Preface With the rapid development of electronic manufacturing technology, integrated circuits are becoming increasingly complex in function while shrinking in size. Therefore, for manufacturers of test electronic components, building the most competitive test platform in the shortest possible time is indeed a significant challenge. In the late 1960s, Hewlett-Packard designed the so-called HP-IB (Hewlett-Packard Interface Bus) as a communication channel between independent instruments and computers. Due to its high data transfer rate (for its time), it was quickly widely accepted, and the IEEE later renamed this interface GPIB (General Purpose Interface Bus). However, GPIB proved inadequate to cope with more complex testing environments and challenges. In 1987, the VXI Association was established and developed the so-called instrument-on-a-card standard, namely VXI (VMEbus eXtensions for Instrumentation). VXI, with its modular and robust architecture, has indeed brought many benefits to the measurement and automation industries. In the past decade, with the dramatic revolution and popularization of personal computers, instrument modules based on the PCI Bus architecture have developed significantly. Therefore, the PXI System Alliance (PXISA) was established in 1998, making PXI (PCI eXtensions for Instrumentation) an open standard architecture. The PXI platform not only has an open architecture and robust form factor similar to VXI, but also features a series of synchronization signals designed for instrument development, making it more suitable as a platform for measurement and test automation. This article mainly aims to introduce how to utilize the advantages of PXI to achieve precise and rapid synchronization between measurement instrument modules. The content includes an introduction and explanation of PXI, commonly used synchronization signals for measurement instrument modules, and application examples. [b]Introduction to PXI[/b] Engineers from test system manufacturers will ask, what is PXI? What are the benefits of using PXI instrument modules and the PXI system as a development platform? How does it differ from CompactPCI or PCI? First, if we want to use the PXI platform as a platform for measurement instruments, we must first understand the architecture and advantages of the PXI platform so that it can work effectively with instrument modules to achieve maximum benefits. In simple terms, PXI is an architecture based on PCI (Peripheral Component Interconnect) and CompactPCI, with some PXI-specific signal combinations. PXI inherits the electrical signals of PCI, giving it the extremely high data transmission capabilities of a PCI bus, achieving transmission speeds from 132 Mbytes/s to 528 Mbytes/s, and is fully compatible in software. Furthermore, PXI uses the same mechanical form factor as CompactPCI, thus sharing the characteristics of high density, robust housing, and high-performance connectors. The relationship between PXI and CompactPCI is shown in Figure 1. A PXI system consists of several components, including a chassis, a PXI backplane, a system controller module, and several peripheral modules. Figure 2 shows an example of a 3U eight-slot PXI system. The system controller, also known as the CPU module, is located in the first slot on the left side of the chassis. Three expansion slots are reserved to its left for the system controller to accommodate larger system cards with complex functions. Slots 2 through 8 are called peripheral slots, allowing users to plug in different instrument modules according to their needs. Slot 2 is also known as the Star Trigger Controller Slot, and its special functions will be explained in later articles. Figure 2 shows a typical 3U high PXI system architecture. The P1 connector on the backplane has 32-bit PCI signals, while the P2 connector has 64-bit PCI signals and PXI-specific signals. So what are these PXI-specific signals? PXI signals include the following, and their complete architecture is shown in Figure 3. [b] 1. 10MHz Reference Clock [/b] The PXI specification defines a low-skew 10MHz reference clock. This reference clock is located on the backplane and distributed to each peripheral slot. Its characteristic is that the wiring length from the clock source to each slot is equal, so the clock received by each peripheral slot is of the same phase. This is a convenient clock source for synchronizing multiple instrument modules. The basic 10MHz reference clock architecture is shown in Figure 4. [b]2. Local Bus[/b] On each peripheral slot, PXI defines a local bus and connects it to the adjacent left and right peripheral slots. There are 13 local buses on each side. This bus can transmit both digital and analog signals. For example, the left local bus on peripheral slot 3 can connect to the right local bus on peripheral slot 2, while the right local bus on peripheral slot 3 connects to the left local bus on peripheral slot 4. The left and right local buses on peripheral slot 3 are not interconnected on the backplane unless the instrument module inserted in peripheral slot 3 connects the signals from both sides. The local bus architecture is shown in Figure 5. **3. Local Bus** On each peripheral slot, PXI defines a local bus and connects it to the adjacent left and right peripheral slots. There are 13 local buses on each side. This bus can transmit both digital and analog signals. For example, the left local bus on peripheral slot 3 can connect to the right local bus on peripheral slot 2, while the right local bus on peripheral slot 3 connects to the left local bus on peripheral slot 4. The left and right local buses on peripheral slot 3 are not interconnected on the backplane unless the instrument module plugged into peripheral slot 3 connects their signals. The local bus architecture is shown in Figure 5. Figure 6: PXI Star Trigger Architecture **4. Trigger Bus** The trigger bus has 8 lines, connecting from the system slot (Slot 1) to the remaining peripheral slots on the backplane, providing a shared communication channel for all instrument modules plugged into the PXI backplane. This 8-bit wide bus allows multiple instrument modules to transmit clock signals, trigger signals, and specific transmission protocols. [b]Introduction to Synchronization Applications of PXI Instrument Modules[/b] After discussing the unique proprietary signals of PXI, we understand that the PXI system simply provides a convenient and simple environment for users. To truly leverage the advantages of the PXI system, these signals must be used in conjunction with instrument modules. Currently, there are hundreds of PXI instrument modules available from various manufacturers, and different types of instruments have different connection architectures and methods. Here, we will use an application example to illustrate how to use PXI's unique signals to achieve synchronization requirements. [b]Example:[/b] A certain detection device is used to detect the structure of an object under test. This device has eight sensors to sense the information transmitted back by the object and send the results as analog signals with a frequency of approximately 7.5MHz. Because these eight signals are time-dependent, when measuring these eight sensor signals, they must start acquiring data simultaneously, and the sampling clocks must be in phase; otherwise, the calculation results will be inaccurate. In addition, this detection device outputs a digital trigger signal simultaneously when the sensor begins transmitting a signal. The relationship between the digital and analog signals is shown in Figure 7. Figure 7: Output Timing Diagram of the Detection Device. To meet the aforementioned measurement requirements, we must select a suitable measurement module. First, the signal frequency returned by the sensor is 7.5MHz. Therefore, according to the Nyquist sampling theorem, the sampling frequency of the measurement module must be above 15MHz, and the input bandwidth of the module itself must be much higher than 7.5MHz to avoid attenuation of the input signal. Considering the above conditions, we selected the PXI-9820 from ADLINK Technology as the measurement module. The PXI-9820 is a high-speed data acquisition module with two sampling channels, a sampling rate of up to 65MS/s, and a front-end analog input bandwidth of up to 30MHz. Furthermore, the PXI-9820 is equipped with a phase-locked loop (PLL) circuit, which can phase-lock the external reference clock. The PXI-9820 can also transmit highly precise trigger signals to the other 13 peripheral slots via the PXI's Star Trigger. Therefore, the PXI-9820 is well-suited for this application. With the appropriate measurement modules in place, we need to plan the measurement process. First, since there are eight sensors to be measured, and each PXI-9820 only has two sampling channels, we need four PXI-9820s. Second, the measurement specifications require that the sampling phase of each channel be the same, so the clocks of each measurement module must be synchronized. Because each PXI-9820 has its own onboard sampling clock, its clocks cannot be guaranteed to be in phase. We use the 10MHz reference clock provided by the PXI backplane as the external reference clock input for the PXI-9820, and use the PXI-9820's own phase-locked loop (PLL) circuit to lock the clock phase. Figure 8 shows the case where the sampling clocks of the various instrument modules are not synchronized. Figure 9 shows the clock result after PLL phase-locking. Finally, since the detection equipment sends out a digital trigger signal when it starts transmitting analog data from the sensors, we use this trigger signal as the trigger condition for each PXI-9820. However, how can this trigger signal reach each PXI-9820 precisely and simultaneously? We inserted one PXI-9820 into the Star Trigger Controller slot, utilizing the unique Star trigger of this slot to transmit the signal to the other three PXI-9820s to achieve the most accurate trigger timing. Conclusion Using PXI instrument modules and the PXI platform as a measurement and testing platform not only fully utilizes the high-speed transmission characteristics of PCI and inherits the user's already familiar software platform, but also utilizes the trigger signals provided by PXI to achieve more precise synchronization functions. PXI developers worldwide offer hundreds of measurement and testing instrument modules, allowing users to complete a PXI system suitable for their application in the most convenient, fast, and economical way. This article clearly explains PXI signals and uses a simple example to illustrate how to synchronize instrument modules using PXI signals. It is hoped that this will provide users preparing to develop PXI systems with a preliminary understanding. Editor: He Shiping