THE WORLD of AVIONICS IN YOUR HAND
BMC COMMUNICATIONS CORP.
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MIL-STD-1553B, which superseded the earlier 1975 specification MIL-STD-1553A, was published in 1978.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.
BMC intelligent boards supports all MIL-STD-1553B modes and can be software programmed as bus controller (BC), as embedded remote terminal or stand-alone remote terminal (RT) , or as bus monitor (BM) .
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).
BMC include in his product line an interface for TWO remote terminals in a single unit ( SBC-UADI-2RT). For more information contact our customer service or email to: firstname.lastname@example.org .
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.
BMC boards combine different 1553 operational modes into a single unit:
Test environments but not only, requires concurrent modes techniques. Not many manufacture include this features in their system.
BC-RT – The unit transmit not only the BC command but also the RT response. It is very useful to test BM mode
BC-BM or RT-BM – BM not only store BUS raw data transfer but add information like timetags making simple real time information analysis.
The concurrent modes should include time tagwith at least 32 bit in the stored data structure.
1553 word Bit Structure
The protocol is based on a 20bit structure word:
1- The first 3 bits are data or command synchronization signals
2- The following 16 bits is data or command information
3- The last bit is even parity bit
Information from the bus controller is called commands. The first word is sub-divided into:
a- RT address – 5 bits
b- Transmit/Receive bit – 1 bit
c- RT sub address - 5 bits
d- Parity bit – 1 bit
Mil-std-1553 couples and cables
The MIL-STD-1553B definition of a data bus is “a twisted-shielded pair transmission line, a 70 ohm coaxial wire, made up of a main bus and a number of attached stubs”. Shielding limits signal interference from outside sources and the twisted pair maintains message integrity through noise canceling.
MIL-STD-1553B allows two methods of coupling terminal devices to the main bus:
a. Direct coupling
Direct Coupling Direct coupling connections are wired directly to the bus cabling. The isolation resistors and transformer are internal to the terminal device, not requiring additional coupling hardware. Direct coupling connections can only be used with stub lengths of less than 1 foot. Isolation resistors provide some protection for the main bus in the event of a stub or terminal short, but MIL-STD-1553B cautions the use of direct coupling because a terminal short could disable the entire bus. Output Voltage signals pick to pick are +-27 Volts.
b. Transformer coupling.
Transformer Coupling a second isolation transformer, located external to the terminal device, in its own housing with the isolation resistors. Transformer coupling extends the stub length to 20 feet and provides electrical isolation, better impedance matching and higher noise rejection characteristics than direct coupling. The electrical isolation prevents a terminal fault or stub impedance mismatch from affecting bus performance. Output Voltage signals pick to pick are +-`19 Volts.
The multiplex data bus and associated equipment which is developed either alone or as a portion of system or subsystem development must follow MIL-STD-1553 rules. The contractor is responsible for invoking all the applicable requirements of this Military Standard and any and all subcontractors he may employ. There are some revisions in the standard.
The developer should select an interface which includes embedded real time development tools like concurrent modes
These tools are crucial during development stage or in case of operational problems.
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