What is MIL-STD-1553?
MIL-STD-1553 is a military standard published by the United States Department of Defense that defines the mechanical, electrical, and functional characteristics of a serial data bus. It was originally designed for use with military avionics, but has also become commonly used in spacecraft on-board data handling (OBDH) subsystems, both military and civil. It features a dual redundant balanced line physical layer, a (differential) network interface, time division multiplexing, half-duplex command/response protocol, and up to 31 remote terminals (devices).
Since its inception in 1973 and in subsequent revisions during the ensuing years, MIL-STD-1553 has evolved into the predominant, internationally accepted networking standard for the integration of military platforms. Today, the standard has expanded beyond its traditional domain of US Air Force and Navy aircraft to encompass applications for combat vehicles, ships, satellites, missiles, and the International Space Station Program, as well as advanced commercial avionic applications. Once considered primarily a military data bus standard, MIL-STD-1553 has caught the attention of commercial aircraft manufacturers who seek to capitalize upon the standard’s inherent reliability, robustness, maturity, and superior EMI performance.
MIL-STD-1553, Digital Internal Time Division Command/Response Multiplex Data Bus (presently in revision B), has become one of the basic tools being used today by the DoD for integration of weapon systems. The standard describes the method of communication and the electrical interface requirements for subsystems connected to the data bus. The 1 Mbps serial communication bus is used to achieve aircraft avionic (MIL-STD-1553B) and stores management (MIL-STD-1760B) integration.
MIL-STD-1553B, which superseded the earlier 1975 specification MIL-STD-1553A, was published in 1978. The basic difference between the 1553A and 1553B revisions is that in the latter, the options are defined rather than being left for the user to define as required. It was found that when the standard did not define an item, there was no coordination in its use. Hardware and software had to be redesigned for each new application. The primary goal of the 1553B was to provide flexibility without creating new designs for each new user. This was accomplished by specifying the electrical interfaces explicitly so that electrical compatibility between designs by different manufacturers could be assured.
The 1553 standard is organized similar to most military standards with a foreword, scope, referenced document section, definitions, general requirements, the appendix, and a tri-service Notice 2. Notice 2, which supersedes Notice 1, was developed to define which options of the standard are required to enhance tri-service interoperability and to further define some of the open-ended timing variables implied within the standard.
Seven change notices to the standard have been published since 1978.
The MIL-STD-1553 standard is now maintained by both the US DOD and the Aerospace branch of the Society of Automotive Engineers.
MIL-STD-1553 Key Elements
Some of the key MIL-STD-1553B elements are the bus controller, the embedded remote terminal (a sensor or subsystem that provides its own internal 1553 interface), the stand-alone remote terminal, bus monitor, and two other devices that are part of the 1553 integration; the twisted shielded pair wire data bus and the isolation couplers that are optional.
The bus controller’s main function is to provide data flow control for all transmissions on the bus. In this role, the bus controller is the sole source of communication. The system uses a command /response method.
The embedded remote terminal consists of interface circuitry located inside a sensor or subsystem directly connected to the data bus. Its primary job is to perform the transfer of data in and out of the subsystem as controlled by the bus controller. This type of terminal usually does not have bus controller capability. However, if the sensor itself is fairly intelligent, it can become a candidate for the backup bus controller function. Generally, an intelligent subsystem (i.e., computer based) can become a backup bus controller if a second computer, equal in function to the primary, is unavailable.
The stand-alone remote terminal is the only device solely dedicated to the multiplex system. It is used to interface various subsystem(s), which are not 1553 compatible with the 1553 data bus system. Its primary function is to interface and monitor transmission in and out of these non-1553 subsystem(s).
The bus monitor listens to all messages, and subsequently collects data, from the data bus. Primary applications of this mode of operation include: collection of data for on-board bulk storage or remote telemetry; or use within a “hot” or off-line back-up controller to observe the state and operational mode of the system and subsystems.
The fourth item is the data bus itself. The standard defines specific characteristics for the twisted pair shielded cable. Notice 2 tightens these requirements and adds a definition for connector polarity.
Data Bus Coupler
The last item to be discussed is the data bus coupler unit that isolates the main bus from the terminals. MIL-STD-1553B allows two types of data bus interface techniques; direct coupling and transformer coupling. Subsystems and 1553 bus elements are interfaced to the main data bus by interconnection buses (called “stubs”). These stubs are either connected directly to the main bus or interfaced via data bus couplers. The data bus couplers contain two isolation resistors (one per wire) and an isolation transformer (with a ratio of 1 to the square root of 2). The purpose of the data bus couplers is to prevent a short on a single stub from shorting the main data bus. The selection of the value of the resistors, the transformer’s turn ratio, and the receiver impedance are such that the stub appears to the main bus as a “clean interface” (i.e., high impedance). This technique reduces the distortion caused on the main data bus by the termination. The characteristics of the data bus couplers are discussed in paragraph 4.2.4 of our designer’s guide. Main buses utilizing direct coupled stubs must be designed to withstand the impedance mismatch of the stubs. This can be reduced by minimizing stub length (less than one foot) and “tuning” the bus by terminal spacing.
Designs not using data bus couplers should be carefully analyzed and tested to determine if waveform distortion is significant enough to cause receiver problems.The other risk associated with direct coupled stubs is a short on a stub will cause the main bus to fail. The obvious advantage to direct coupled stubs is the elimination of the logistical problems associated with another device and the installation problem of locating these small devices (approximately 1 inch cube) in the aircraft. Today, data bus couplers and line terminating resistors are available in molded packages, which can become part of the wiring harness, thus eliminating some of the installation problems. Also, multiple data bus couplers and data bus line terminating resistors are available in single packages, which reduces the number of unique units installed per aircraft.