I. History of the 1553B Bus
The 1553B bus is short for MIL-STD-1553 bus, where "B" stands for BUS. The MIL-STD-1553 bus is an internal time-division command/response multiplexed data bus for aircraft. The 1553B data bus standard is a serial multiplexed data bus standard published by the United States in the 1970s. The 1553B bus can connect up to 31 remote terminals. It uses a command/response communication protocol and has three terminal types: Bus Controller (BC), Remote Terminal (RT), and Bus Monitor (BM). Information formats include BC to RT, RT to BC, RT to RT, broadcast, and system control. The transmission medium is shielded twisted-pair cable. The 1553B bus uses direct coupling and transformer coupling. It has a multi-redundancy bus topology, bidirectional transmission capabilities, a transmission speed of 1 Mbps, and uses half-duplex transmission with Manchester encoding. This encoding method is used because it is suitable for transformer coupling. Direct coupling is not conducive to terminal fault isolation and can cause the entire bus network to be completely paralyzed due to the failure of one terminal. Therefore, the protocol explicitly states that direct coupling is not recommended.
Prior to the 1960s, aircraft avionics systems lacked standardized, universal data channels, often requiring extensive cabling to connect various electronic units. As avionics systems became increasingly complex, the cabling used in this communication method became space-consuming and heavy, and the definition and testing of transmission lines were complex and costly. To address this issue, the U.S. SAEA2K Committee, with support from the military and industry, decided in 1968 to develop a standard signal multiplexing system, and published the MIL-STD-1553 standard in 1973. The 1553B multiplexed data bus of 1973 became the technology adopted by future military aircraft. It replaced the bulky equipment used to transfer data between sensors, computers, indicators, and other aircraft devices, significantly reducing aircraft weight and offering simplicity and flexibility. A revised version of this standard, MIL-STD-1553, was published in 1978. In 1980, the U.S. Air Force made further modifications and additions to the standard. This standard, as one of the foundations of the U.S. Department of Defense's weapon system integration and standardization management, is widely used in aircraft integrated avionics systems, external stores management and integration systems, and is gradually being expanded to flight control systems, as well as tanks, ships, aerospace, and other fields. Originally used by the U.S. Air Force for aircraft avionics systems, it is now widely used by the U.S. and European Army, Navy, and Air Force, and is becoming an international standard. my country promulgated its own military standard in 1987.
II. Features of the 1553B Bus
The 1553B bus is a centralized time-division serial bus, characterized by distributed processing, centralized control, and real-time response. Its reliability mechanisms include fault prevention, fault tolerance, error detection and location, error isolation, error correction, system monitoring, and system recovery. Employing a dual-redundancy system with two transmission channels, it ensures good fault tolerance and fault isolation. In summary, the 1553B bus has the following characteristics:
First, it offers excellent real-time performance. The 1553B bus has a data transmission rate of 1 Mbps, each message contains a maximum of 32 words, and the time required to transmit a fixed message is short. Its data transmission rate is higher than that of typical communication networks.
Secondly, the 1553B employs reasonable error control measures and unique mode commands to ensure data transmission integrity. It utilizes a feedback retransmission error correction method. When a BC issues a command or sends a message to a RT, the terminal should send back a status word within a given response time. If the transmitted message is incorrect, the terminal refuses to send back the status word, thus reporting the previous message transmission as invalid. Furthermore, the unique mode commands not only enable the system to complete data communication control tasks but also check for faults and perform fault tolerance management functions.
Third, it has high bus efficiency. The bus topology has high requirements for bus efficiency. Therefore, the 1553B has strict limits on certain mandatory requirements related to bus efficiency indicators, such as command response time, message interval time, and the length of the maximum and minimum data blocks for each message transmission.
Fourth, it features command/response and "broadcast" communication modes. The BC can broadcast a time synchronization message to all RTs, thus controlling all message transmissions on the bus via commands issued by the bus controller. Relevant terminals should respond to these commands and execute the corresponding operations. This approach is well-suited for centrally controlled distributed processing systems. However, the high price of the 1553B bus limits its widespread application in industrial sectors.
III. 1553B Bus Message Transmission Mechanism
Information on the 1553B bus is transmitted in Manchester code as messages. Each message consists of a maximum of 32 words, divided into three categories: command words, data words, and status words. Each word is 20 bits long, with 16 bits being the valid information bits. The first 3 bits of each word are the synchronization header, and the last bit is the parity bit. The valid information (16 bits) and parity bit are transmitted on the bus in Manchester code, with each bit taking 1 second to transmit (i.e., a code rate of 1 MHz). The synchronization header occupies 3 bits; positive bits followed by negative bits indicate command and status words, while negative bits followed by positive bits indicate data words.
Of these three types of words, the command word is located at the beginning of each message, and its content specifies the requirements for that transmission. The status word can only be issued by the RT, and its content represents the RT's feedback to a valid command issued by the BC. The BC can determine its next action based on the content of the status word. The data word can be transmitted from the BC to an RT, from an RT to the BC, or from one RT to another; its content represents the transmitted data.
The message transmission process on the 1553B bus is as follows: the bus controller issues a receive/send command to a terminal, and the terminal sends back a status word within a given response time range and performs message reception/send. The BC verifies the success of the transmission by accepting the status word replied by the RT and performs subsequent operations accordingly.
A message is the basic unit of communication on the 1553B bus. To perform a specific function, multiple messages must be organized into a new structure called a frame. The structure of a frame is shown in Figure 2. In the figure, the time to complete a message is called the message time, the interval between two messages is called the message interval time, and the time to complete a frame is called the frame time. In practical applications, all three times can be set through programming.
IV. Application of the 1553B bus in weapon communication
Driven by military needs, the electronic equipment on weapons is constantly increasing. Effectively integrating this electronic equipment to achieve resource and functional integration has become an inevitable requirement for weapon development. The foundation of an integrated weapon electronic system is the adoption of a data bus structure. This data bus allows the processor (including hardware and software), information transmission, and control display subsystems to be shared by various tasks. This offers the following advantages: reduced weapon size and weight, improved weapon system reliability, lower costs, and increased detection accuracy. Modern weapons generally have the following requirements for their communication systems:
First, it can effectively realize data transmission between subsystems and meet specific communication characteristics;
Secondly, the communication subsystem operates relatively independently, is as transparent as possible to the application software, and occupies as little time as possible on the host.
Third, the communication system is flexible and easy to modify.
Fourth, the communication subsystem has strong anti-interference capabilities.
The excellent performance of the 1553B bus perfectly meets the above requirements, which has led to its increasing importance in modern weapon systems. It has become the main working pillar of electronic systems on weapon platforms such as armored vehicles, ships, and aircraft.
Avionics systems typically comprise more than a dozen onboard computer subsystems. Effective data communication between these subsystems is crucial to the success of the entire aviation system. Since the United States published the military standard MIL-STD-1553B bus in 1973, it has been rapidly adopted by the Air Force and used on various aircraft such as the F-16, F-18, B-1, and AV-SB.
Currently, there are many types of shipboard tactical data buses in the world that can be used as military standards and specialized systems, but the most widely used is undoubtedly the American MIL-STD-1553B. The 1553B uses coaxial cable, shielded twisted pair cable, and optical fiber as transmission media, and terminals are coupled to the bus via transformer coupling or direct coupling. This data bus's transmission rate, transmission distance, and number of remote terminals can well meet the communication requirements of various small and medium-sized ships and submarine systems, hence its widespread application.
Military vehicles and various armored vehicles, as combat platforms for ground weapons, frequently operate in harsh environments characterized by strong vibrations, high noise levels, high dust levels, and large temperature variations. Therefore, data communication between their internal electronic devices requires rigorous fault detection to achieve high reliability, survivability, and fault tolerance. In terms of real-time performance, integrated powertrain control requires separate control of the engine and transmission. Data communication between these two systems demands extremely short maximum response times for a single message to achieve real-time control of the engine and transmission, thereby improving the overall performance of the powertrain system. Furthermore, there are specific requirements for data communication, such as protocol simplicity, short frame transmission, frequent information exchange, network load stability, high security, and cost-effectiveness. The 1553B bus offers high reliability and excellent real-time performance. For integrated powertrain control systems—characterized by diverse data communication types, large data volumes, high real-time requirements, and a small number of network nodes—the 1553B bus provides significant performance advantages over most existing buses.
The key technologies for the 1553B bus in weapon communication systems generally include the following:
First, the design of the bus interface hardware and software. Interface cards or controllers are used to connect to the hardware of each weapon subsystem. Simultaneously, corresponding communication control software needs to be developed, including transmission layer software and driver layer software, to achieve true functional integration at the application layer through information and resource sharing, in accordance with the weapon's operational objectives.
Second is the Interface Control Document (ICD). The ICD consists of interface signals that interconnect various electronic devices within the weapon via the 1553B data bus. Based on the weapon's control strategy and objectives, a compliant ICD file must be written to define the periodic and random data transmitted on the bus. Only in this way can the interrelationships between data streams be determined, enabling efficient integration of functions and effectively improving the weapon's combat performance.
Third is the bus list. The bus list refers to the set of all possible bus commands that can be transmitted within a cycle. Based on the control requirements of the weapon platform, the command and message queues to be transmitted within a cycle are determined, time slices are divided according to the size of the cycle, and the message queues are sorted and optimized to balance the bus load and improve the utilization of the bus and the real-time performance of data transmission.
