The Chief Components of the MIL-STD-1553 Bus Standard
The United States Department of Defense (DOD) defines its standard serial data bus in MIL-STD-1553. The standard was initially intended to serve as the primary military avionics data bus specification, but has since expanded to civil and military spacecraft systems as well. The electrical, mechanical, and functional characteristics of the data bus – including the physical layer requirements and communications protocols – are described in the 1553 standard. A typical system layout consists of a main data bus connecting various remote terminals via a data bus coupler, a bus controller to manage communication, and a bus monitor to observe and collect transmissions. These principal components are described in further detail below.
The Data Bus
The MIL-STD-1553 data bus specification describes a shielded twisted pair cable in the physical layer. The current standard requires an impedance between 70 and 85 ohms at 1 MHz. Allowances are made for a secondary or tertiary communication bus for redundancy, consisting of independent cables connected to all terminals. The required bit rate for the bus is 1 megabit per second. The newest revision also specifies end-connector polarities.
The Remote Terminals
Any subsystems connected to the data bus with appropriate connectors and interface circuitry are considered remote terminals. Additionally, a bridge between two separate MIL-STD-1553 systems behaves as a remote terminal. The remote terminal interface circuity must translate sensor data or other subsystem information into 1553 standard communication protocol in order to relay messages to other subsystems through the bus controller. Receiving terminals can then translate the code into display data or other commands
The Data Bus Coupler
The Data Bus Coupler is the interface between the main MIL-STD-1553 data bus and any components at the remote terminals. The interfacing can either be direct or through transformer coupling. In either case, interconnection (stub) buses serve to connect the main bus to the subsystems. Transformer data bus couplers consist of an isolation resistor on each of the two bus leads and an isolation transformer.
The Bus Controller
The Bus Controller (BC) initiates bus communication and manages all data flow and transmission. There can only be a single bus controller on any given bus, and it is the only communication source of any kind present. The controller responds to communication requests from the terminals, issues commands to destination terminals, and performs error recovery and tracking. The specification allows for a Backup Bus Controller (BBC) to which control can be transferred in the event of the presence of error flags within the transmission code.
The Bus Monitor
The Bus Monitor (BM) is a passive system that monitors and collects all transmission data being sent over the data bus. The collected records are stored for later recovery or analysis, but they are also used in the event that the Backup Bus Controller must be initiated so that the backup controller will be instantly familiar with the current state of all subsystems on the bus. The Bus Monitor makes no transmissions of its own on the system.
The most recent major revision of MIL-STD-1553 is MIL-STD-1553B, which defined various options explicitly rather than leaving them to the user. Six more minor revisions, known as notices, have been issued since the publication of 1553B.