MIL-STD-1553 Specification Tutorial
MIL-STD-1553B is the military specification defining a Digital Time Division Command/Response Multiplexed Data Bus. The 1553 databus is a dual-redundant, bi-directional, Manchester II encoded databus with a high bit error reliability. All bus communications are controlled and initiated by a main bus controller. Remote terminal devices attached to the bus respond to controller commands.
MIL-STD-1553B defines specifications for terminal device operation and coupling, word structure and format, messaging protocol and electrical characteristics.
MIL-STD-1553 Specification Tutorial
MIL-STD-1553B was developed from the growing complexity of integrated avionics systems and the subsequent increase in the number of discrete interconnects between terminal devices. Direct point-to-point wiring became complex, expensive and bulky, requiring definition of a multiplex data bus standard. The first draft of a standard in 1968 by the Aerospace Branch of the Society of Automotive Engineers (SAE) laid the foundation for the US Air Force’s adoption of MIL-STD-1553 in 1973.
A tri-service version, MIL-STD-1553A was released in 1975 and utilized in the Air Force F-16 and the US Army AH-64A Apache Attack Helicopter. Notice 2, released in 1986 and superceding Notice 1 released in 1980, is a tri-service standard for RT design specs and defines how some bus options are to be used. Currently revised to MIL-STD-1553B, Notice 2, it has become the internationally accepted networking standard for integrating military platforms.
Military services and contractors originally adopted MIL-STD-1553 as an avionics data bus due to its highly reliable, serial, 1 Megabit per sec transfer rate and extremely low error rate of 1 word fault per 10 million words, on a dual-redundant architecture.
This reliability has proven equally effective on communication networks in submarines, tanks, target drones, missile and satellite systems, land-based and launch vehicles, and space system including the current International Space Station and Shuttle programs.
MIL-STD-1553B defines the data bus structure for interconnection of up to 31 remote terminal devices. A single controller device on the bus initiates the command/response communication with the remote devices. The remote and control devices are interconnected over two, separate buses. Normal operation involves only the primary bus with the secondary bus available as redundant backup in the event of primary bus damage or failure.
MIL-STD-1553 Hardware Components
MIL-STD-1553 Bus Controller
The main function of the bus controller (BC) is to provide data flow control for all transmissions on the bus. In addition to initiating all data transfers, the BC must transmit, receive and coordinate the transfer of information on the data bus. All information is communicated in command/response mode - the BC sends a command to the RTs, which reply with a response.
The bus controller, according to MIL-STD-1553B, is the “key part of the data bus system” and “the sole control of information transmission on the bus shall reside with the bus controller, which shall initiate all transmission”. The bus can support multiple BCs, but only one can be active at a time.
Normal BC data flow control includes transmitting commands to RTs at predetermined time intervals. The commands may include data or requests for data (including status) from RTs. The BC has control to modify the flow of bus data based on changes in the operating environment. These changes could be a result of an air-to-ground attack mode changing to air-to-air, or the failure mode of a hydraulic system. The BC is responsible for detecting these changes and initiating action to counter them. Error detection may require the BC to attempt communications to the RT on the redundant, backup bus.
MIL-STD-1553 Remote Terminal
The remote terminal (RT) is a device designed to interface various subsystems with the 1553 data bus. The interface device may be embedded within the subsystem itself, or be an external interface to tie a non-1553 compatible device to the bus. As a function of the interface requirement, the RT receives and decodes commands from the BC, detects any errors and reacts to those errors. The RT must be able to properly handle both protocol errors (missing data, extra words, etc) and electrical errors (waveform distortion, rise time violations, etc). RTs are the largest segment of bus components.