAP) and the intelligent sensor module (STIM) comply with the IEEE 1451.2 standard, intelligent 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 intelligent sensor interfaces. The IEEE 1451.2 protocol proposes a wired transmission interface scheme from a digital point-to-point intelligent 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 Transducer Independent Interface is a 10-wire interface, functionally divided into four groups, as listed in Table 1. The communication protocol specifies the sampling triggering mechanism and two data transmission modes: byte read/write and frame read/write. IEEE 1451.2 stipulates that intelligent sensor interface modules must be plug-and-play, which is implemented in software through sensor electronic data forms and requires the interface to be hot-swappable in hardware. 2. TII Interface Circuit Design Based on the above standards, the hardware requirements for the TII interface have two functions: first, it must implement data transmission based on the existing microprocessor bus; second, it must have surge current control function to support hot-plugging. 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 working 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 the order of most significant bit first, least significant bit last. 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 intelligent sensor interface modules with different operating frequencies. The module's SPI interface transmission rate is as high as 1.5 Mbps, far exceeding the protocol's recommended 6 kbps, which allows the SPI-based TII interface technology to meet the requirements of higher data transmission rates. Figure 1 shows the TII interface circuit diagram. On the left is the Smart Sensor Interface Module (STIM), and on the right is the Network Adapter (NCAP) supporting hot-swapping. GPIO refers to the general-purpose input/output pins of the microprocessor, and SN74ALVCl64245 is a bidirectional 5-3.3V level converter chip. In the power system sensor network designed in our laboratory, the two modules use the AT89S53 and AT91SAM9261 chips respectively. The diagram also shows the data transmission and power supply wiring design between them. Relative to different sensor operating modes, the TII interface also has multiple transmission modes. The following describes its operation using the sensor mode as an example: When the network adapter requests the STIM module to perform a certain task, it first writes the channel address and command to the STIM module, then triggers the action with the NTRIG signal, waits for a data setup time, and then reads data from the STIM module. When the network adapter wants to write data to or read data from the STIM 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 through the NIOE_S pin, it promptly drives the NACK signal based on the status of the smart sensor interface module, responding to the network adapter's read/write requests. When the network adapter receives the NACK signal, it begins sending or reading data. The IEEE 1451.2 protocol requires the NIOE signal to remain valid throughout data transmission. Therefore, during data transmission, when the STIM reads or writes data from the SPI shift register, it must check 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. Power system synchronous phasor measurement requires sampling time accuracy up to 1μs. To ensure the accuracy of action execution time, the NTRIG signal is simultaneously connected to multiple sensors or actuators within the STIM. As shown in Figure 2, a smart sensor interface module has multiple sensor channels, each acquiring one signal. When the network application adapter opens a sensor or actuator channel, the AT89S53 enables the corresponding sensor or actuator's enable signal. The output of the AND operation between this enable signal and the NTRIG signal then enables the corresponding sensor or actuator. This allows the NTRIG signal to accurately trigger the correct channel action. 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 that the design of the sensor's independent interface circuit consider the impact of the instantaneous current during the hot-swapping process. When the smart sensor interface module is inserted into the network adapter, the network adapter is already in a stable operating state, and all capacitors are fully charged. The smart sensor interface module is uncharged, and its capacitors contain no charge. 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 powered smart sensor interface module is unplugged from the network application adapter, a low-resistance path is formed between the powered smart sensor interface module and the network adapter due to the discharge of the bypass capacitor, which can also lead to a large instantaneous current. In severe cases, the large instantaneous current during hot-plugging can cause a momentary drop in power supply voltage, resulting in system reset, and even damage to connectors, electronic components, and circuit board wiring. For the safe and reliable operation of the system, excessive instantaneous current must be suppressed. Therefore, the UCC3918 chip is used in the interface circuit design. The UCC3918 low-resistance hot-swap power controller is a hot-swap controller manufactured by TI. The UCC3918 operates at 3–6 V, has an on-resistance as low as 0.06 Ω, and a maximum current limiting of 5 A. With very few external components, the UCC3918 can provide complete power management, hot-swap current limiting, and circuit breaker functions. The basic operating principle of the UCC3918 chip is: when the output current is lower than the maximum allowable current value IMAX, the UCC3918 operates in a low-resistance 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 CT voltage reaches the preset threshold, the current output will be shut off for 30 times the charging time. When the output current drops below the maximum allowable current, 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 shut off the current output. This function provides effective protection for the device under extreme conditions such as short circuits. 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. In the formula, IMAX is the maximum load current. When setting the current threshold value for TII, IMAX is set to 1.2 to 1.5 times the normal load current of the intelligent sensor interface module, the fault current IEAULT is set to 4 times the normal load current of the intelligent sensor interface module, and CT is taken as one load capacitance. To verify the effectiveness of the above design, experiments were conducted on the TII interface, and the results are shown in Table 2. One set of experimental conditions was without a hot-swap control circuit, and the other set of experimental conditions used a UCC3918 hot-swap controller. The normal operating current of the intelligent sensor interface module as a load is 650mA. The maximum instantaneous current of the TII interface with hot-swap functionality is 2.0A, 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 lead to a sudden voltage drop in the system power supply or damage to devices. Figure 4 shows a comparison of the current waveforms during hot-swap operation. The top waveform is the current waveform with the hot-swap control circuit activated, and the bottom waveform is the current waveform without the hot-swap control circuit activated. Conclusion This paper introduces the design and implementation of an independent interface for intelligent sensors 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 a power system sensor network.