0 Introduction GaAs photocathode is a negative electron affinity photocathode with the advantages of high quantum efficiency, concentrated emission electron energy and angular distribution. Therefore, it has been widely used in many fields such as low-light image intensifiers, semiconductor sensitive devices, and spin polarized electron sources [1]. However, the preparation process of GaAs photocathode is extremely complex and has strict requirements on preparation process and conditions. At present, the preparation of GaAs photocathode mainly relies on skilled operators to carry out manual operation. This operation method not only wastes a lot of manpower and material resources, but also cannot guarantee the preparation quality and efficiency. In the preparation process, the online spectral response tester [2] which is commonly used can only be used to test the spectral response curve after the cathode is prepared and to evaluate the preparation quality of the cathode. It does not have the function of real-time acquisition of many other information in the cathode preparation process, such as vacuum degree, cesium source and oxygen source current, etc., and it does not have computer control of cesium (oxygen) source current and automation of the preparation process. Therefore, it greatly restricts the theoretical research, preparation quality and efficiency improvement of GaAs photocathode preparation process in China. This paper utilizes the advantages of CAN bus, such as high reliability, low cost, flexible configuration, and fast transmission speed [3-5], to design a GaAs photocathode preparation measurement and control system based on CAN bus, which can realize real-time testing of the above information and automatic control of cesium (oxygen) source current. 1. Composition of Measurement and Control System According to the requirements of GaAs photocathode preparation process, we designed the principle block diagram of GaAs photocathode preparation measurement and control system shown in Figure 1. The measurement and control system consists of three main parts: an ultra-high vacuum activation system, a computer, and peripheral measurement and control equipment. The ultra-high vacuum activation system is used for GaAs photocathode preparation (heating purification and cesium oxygen activation). Various information quantities in the preparation process can be connected to the computer through peripheral measurement and control equipment. The measurement and control equipment is divided into two parts: one part completes the testing of photocurrent and spectral response curve, and the other part completes the testing of vacuum degree and the testing and control of cesium (oxygen) source current signal. The first part is equivalent to a spectral response tester. When testing the spectral response curve, the computer controls the grating monochromator to output monochromatic light of a certain wavelength and irradiate the cathode surface. The cathode generates a weak photocurrent. After the photocurrent is amplified, it is converted into a digital signal by A/D. The computer processes the signal and the corresponding monochromatic light radiation power to obtain the spectral response curve of the cathode [2]. The CAN bus is not used in this part, so it will not be introduced in this paper. The focus is on the second part. In the second part, the vacuum degree and cesium (oxygen) source current signal are measured and controlled through the CAN bus. The measurement and control signal is connected to the computer through the USB-CAN converter. The design of this part combines the advantages of CAN and USB buses, so as to realize more flexible communication tasks and more powerful signal measurement and control functions. [align=center] Figure 1 Block diagram of GaAs photocathode preparation measurement and control system[/align] 2 Hardware design of measurement and control system 2.1 Hardware design of USB-CAN converter The USB-CAN converter realizes the protocol conversion between USB and CAN buses. Figure 2 shows its structural block diagram. In the diagram, the 89C52 microcontroller is responsible for monitoring the converter and communicating with the CAN and USB buses. The CAN controller interface circuit uses the SJA1000 and 82C250, and the USB controller interface circuit uses the CH372 USB universal device interface chip. In the microcontroller, both USB and CAN bus message reception use interrupt mode, which minimizes latency and improves the system's real-time communication capability. 2.2 Hardware Design of Multi-Information Measurement and Control Equipment The original system's vacuum gauge and analog power supply operate independently and lack the ability to communicate with a computer. To achieve the measurement and control of vacuum level and cesium (oxygen) source current information, and also to save costs, this paper adopts a solution of adding a multi-information measurement and control module to the original equipment, enabling it to have digital measurement and control and communication functions. The task of the multi-information measurement and control module is to extract the signals displayed by the equipment and transmit them to the computer, or to transmit control commands sent by the computer to the equipment. Therefore, the measurement and control module circuit actually consists of two parts: one part completes the testing of vacuum level and cesium (oxygen) source current information, and the other part controls the magnitude and on/off state of the cesium (oxygen) source current. In testing vacuum level and cesium (oxygen) source current information, to maintain consistency between the acquired data and the data displayed on the device, the method used is to directly extract the information displayed on the device's digital tube. Both the vacuum gauge and the analog power supply display consist of a 3-digit digital tube, so they can use the same test circuit. Figure 3 shows the block diagram of the multi-information test module. In the figure, the 3-digit digital tube of the vacuum gauge or power supply is connected to a dual 4-to-1 electronic switch CD4052. The microcontroller outputs an address selection signal from the latch, controlling the CD4052 to sequentially output two segments of the 8-segment display information from the digital tube, which are connected to the comparator LM358. After comparison and conversion to standard logic levels, the output is sent to the microcontroller. Finally, the microcontroller combines the signals from the three digital tubes to obtain the data displayed on the vacuum gauge or power supply. The main advantage of using a comparator here is that it allows for flexible adjustment of the comparison voltage according to the drive level of different digital tubes, and the output logic level is stable, thus making the test circuit more adaptable. After acquiring the display information, it can be transmitted to the computer via the CAN bus under the control of the microcontroller. [align=center]Figure 2 USB-CAN Converter Block Diagram[/align] [align=center]Figure 3 Multi-Information Test Module Block Diagram[/align] In the original system, the cesium (oxygen) source current was controlled by manually adjusting the analog potentiometer of the power supply. To achieve digital control of the current by the computer, the most direct method is to replace the analog potentiometer with a digital potentiometer. The block diagram of the current control module designed according to this scheme is shown in Figure 4. The power supply has a 10K coarse adjustment potentiometer and a 1K fine adjustment potentiometer, which are replaced by 10K and 1K digital potentiometers X9C103 and X9C102 respectively. In Figure 4, the two digital potentiometers are connected in series. This connection method allows for higher current control accuracy, and the actual control accuracy exceeds the three-digit display accuracy of the analog power supply, which fully meets the system's requirements for current control accuracy. The selection of the digital or analog potentiometer can be determined by the microcontroller outputting a control signal, which, after being driven, controls a double-pole double-throw relay. The microcontroller can also output control signals to control the relay to connect or disconnect the external 220V power supply of the analog power supply, thereby realizing the switching on and off of the cesium (oxygen) source current. Since the control is of 220V AC power, opto-isolation is necessary between the microcontroller and the driver. The computer can achieve automatic cesium-oxygen alternation in the cathode preparation process through the above method. [align=center] Figure 4 Block diagram of the cesium (oxygen) source current control module[/align] 3 Measurement and Control System Software Design 3.1 USB-CAN Converter Software Design The USB-CAN bus converter software mainly performs protocol conversion between USB and CAN buses, enabling communication between the measurement and control equipment and the computer. Its software design mainly includes the initialization of the CAN and USB interface chips, as well as the sending and receiving of CAN and USB messages. The USB and CAN bus controller chips are CH372 and SJA1000 respectively. CH372 has a built-in firmware mode, making it very convenient to use. The reception of USB and CAN bus messages is achieved using interrupt mode, which ensures the fastest response speed and improves communication efficiency. Received USB messages can be directly sent to the measurement and control equipment via the CAN bus, while received CAN messages are sent to the computer in two steps: first, the computer's interrupt service routine is triggered by a USB interrupt transmission, and then the CAN messages are sent to the computer in batch transmission. This method can trigger the computer to process the acquired data in real time, improving the computer's response speed. 3.2 Multi-Information Measurement and Control Software Design The multi-information measurement and control software consists of test software and control software. The function of the test software is to extract the data displayed on the device's digital tube, combine and decode it to convert it into the measured information. The key to the test software design is to control the CD4052 to sequentially select each digital tube and output the displayed information multiple times, then combine these information together, and then obtain the vacuum degree or current value displayed by the device by looking up the pen type code table. The function of the control software is to control the magnitude and on/off of the cesium (oxygen) source current through digital potentiometers and relays. The focus of the control software design is the control of the current magnitude. Since the power supply output current and the potentiometer resistance are not necessarily linearly related, and the situation is different for different power supplies, the control algorithm should have a certain degree of adaptability. Drawing on experience from manual current adjustment, the control software employs a two-step adjustment algorithm: first, a "coarse adjustment," then a "fine adjustment." The two-step adjustment method first calculates the difference between the current setpoint and the current value. If the difference is large, a "coarse adjustment" (adjusting X9C103) is performed. If the difference falls within the adjustment range of X9C102, a "fine adjustment" (adjusting X9C102) is performed. During fine adjustment, a "half-splitting" adjustment method (similar to the half-splitting search method) is used to quickly bring the current close to and reach the setpoint. This two-step adjustment algorithm ensures short adjustment time and high control accuracy for the output current. 3.3 Computer Software Design The computer software consists of three main modules: a multi-information measurement and control module, an information display module, and a performance parameter calculation module. The multi-information measurement and control module includes A/D acquisition of photocurrent, acquisition of vacuum level, and acquisition and control of cesium (oxygen) source current. Photocurrent is collected at regular intervals under computer control. Vacuum level and cesium (oxygen) source current are collected differently; they are actively sent to the computer by the vacuum gauge or power supply when their displayed information changes. The computer receives the data, updates the display, or processes the data. To ensure the computer receives information from the devices in a timely manner, we adopted pseudo-interrupt and multi-threading techniques in the software. The pseudo-interrupt service routine is simulated at the application layer by a dynamic link library (DLL) after the CH372 driver interrupts, enabling timely response to USB interrupt transmissions. Multi-threading allows for parallel processing of multiple tasks, improving response speed and processing efficiency. The performance parameter calculation module mainly analyzes and processes the test results to obtain the desired performance parameters for various preparation processes or cathodes. 4. Conclusion The application of the CAN bus in the GaAs photocathode preparation measurement and control system successfully solved the problems of large amounts of system measurement and control information and high requirements for reliability and real-time performance. The successful development of this measurement and control system will facilitate in-depth theoretical research on GaAs photocathode preparation processes and promote the improvement of GaAs photocathode preparation levels. This system has good application prospects and promotional value. References [1] Du Xiaoqing, Chang Benkang, Zong Zhiyuan. Theoretical optimization of p-type doping concentration of GaAs photocathode. Vacuum Science and Technology, 2004, 24(3): 195-198 [2] Qian Yunsheng, Zong Zhiyuan, Chang Benkang. 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