Abstract: In recent years, China's railway locomotive maintenance system has gradually transitioned from planned maintenance to condition-based maintenance, and locomotive fault diagnosis systems have been continuously developing. In an expert system-based locomotive fault diagnosis system, the locomotive's intelligent instrument for testing needs to send testing data to the expert system, and the expert system sets various parameters for the intelligent instrument for testing. To enable effective transmission of information and data between the expert system and the intelligent instrument for testing, this paper discusses the communication implementation between the locomotive fault diagnosis system and the intelligent instrument for testing. Keywords: locomotive fault diagnosis communication **Communication Module in Locomotive Fault Intelligent Diagnose System** Wang Li, Zang Haihe ** Abstract: ** In recent years, the locomotive service system has gradually transitioned from planned servicing to status servicing, and the locomotive fault diagnosis system has developed rapidly. In the locomotive fault diagnosis based on the expert system, the locomotive intelligent instrument transmits data to the expert system, and the expert system sets parameters to the intelligent instrument. To ensure that information and data are effectively transmitted between the expert system and the examination intelligent measuring appliance, this text discusses the communication between the locomotive fault intelligent diagnosis system and the intelligent instrument. **Keywords: ** locomotive fault diagnosis communication **1 Introduction** Railways are a major mode of transportation in China, and locomotives are the main traction power. The quality of locomotives directly affects the organization of railway transportation. Therefore, for decades, improving locomotive quality and operational efficiency while reducing maintenance costs has been an important part of locomotive maintenance work. The transition from traditional planned maintenance to condition-based maintenance is the future direction of locomotive maintenance. Planned maintenance plays a crucial role in locomotive maintenance, involving regular shutdowns for inspection and replacement of parts according to a maintenance plan, which can prevent certain accidents. However, with improved management, planned maintenance has increasingly revealed its weaknesses, such as blind maintenance, susceptibility to human error during maintenance, and high maintenance costs. Condition-based maintenance, on the other hand, assesses the severity and development trend of locomotive faults based on online detection and diagnostics, determining the timing and content of maintenance. The automatic locomotive fault diagnosis system was specifically developed to adapt to the changing locomotive maintenance system and for condition-based maintenance. Based on various fault detection instruments, it utilizes modern fault diagnosis technology to discover and determine the location and nature of locomotive faults, predict fault trends, and propose corresponding decisions. 2. Automatic Locomotive Fault Diagnosis System The automatic locomotive fault diagnosis system determines whether a system component is faulty by comparing the actual output of the system under test with typical fault values. If a fault is found, it isolates fault symptoms from the detected fault information, identifies the cause of the fault, locates the fault source, and takes corresponding corrective measures. Faults in railway locomotive equipment directly affect the safe and economical operation of the locomotive and even the entire railway system. Fault diagnosis is a complex and highly experience-based technical task. Locomotive equipment faults have many causes, requiring rapid, effective, and accurate fault identification and timely troubleshooting. Utilizing expert systems for fault diagnosis and providing corrective measures to assist maintenance personnel in handling accidents and improve the safe and economical operation of locomotives is a specific application of expert systems in railway locomotive fault diagnosis systems. 3. System Communication Implementation Due to the inherent limitations of microcontroller systems, complex functions must be implemented by the system host. Therefore, when performing multi-point measurement and control tasks in industrial settings, the system generally adopts a master-slave structure. This means that the system is centered on a master PC, composed of distributed lower-level microcontroller systems. Each lower-level system exists independently with different tasks, but all are controlled, scheduled, and managed by the master host. The locomotive fault detection and diagnosis system also uses this approach. Because a locomotive is a moving piece of equipment, and given the limited level of locomotive testing capabilities in China, new testing equipment must undergo rigorous review and approval before it can be used on locomotives. Therefore, locomotive testing differs from general equipment testing. Online testing is not feasible in the near future; currently, only offline testing methods are available. Considering the current situation of locomotive testing, the development and design of the intelligent circuit testing instrument system and the locomotive fault diagnosis system must ensure that the intelligent circuit testing instrument has data communication capabilities, enabling communication with the locomotive fault diagnosis system. 3.1 Communication Protocol This protocol is designed to enable the efficient transmission of information and data between the locomotive fault diagnosis system (host) and various intelligent instruments (microcontrollers). It includes: allowing the host to transmit and access all setting parameters to all microcontrollers and allowing the host to access all measurement data of the microcontrollers. The communication rules between the locomotive fault automatic diagnosis system and various intelligent instruments are as follows: (1) Communication should follow the master-slave mode. (2) The host will initialize and control the information transmitted on the communication loop. (3) Communication will never be initiated by the microcontroller. (4) All communication actions occur in frames. A frame is a simple string, which can contain up to 160 bytes. These bytes that make up the frame constitute standard asynchronous serial communication and are transmitted in the form of 8 data bits, 1 stop bit, and no parity bit. (5) All frames have only two types, namely from host to microcontroller and from microcontroller to host. The two types of frames are distinguished by the synchronization byte. (6) If the host or any microcontroller receives a frame containing an unknown command, the frame is invalid. The communication protocol between the automatic locomotive fault diagnosis system and various intelligent instruments is as follows: (1) The microcontroller receives the read data command frame sent by the host. SYN: Synchronization byte, 14H, 1 byte. FADDR: System function module address, different modules of the system correspond to different detection instruments, 1 byte. YADDR: Intelligent instrument address, 1 byte. CODE: Password, 2 bytes. DFA: Data start address, 2 bytes. DLA: Data end address, 2 bytes. CRC: CRC check code, CRC check of all bytes from the synchronization byte to the data end address, 1 byte. (2) The microcontroller sends the data frame. SYN: Synchronization byte, 27H, 1 byte. DLEN: Length, the number of data bytes from the start address to the end address of the data area, 1 byte. DATA1: First data, 1 byte. DATA2: Second data, 1 byte. ……… DATAn: Last data, 1 byte. CRC: CRC check code, CRC check of all bytes from the synchronization byte to the last data, 1 byte. (3) The microcontroller receives the data frame transmitted by the host. SYN: Synchronization byte, 14H, 1 byte. FLAG: Threshold parameter/Chinese character library selection flag, 1 byte. FADDR: System function module address, 1 byte. YADDR: Intelligent instrument address, 1 byte. CODE: Password, 2 bytes. DLEN: Data length, 1 byte. DATA1: First data, 1 byte. DATA2: Second data, 1 byte. ……… DATAn: Last data, 1 byte. CRC: CRC checksum, 1 byte. (4) The microcontroller sends the response frame. SYN: Synchronization byte, 27H, 1 byte. DLEN: Length, number of data bytes from the start address to the end address of the data area, 1 byte. YADDR: Intelligent instrument address, 1 byte. FADDR: System function module address, 1 byte. ACK/NAK: Response/Non-response, if all data is received correctly, this byte is FFH, otherwise it is 00H, 1 byte. CRC: CRC checksum, a CRC check of all bytes from the synchronization byte to ACK/NAK, 1 byte. 3.2 Implementation of Serial Communication To achieve communication between the locomotive intelligent detector and the system, a communication key was specifically set in the detector, and a communication function was set in the locomotive fault diagnosis system. Here, only the communication function in the microcontroller is described. The program flow of the communication key is shown in Figure 1. During initialization, the detector's communication interface is in the receiving state. Only after the detector receives its own information is it set to the sending state, and after sending, it is set back to the receiving state. According to the communication protocol, all communication actions occur in frame format. There are only two cases: transmission from the host to the microcontroller and transmission from the microcontroller to the host, distinguished by the synchronization byte. Therefore, the flowchart first determines the synchronization byte, then determines whether it is receiving or sending data. If it is receiving data, it enters the receive interrupt service routine to receive the data. After receiving, a CRC check is performed. If incorrect, a NAK signal is sent, waiting for re-reception; if correct, an ACK response signal is sent, and it is determined whether the data is a threshold parameter or Chinese character library data, and then the corresponding data initialization is completed. [align=center]Figure 1 Communication Key Flowchart[/align] 4 Conclusion Locomotive equipment failures have many causes, requiring rapid, effective, and accurate fault identification and timely troubleshooting. This necessitates that various locomotive testing instruments transmit their data to the locomotive intelligent fault diagnosis system in a timely manner. Following the communication protocol designed in this paper, the locomotive intelligent fault diagnosis system completes data communication with various locomotive testing instruments. Practice shows that this communication system ensures the reliability and real-time performance of data transmission, providing strong support for the comprehensive automation of railway locomotive testing, diagnosis, and maintenance. The author's innovation lies in designing a communication protocol between the diagnostic system and various locomotive testing instruments, addressing the communication requirements between the locomotive intelligent fault diagnosis system and testing instruments, as well as with the railway data network, thus satisfying communication needs between different systems. References: [1] Yang Shizhong, Xing Lijuan. Parallel communication protocol between PC and microcontroller. Automation Instrumentation [J]. 2006.02 [2] Ding Qingyan, Zhang Xiuyuan. Basic structural design of railway signal fault diagnosis expert system [J]. Railway Computer Application, 2005 (5): 29-30. [3] Hong Jiaping. 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