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1553 Terminology - MIL-STD-1553

1553 Terminology

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Address Programming
The method by which an RT is made unqiue from all others.

The Avionics Full Duplex Switched Ethernet standard, known as AFDX or ARINC 644, is a data network for safety-critical applications that utilizes dedicated bandwidth while providing deterministic Quality of Service (QoS). AFDX is based on IEEE 802.3 Ethernet technology and utilizes commercial off-the-shelf (COTS) components. It is described specifically by Part 7 of the ARINC 664 Specification, as a special case of a profiled version of an IEEE 802.3 network per parts 1 & 2, which defines how Commercial Off-the-Shelf networking components will be used for future generation Aircraft Data Networks (ADN). The six primary aspects of AFDX include full duplex, redundancy, deterministic, high speed performance, switched and profiled network. AFDX is widely becoming adopted within the avionics industry as the successor to ARINC 429 point-to-point communications.

Analog Front End of a Terminal
The analog front-end portion of a 1553B terminal consists of one or more channels, each of which contains an interface to translate the 1553B bus signal into a digital signal with voltage levels appropriate for the remainder of the terminal. The fact that most terminals being designed today use purchase hybrid of VLSI IC parts has gone far toward simplifying terminal design.

The American National Standards Institute or ANSI is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organization also coordinates U.S. standards with international standards so that American products can be used worldwide.

ANSI accredits standards that are developed by representatives of standards developing organizations, government agencies, consumer groups, companies, and others. These standards ensure that the characteristics and performance of products are consistent, that people use the same definitions and terms, and that products are tested the same way. ANSI also accredits organizations that carry out product or personnel certification in accordance with requirements defined in international standards.

A treatment which renders nonconductive material receptive to electroless deposition.

Applications Software
Electronic components, such as transistors, diodes, thyristors, etc., which can operate on an applied electrical signal so as to change its basics character; i.e., rectification, amplification, switching, etc.

Aeronautical Radio, Incorporated (ARINC), established in 1929, is the leading provider of transport communications and systems engineering solutions for major industries (aviation, airports, governement, and transportation).

ARINC 429 is the technical standard for the predominant avionics data bus used on most higher-end commercial and transport aircraft. It defines the physical and electrical interfaces of a two-wire data bus and a data protocol to support an aircraft's avionics local area network. Messages consist of a single data word, 32 bits in length.

The ARINC 561 specification provides a standard communications method for the "Air Transmort Inertial Navigation System". It uses 6 wires in 3 pairs to transmit coloc, sync and data signals using NRZ encoding with voltage levels of 12V, and a word length of 32 bits.

ARINC 575 is the precursor to ARINC 429. It is a 2 wire version, which was used in the Digital Air Data System, later evolved to be ARINC 429. It uses a low speed data rate with 32 bit words.

ARINC 615 is a family of standards covering "data loading", commonly used for transferring software and data to or from avionics devices. ARINC 615 is an ARINC 429 compliant system, which uses a high-speed data loader to transfer information to and from on-board digital systems. This software protocol layered on top of an ARINC 429 physical layer has two versions of the loader, PDL - a portable flight line peice of test equipment, and ADL - designed to fit in commercial aircraft instrument panels.

ARINC 664 is based on the IEEE 802.3 Ethernet standard. It has been modified to remove the non-avionics-quality services and add new features such as bounded latency and guaranteed bandwidth. This first popular implementation is known as AFDX ( Avionics Full Duplex Switched Ethernet).

ARINC 708 is an avionics standard pecific to airborne weather radar systems. It is used as the output from the radar to the radar display. The bus uses 2-wires, is simplex, Manchester encoded and runs at a one-megabit data rate. It was originally based upon a simple derivative of MIL-STD-1553 technology. The data words are 1600 bits long which is composed of one, 64-bit status word and 512, 3-bit data words.

ARINC 717 specification describes the protocol for communication between the Digital Flight Data Acquisition Unit (DFDAU) and the Digital Flight Data Recorder (DFDR). The Flight Data Recorder sends a continuous data stream of Harvard Bi-Phase encoded 12 bit words which is encoded in frames. The frames are broken into four sub-frames, which contain a one-second sequesnce of 12-bit words. Each subframe is marked by a unqiue sync word that is used by the receiver to synchronize with the incoming data. ARINC 717 also provides an alternate output data stream that is identical to the primary Harvard Bi-Phase encoded stream, except that it is encoded in BPRX format (the same as ARINC 429). ARINC 717 data is organized by words, sub-frames, frames, and in some cases super-frames.

Asynchronous Operation
An electrical element capable of modifying an input voltage in such a way as to achieve rectification, amplification, or switching action, e.g., transistors. Discrete devices such as diodes or transistors; or integrated devices, such as analog or digital circuits in monolithic or hybrid form.

Avionics comprise of electronic systems for use on aircraft, artificial satellites and spacecraft, comprising communications, navigation and the display and management of multiple systems. It also includes the hundreds of systems that are fitted to aircraft to meet individual roles

Avionics Hot Bench
A network containing active and passive elements.


Contraction of binary digit, a bit is the basic unit of information in computing and telecommunications. The bit may be either zero or one. In information theory, a bindary digit is equal to one binary decision or the designation of one of two possible values or states of anything used to store or convey information.

The symbol for bit, as a unit of information, is either simply "bit" (recommended by the ISO/IEC standard 80000-13 (2008)) or lowercase "b" (recommended by the IEEE 1541 Standard (2002)).

Bit Rate
The number of bits transmitted per unit of time, usually per second.

The designation applied to all wirelines, components, equipment, and systems which handle only encrypted or unclassified signals, and areas in which no uncrypted or classified signals occur.

Operation of a data bus system such that information transmitted by the bus controller using a unique broadcast address is addressed to all of the terminals connected to the data bus.

Broadcast Command Received Bit
This bit is set by an RT that implements Broadcast Commands any time that a valid Broadcast Command has been received. Since there is no response to a Broadcast Command, the setting of this bit allows the Bus Controller to subsequently check taht teh command was received properly by issuing a "Transmit Status" or "Transmit Last Command" Mode Command.

Built-In-Test (BIT)
The capability of a line-replaceable unit (LRU)to perform some form of self-test.

A subsytem that transfers data between components. Refering to the 1553 data bus, it is the part of the network, which is terminated in its characteristics impedance, and to which stubs are attached.

Bus Controller
The Bus Controller (BC) is the terminal responsible for directing the flow of information on a MIL-STD-1553 bus. There is onlt one BC on a 1553B bus at any given time, although other terminals on teh bus may also have the capability to operate as a BC. The BC is the only terminal allowed to issue commands onto the bus, and no other terminal can transmit anything unless instructed by the BC. Commands issued by the BC include commands for the transfer of data, or for the control and management of the Bus.

Bus Coupler
Bus Couplers provide electrical isolation ont he 1553 bus. The circuit is used to couple signals between the main bus cable and transformer coupled stub cables. A single stub bus coupler contains two isolation resustors (one per wire), an isolation transformer, and a sheilded enclosure. The purpose of the bus coupler is to prevent a short on a single stub from shorting the main data bus. Bus couplers are avialable with different options, such as multiple stub connections and internal terminations.

Bus Interface Unit (BIU) Function
This term is generally interchangeably with "terminal", as defined in 1553B: "The electronic module necessary to interface the data bus with the subsystem and the subsystem with the data bus. Terminals may exist as separate line replaceable units (LRUs) or be contained within the elements of the subsystem.

Bus Interface Unit (BIU) Hardware
This term describes a particular set of hardware that performs the interface between the data bus and the internal portion of an embedded or standalone remote terminal. As a minimum, it refers to the digtial decode and encode logic that expands to the complete analog-to-digital interface between the data bus and the internal remote terminal electronics or the subsystem for embedded terminals.

Bus Monitor
The Bus Monitor (BM) is the terminal assigned the task of receiving bus traffic and extracting selected information to be used at a later time. The monitor does not transmit status words or anything else on the bus. It may not have a terminal address, but can receive information addressed to any (or all) of the terminals on the bus. A Bus Monitor may have an assigned terminal address, allowing it to act like an RT for commands to that address. Two common applications Bus Monitors are used are: 1. Instrumentation—for recording bus traffic from many or all terminals for off-line analysis, 2. Backup BC—to provide a terminal with enought information to become the BC on the bus if commanded to do so, either with a Dynamic Bus Control Mode command, or some other method.

Bus Repeater
A Bus Repeater is a device which, when inserted in a bus, boosts the weak inout signal back up to its specified levels on the output—enabling longer cable lengths than are normally used.

Bus Switch
A Bus Switch allows for switched connections between multiple buses.

Busy Bit
An RT that is functional, but cannot transfer data to or from the subsystem on command from the Bus Controller is  busy. An RT that is busy should set the "Busy" bit in its Status Word responses on the bus.


CAN (Controller Area Network) Bus is a vehicle bus standard designed to allow microcontrollers and devices to communicate with each other within a vehicle without a host computer. CAN is a message-based protocol, developed by Bosch GmbH in 1983. It uses a differential two wire interface running over shielded twisted pair cable, at a maximum speed of 1Mbps.

Characteristic Impedance (Z0)
 The value of impedance which, if it terminates a transmission line, results in no reflection along the line. Z0 is usually specified at a certain frequency (1.0 MHz in 1553B). Zo is approximately equal to the square root of L/C, where L = inductance and C = capacitance per unit length of cable.

A fixed-size datum computed from an arbitrary block of digital data for the purpose if detecting accidental errors that may have been introduced during its transmission or storage. The integrity of the data can be checked at any later time by recomputing the checksum and comparing it with teh stored one. If the checksums match, the data were almost certainly not altered.

Clock Rate
The signal from the 1553B bus is asynchronous with any clock in the terminal, since a Manchester code is by its nature self-clocking. That is, all Manchester-encoded bits have a zero-crossing in the middle, and it is to this zero-crossing time that the data is referenced.

Operation of a data bus system such that remote terminals receive and transmit data only when commanded to do so by the bus controller.

Compact PCI
Compact PCI (cPCI) is the standard used for PCI based industrial computers. It is electrically a superset of desktop PCI with a different physical form factor.  Compact PCI utilizes the Eurocard form factor popularized by the VME bus. Due to its excellent shock and vibration characteristics, it is often used for ruggedized embedded systems.

A basic electronic element, some available in a discrete form, having two or more electrical terminals (leads). These are intended to be connected together, usually by soldering to a printed circuit board, in order to create an electronic circuit with a particular function. Basic electronic components may be packaged discretely, as arrays or networks of like components, or integrated inside of packages such as semiconductor integrated circuits or thick film devices.

COM Port
A COM Port, or serial port, is a physical port on desktop computers that enable applications running on the desktop to communicate with an external device.

Conduction Cooled
A method of transferring heat via thermally conductive solid materials, such as heatsinks. It is the most common method of cooling used in operational embedded systems, since the solid heatsinks also provide mechanical ruggedization against vibration and shock.

The specific functional structure of a given integrated system consisting of physical interconnection (topology) and system control.

Conformal Coating
A dielectric material applied to electronic circuitry to act as protection against moisture, dust, chemicals, corrosion, abrasion, temperature extremes, and other environmental stresses that if uncoated could result in a complete failure of the electronic system. Common conformal coatings include silicone, acrylic, polyurethane, epoxy, and parylene. these coatings can be applied via brushing, spraying, and dipping.

Convection Cooled
A method of transferring heat away from hot components by dissipating it into the surrounding air. It is most commonly used in labrotory environments, or embedded systems that have forced air flow.


Data Bus
The Data Bus, or bus, refers to the hardware required to provide a single data path between the bus controller and all of the associated remote terminals.

Data Bus Connectors
The two physically seperate connectors provided on the RT interface to the data bus.

Data Bus Interface
The part of the digital interface which is concerned with transferring data.

Data Bus Loading
The percent utilization of the total information transfer capacity of a multiplexed data bus.

Data Latency
The age of the data, or how long it has been since the data was measured or calculated to the point where it is used.

Data Recorder
A device that monitors and records data on a communciations bus. It may record all data, or just specified data. These are often used in aircraft avionics systems to verify correct communications, and to allow an analysis of the buses performance to be carried out post-flight.

Data Wrap-Around
A function included in many RTs, where data words that are sent to the RT with a receive command are sent back to the bus controller with a subsequent transmit command.

Driver Development Kit (DDK)
A set of programs and related files that are used to develop a new software or hardware driver or to update an existing legacy application driver for an operating system. Typically, DDKs are used by device manufacturers and software application developers. They typically include sample drivers, source code, a debugging utility, a compiler, testing tools, other utilities, and documentation.

Digital Discrete I/O
Connector pins in a COM Port, that are programmable to receive or transmit discontinuous values. The information is represented discrete, such as number, letters, or icons.

Digital Interface
Embedded, high speed interface to the subsytem used to transfer control, status information, and data to and from the subsystem.

Digital Section
The remainder of the terminal other than the analog front end.

Direct Coupled
A method of connecting terminals to the 1553 data bus using only a wire splice.

Transmission line effects on lossy transmission lines on propagating waveforms. It is a result of frequency-dependent velocity and frequency dependent attenuation which distorts the propagating wave.

DMA Interface
DMA (Direct Memory Access) hardware and CPU protocol establish the means by which data may be transferred to and from memory without direct CPU intervention.

A collection of software functions (computer program) which interact with the hardware device. These are are hardware-dependent and operating-system-specific.

The transient exponential decay of voltage across an inductor (typically a transformer winding) due to voltage drop across the output impedance of the source driving the inductor as the current in the winding increases (proportionally to the integral in time of the voltage).

Use of two twisted, shielded cable pairs and interfaces for the purpose of greater reliability.

Dynamic Bus Control
The operation of a data bus system in which designated terminals are offered control of the data bus, i.e.: they become a BC when the terminal offering control relinquishes control.


Enhanced Bit Rate 1553 is an enhanced version of MIL-STD-1553, which enables 1553 data rates of 10Mb/s.  

Embedded Interface
MIL-STD-1553 interface circuitry housed withing a subsystem.

Embedded System
A special-purpose computer system designed to perform one or a few dedicated functions, often with real-time computing constraints. It is usually embedded as part of a complete device including hardware and mechanical parts.

Encryption Designs
Encryption techniques used by the data bus network and its associated terminals and processors to convert RED data into BLACK data and to isolate multiple classification levels and compartments of RED data. The specific encryption technique and system design must be approved by the government agency responsible for encryption certification.

Engineering Units
A measureable attribute or characteristic within the data of some message, where the data is defined by the communications protocol under which the message is being transmitted. A defined portion of the data, i.e.: some number of bits within some data word(s), comprises an Engineering Unit.

Error Management
General term used to describe the detection of transient events that temporarily degrade bus timing or performance and the step-by-step sequence to branch to alternate functions, procedures, or equipment use.

A family of frame-based computer networking technologies for local area networks (LAN). The most common are Ethernet over twisted pair to connect end systems, and fiber optic versions for site backnones. The official standard is IEEE 802.3, but is more commonly referred to as 10/100BASE-X.

A single occurrance at a precise time.

An interface to allow peripheral devices to be connected to a computer, usually a laptop computer. The Expresscard standard specifies the design of slots built into the computer and of cards which can be inserted into ExpressCard slots. The cards contain electronic circuitry and connectos to which external devices can be connected. The ExpressCard standard replaces the PC card (also known as PCMCIA). The standard specifies two form factos, ExpressCard/34 (34 mm wide) and ExpressCard/54 (54 mm wide, in an L-shape)—the connector is the same on both (34 mm wide). Standard cards are 75 mm long and 5 mm thich, but may be thicher on sections that extend outside the standard form for antennas, sockets, etc.


Fail-Safe Timer
MIL-STD-1553B requires that every RT and BC contain a hardware timer to prevent any transmission on the bus longer than 800 µS. Since no valid transmission is longer than 660 µS, only a failure in the terminal could result in a transmission of 800 µS or longer. The fail-safe timer is required to prevent such a failure from causing a continuous transmission on the bus, and thus rendering it unusable for other transmissions.

Fault Management
General term used to describe the detection of intermittent or permanent events that require changes to system structure of operation and the step-by-step sequence to branch to alternate functions, procedures, or equipment use.

FireWire (IEEE 1394)
A high-speed serial bus standard for transferring data to and from digital devices at data transfer rates of up to 400 Mbps. FireWire is Apple Computer's trademark name for their implementation of the IEEE 1394 standard. Other companies follow this standard, but have names such as Lynx and I-Link.

The special work done by a subsystem or a software task.

Fundamental Waveform
Defined in this glossary to be the original impinging waveform. The waveform that is transmitted.


A unit that passes data between two data buses of similar bus type.

Gigabit Ethernet (GbE)
The latest version of the popular packet based network protocol, running at a data transfer rate of 1Gb/s. The inital standardized version IEEE 802.3z from 1998 has evolved into the current IEEE 802.3-2005, and is commonly referred ti as 1000Base-X. GbE can operate over copper cables, or single/multi-mode optical fibers.

Graphical User Interface (GUI)
A type of user interface that allows users to interact with electronic devices with images rather than text commands. A GUI represents the information and actions available to a user through graphical incons and visual indicators such as secondary notation, as opposed to text-based interfaces, typed command lables or text navigation. The actions are usually performed through direct manipulation of the graphical elements.


Half Duplex
Operation of a data transfer system in either direction over a single line, but not in both directions on that line simultaneously.

Hierarchical Control
A form of distributing all subsytem control in a system, where one level of control is subordinate to a higher level of control.

Hierarchical Network
A description of a physical topology that has both global and local levels of data buses.


Illegal Commands
A valid command that is not implemented in the receiving RT.

Inline Coupler
A small, lightweight bus coupler as used on aircraft, missiles, etc.

Input Threshold Adjustment
Some receivers allow the input voltage thresholds to be adjusted. It may be desirable to adjust the voltage thresholds to alter the noise performance for some special applications. Receivers are generally supplied with the input voltage threshold adjusted for optimum performance, and the range of acceptable threshold values is not wide.

Input/Output (I/O)
This term is used to describe both the function of hardware and software to receive and transmit data and the physical hardware section that is the interface between a 1553 interface and subsytems of a remote terminal or bus controller.

The purpose of the "instrumentation" bit is to enable the differntiation of Status Words and Command Words, which are otherwise differentiated only by their position in a message.

The cooperative need for shared information and the means for achieving that cooperation.

Intermessage Gap (IMG)
The response time between a BC and RT, or from an RT to another RT.

Intermessage Routine (IMRS)
A set of tasks executed in real-time after any configured 1553 message. Multiple IMRS can be grouped together for one message, however caution must be taken in order to avoide conflicts between the IMRS and sufficent intermessage gap time must be available for proper operation.

Intersymbol Interference (ISI)
The effect seen where a waveform is distored by passing it through a network which either has less bandwidth than the signal, or is dispersive.

Invalid Command
A command in which the command word fails to meet validation criteria.

IRIG-106 Chapter 10
The Inter Range Instrumentation Group (IRIG) is the standards body of the Range Commanders Council (RCC). They publish a number of standards through the RCC Secretariat at White Sands Missile Range (WSMR).

IRIG-106 is a comprehensive  telemetry standard to ensure interoperability in aeronautical telemetry application at RCC member ranges. IRIG-106 Chapter 10 is the standard for digital flight data recorder.

IRIG is a serial time code format used to provide a common time source to many different units in a system. This allows all of the units and applications to be synchronized to a common time cource. IRIG has six different time code formats, of which IRIG-B is the most commonly used format in avionics applications. IRIG is controlled by the Range Commanders Council.

Industry Standard Architecture. A computer bus standard for IBM compatible computers. It was superseded by PCI in 1993.

Isolation Resistors
Terminals for direct-coupled stubs require two isolation resistors between the terminal output and the bus connection. Their function is to isolate the bus from a terminal that has shorted, such as a terminal that, due to some failure, is presenting an abnormally low impedance to the bus.

International Traffic In Arms Regulation is a set of United States governement regulations that control the export and import of defense-related articles and services on the United States Munitions List. For practical purposes. ITAR regulations dictate that information and material pertaining to defense and military related technologies—listed on the U.S. Munitions List—may only be shared with U.S. Persons unless authorization from the Department of State is received, or a special exemption is used. U.S. Persons (including organizations) can face heavy fines if they have, withouth authorization or the use of an exemption, provided foreign (non-US) persons with access to ITAR-protected defense articles, service, or technical data.


Jount Test Action Group (JTAG) is the name normally used for the IEEE 1149.1 standard called "Standard Test Access Port and Boundry Scan Architecture". The Boundary Scan Description Language (BSDL) is used to test and debug on-chip blocks via the JTAG interface, which is a 5 pin electrical connection to internal electronics in silicon chips.


Kernel Driver
A  low level driver that provides the functional interface between the software drivers and the underlying operating system calls. the function calls provide access to system memory, provide a pointer to the memory on the board, handle interrupts, etc.


LabVIEW (Laboratory Virtual Instrumentation Engineering Workbench) is a platform and development environment for a visual programming language from National Instruments. Commonly used for data acquisition, instrument control, and industrial automation; it is also used in development of scalable test, measurement, and control applications. LabVIEW programs/subroutines are called virtual instruments (VIs). Each VI has three components: a block diagram, a front panel (user interface) and a connector pane.

LabVIEW Real-Time Module
A graphical programming module that provides users with an easy way to program robust real-time applications. This module can be used to create advanced deterministic  applications with precise timing that run on a variety of hardware platforms. It allows users to utilize a majority of the base LabVIEW palette to implement complex algorithms on real-time systems. The module also provides a number of real-time specific tools, including the Real-Time Execution Trace toolkit, LabVIEW MathScript RT Module, Real-Time FIFOs, deterministic Real-Time timed loops, and low level driver access to real-time hardware. It also allows the user to perform Real-Time multicore processing to further increase a systems throughput.

LabWindows CVI
LabWindows/CVI (C for Virtual Instrumentation) is an event-driven, ANSI C programming environment developed by National Instruments. It is a programming environment for developing measurement applications, in addition to being a powerful tool for writing data acquisition programs. It includes a large set of run-time libraries for instrument control, data acquisition, analysis, user interface, and features code generation tools and prototyping utilities.

Linked Lists
In linked list message processing, each message points to the next message to be transmitted. This method makes it easy to insert messages into the middle of a particular minor frame's message stream.

Also known as GNU Linux, this Unix like operating system is on of the most prominent examples of open source development and free software. All of its underlying source code is available to the general public for use, modification and redistribution free of charge. It can be found in systems ranging from supercomputers to mobile phones, with its major advantages being its security, reliability, low cost, and freedom from vendor lock-in.

In the context of lossy transmission line, the term lossy acknowledges the fact that transmission lines do not have infinite, bandwidth, and contribute to frequency shaping of a propagating pulse above and beyond ideal reflective effects.

A Real Time Operating System (RTOS) from LynuxWorks. It features full POSIX conformance and Linux compatibility. It can be found in real time embedded flyable avionics systems, aerospace, military and industrial controllers.


Major Cycle
A period of scheduled time during which all periodic transmissions and computations occur at least once. Major cycles are divided into subcycles called minor cycles.

Manchester Code
A line code, also known as phase encoding or PE, which the encoding of each data bit has at least one transition and occupies the same time. It therefore has no DC component, and is self-clocking (inductively or capacitively coupled), and that a clock signal can be recovered from the encoded data. This code ensures frequent line voltage transitions, directly proportional to the clock rate, which helps clock recovery.

Manchester-Coded Format Encoding and Decoding
Each channel of the digital section contains an encoder/decoder function that deals with the data on a bit and single-word level.  Its purpose is to change the data from its Machester-coded format into the proper digital data format (typically 16-bit parallel) needed by the rest of the terminal (and vice-versa), and to perform error detection for word-level 1553B errors, such as bit count errors, Manchester coding errors, etc. There must be a seperate decoder for each channel, but there may be only one encoder.

In 1553 terms, a message is a part of an information transfer format, which as 1 to 32 data words. A message may also refer to entire transmission by both bus controller and responding remote terminal, which includes not only the data words, but the overhead. This second usage is more correctly called an information transfer format. 

Message Stacks
The stack method is the simplest to implement and allows for the implementation of minor and major frames by use of seperate stacks for each minor frame. The subsystem processor simply reinitializes the stack pointer to the appropriate stack each time the particular minor frame is to begin.

A military standard published by the United States Department of Defense that defines the mechanical, electrical, and functional characteristics of a serial data bus. The standard has two cariations, 1553A and 1553B. the 1553B standard increases compatibility between designs by different manufacturers by enabling them to be electrically interchangeable.

A military standard published by the United States Department of Defense that defines a standardized electrical interface between a military aircraft and its carriage stores. These carriage stores range from weapons, to pods, to external fuel tanks. The standard defines the electrical characteristics of the signal at the interface, as well as the connector and pin assignments of all of the signals used in the interface. This enhancement to the MIL-STD-1553 digital interface for the transfer of digital messages to a remote terminal include additional error detection in the form of a checksum. A checksum is mandated on critical control messages and provisional on the remainder of the messages. Implementing this level of error detection esnures a higher degree of error free data.

Minor Cycle
A period of scheduled time during which the most frequently occurring periodic transmission or computation will occur, or a period scheduled for a frequently occurring transmission or computation. Multiple minor cycles may be required to achieve a major cycle.

Maritime Mobile Service Identity (MMSI) is a series of nine digits which are sent in digital form over a radio frequency channel in order to uniquely identify ship stations, ship earth stations, coast stations, coast earth stations, and group calls. These identities are formed in such a way that the identity or part thereof can be used by telephone and telex subscribers connected to the general telecommunications network to call ships automatically.

Mode Code
A means to manage the flow of communication and information between the bus controller and the remote terminals on the bus.

Mode Command
An information transfer with the subaddress/mode field in the command word set to indicate that the next following field is a mode code. An RT that implements mode commands is required to know that a subaddress/mode field in a command word equal to 00000 or 11111 defines a mode command, and that the word count field is to be treated as the mode code rather than the number of words.

The signaling method used to convey data on the data bus.

Allows operation in all modes simutaneously: BC (Bus Controller) Mode, multiple RT (Remote Terminal) Mode, Bus Monitor Mode, and RT/Concurrent Bus Monitor Mode. It allows you to test and montior up to 32 RTs simutaneously, and includes error injection.

Multiple-Message Terminals
A processor or sequencer in its own right. This type of terminal only makes sense as a BC, although some multiple-message BCs are capable of being configured to act as RTs upon command from the subsystem or with a discrete signal. It is capable of chaining several messages together, maintaining a schedule of messages required on the bus, and initiating  all transfers at the required times and in the required sequence. In system terms, the multiple-message BC would by programmed with a whole minor frame or even major frame at a time.

Allows the use of many different communication protocols at the same time, on the same board. This can include MIL-STD-1553A/B, MIL-STD-1760, Discrete, MMSI, EBR-1553, ARINC 429, ARINC 615, ARINC 717, Serial (RS-232/422/485) and CANbus.

Multiplex Sustem Topology
A network of data bus terminals, the components that comprise the data bus, and the physical arrangement of redundant elements—whether terminals, bus controllers, or bus cables, couplers, or terminators. It includes all terminals and data buses involved in integrating the data buses into the vehicle.

The transmission of information from several signal sources through one communication system.


A group of electronic components or systems connected in such a fashion that they may exchange data with each other. They can come in many different forms, such as buses, point-to-point, star topology, and wireless.

An unwanted perturbation to a wanted signal. It is mainly caused due to reflections from impedance discontinuities in the bus network. Another source is electromagnetic interference (EMI) or coupling of signals into the cable from other parts of the system. Both of these types of noise are mostly higher in frequency that the 1553B fundamental frequency of 1 MHz. This noise and signal distortion can cause multiple zero-crossings to occur in a bit time (1 microsecond) and can also cause a large error in the time of a zero-crossing (shift error). This could cause the word to be misinterpreted by the decoder. It is most likely that the decoder would detect a Manchester error, which is a bit that does not have opposite values in the two halves of the bit time. Also, if one bit is distorted sufficiently that is decoded as valid but of the wrong value, the decoder detects this error with the parity bit. Experience has shown that there is little need for filtering of low-frequency noise. Good performance of the terminal in the presence of noise on the bus depends on input filtering noise and the proper setting of the input voltage thresholds.


Operation Code. The portion of a machine language instruction that specifies the operation to be performed. Their specification and format are laid out in the instruction set architecture of the processor in question (which may be a general CPU or a more specialized processing unit).  Apart from the opcode itself, an instruction normally also has one or more specifiers for operands (i.e. data) on which the operation should act, although some operations may have implicit operands, or none at all. There are instruction sets with nearly uniform fields for opcode and operand specifiers, as well as others with a more complicated, varied length structure.

Depending on architecture, the operands may be register values, values in the stack, other memory values, I/O ports, etc, specified and accessed using more or less complex addressing modes. The types of operations include arithmetics, data copying, logical operations, and program control, as well as special instructions (such as CPUID and others).

Operating Temperature
The specified temperature range that equipment may be operated in.

Output Short Circuit Protection
Some devices are protected against short circuits applied to their outputs. This could be used to protect parts in debugging or breadboarding activities. Note that this is somewhat inherent in the design of a current-mode transmitter.

Output Voltage Adjustment
Some transmitters offer an adjustable output voltage. This feature could be very useful in the design of test equipment, but is of limited usefulness otherwise.

Over-Temperature Shutdown
Some devices include a temperature sensor that shuts down the transmitter if it gets too hot.  This is desirable to protect the part from damage, but also increases the device's cost and slightly decrease the device's reliability.


A formatted unit of data carried by a packet mode computer network. Computer communications that do not support packets, such as traditional point-to-point telecommunications links, simply transmit data as a series of bytes, characters, or bits alone. Where data is formatted into packets, the bitrate of the communication medium can be better shared among users that if the network were circuit switched. By using oacket switched networking, it is also harder to guarantee a lowest possible bitrate. These packets consist of two types of data: control information and user data. The control information provides data the network needs to deliver the user data (such as source and destination addresses, error detection codes like checksums, and sequencing information). typically, control information is found in packet headers and trailers, with user data in between.

The method used to divide a complex system or function into manageable sizes before allocating these smaller pieces to devices to perform the required job.

An embedded computer standard that defines both a form factor and computer bus. This standard is intended for environments that require reliable data transfer in harsh environmental conditions. The computer bus utilizes 104 pins, These pins include all the normal lines used in the ISA bus, with additional ground pins added to ensure bus integrity.

An embedded computer standard based on PC/104 with the addition of a high-speed PCI bus.

Peripheral Component Interconnect. A high speed I/O bus connection for devices including SCSI cards, video cards, modems, video capture cards, etc. This is the primary way of adding peripheral devices to your computer. PCI  may be a 64-bit bus, though it is usually implemented as a 32-bit bus. It can run at clock speeds of 33 or 66 MHz.

An embedded computer standard that inclides the PCI connector, but not the ISA connector, in order to increase the available board real estate. This standard is incompatible with PC/104 boards.

PCI-Extended is an enhanced version of  the PCI bus, running at twice the speed. The 32 or 64 bit bus speed has been doubled to 133MHz, giving a theoretical maximum throughput of 1.06Gb/s. PCIe is backwards compatible with PCI, meaning that a regular PCI card can run in a PCIe slot.

Pulse Code Modulation. A standard method of encoding analog audio signals in digital form. The form of moduclation in which the modulation signal is sampled, quantized, and coded so that each element of information consists of different types or numbers of pulses and spaces.

Personal Computer Memory Card International Association. It is a standard for small memory cards or devices, designed for portable computers or laptops. The dimension of the card is 85.6 by 54 millimeters. The thickness varies depending on the type: Type 1 - up to 3.3 mm, Type 2 - up to 5.5 mm, Type 3 - up to 10.5 mm.

Event(s) recurring at specific time intervals.

PCI Mezzanine Card is defined by the IEEE P1386.1 standard. It combines the electrical characteristics of the PCI bus, with the mechanical form factor of the Common Mezzanine Card (CMC) defined in IEEE 1386. PMC cards are small, light, and rugged, making them ideal for embedded operational systems.

This is the method of communicating with multiple terminals within a system to determine information transfer priorities or servicing needs. RTs might be polled to determine whether they have aperiodic or high priority messages to transmit, state of health, or capability of accepting bus control.

Portable Operating System Interface for Unix (POSIX) is the collective name of a family of related standards specified by the IEEE to define the application programming interface (API) for software compatible with variants of the Unix operating system.

The conventions imposed on serial data to ensure that the receiver correctly interprets the transmitted data; also, the procedures used for initiating messages and responding to them.

A Peripheral Unit (PU) is the H009 equivalent of a Remote Terminal (RT) in MIL-STD-1553. Up to 16 PUs are allowed on a H009 bus.

PCI eXtensions for Instrumentation (PXI) is one of several modular electronic instrumentation platforms in current use, originally introduced in 1997 by National Instruments. These platforms are used as a basis for building electronic test equipment or automation systems. Based on industry-standard computer buses and designed with extra features to facilitate electronic test, they permit flexibility in building the extact test equipment or automation system required. They are often fitted with  custom software to manage the system.


A Quality Management System (QMS) is a system which defines procedures and policies to govern the various processes that occur in business. They often include feedback and controls, so that continual improvements can be made, thus resulting in a better process.


Receiver Input Filtering
A method of electroplating in which the parts are affixed to a rigid rack.

The receiving end of a communiation channel. It receives decoded messages/information from the sender, who first encoded them.

Random access memory; a type of memory which offers access to storage locations within means of X and Y coordinates.

Circuit failures which occur randomly with the overall failure rate for the sample population being nearly constant.

Redundant Data Bus
A network composed only of resistors and capacitors.

Metals that readily form compunds.

Reflection Coefficient
The surface area of an integrated circuit or of a hybrid or PCB substrate. The surface area required for a component or element.

Remote Terminal (RT)
An electronic unit which transfers data between the 1553 bus and its own subsytem. An RT may only respond when it has been specifically commanded by the Bus Controller. A maximum  of 32 RTs may be used on a 1553 bus.

A configurable option that enables the information on the bus to be replayed, usually emulating the RTs and enabling the review of unknown errors on the bus.

A second bond made on top of a removed or damaged bond or a second bond made immediately adjacent to the first bond.

The traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE (Data Terminal Equipment) and a DCE (Data Circuit-Terminatingg Equipment). the standard defines the electifcal characteristics and timing of signals, the meaning of signals, and the physical size and pinout of connectors.

A common short form and former official title of American National Standards Institute (ANSI) standard ANSI/TIA/EIA-422-B and its international equivalent ITU-T Recommendation T-REC-V.11, also known as X.27. These technical standards specify the electrical characteristics of the balanced voltage digital interface circuit. RS-422 provides for data transmission, using balanced or differential signaling, with unidirectional/non-reversible, terminated or non-terminated transmission lines, point to point, or multi-drop.  EIA-422/V.11 does not allow multiple drivers but only multiple receivers.

Also known as EIA-485 or TIA/EIA-485. A standard defining the electrical characteristics of drivers and receivers for use in balanced digital multipoint systems. The standard is published by the ANSI Telecommunications Industry Association/Electronic Industries Alliance (TIA/EIA). Digital communications networks implementing the RS-485 standard can be used effectively over long distances and in electrically noisy environments. Multiple receivers may be connected to such a network in a linear, multi-drop configuration. These characteristics make such networks useful in industrial environments and similar applications.

Real Time Operating System. This is a type of operating system which provides predicatble latency for software function calls. RTOS is normally used in mission critical systems.


Sample Consistency
Relates to the consistency of data in a message. Messages transmitted shall contain only mutually consistent samples of information, (i.e., all parameters shall be of the same sample set).

Small Computer System Interface. A set of standards for physically connecting and transferring data between computers and peripheral devices. The standards define commands, protocols, and electrical and optical interfaces.

The hardware and sofrware required to perform a specific system function, such as inertial, measurement, and radar detection.

The process of sending data one bit at a tile, sequentially, over a communication channel or computer bus.

Serial Port
A serial communication physical interface where information transfers in or out one bit at a time. The serial port usually identifies hardware more or less compliant to the RS-232 standard, inteded to interface with a modem of with a similar communication device.

Service Request Bit
The purpose of this bit is to inform the Bus Controller that the RT wants the Bus Controller to request a particular message to be transmitted from the RT to the BC.

Single Function
Allows operation in one mode at a time: BC (Bus Controller) Mode, RT (Remote Terminal) Mode, Bus Monitor Mode, or RT/Concurrent Bus Monitor Mode.

Single-Message Terminals
This terminal has enough capability to construct or process a complete message without any achtion by the subsytem. Subsystem action is required only at the beginning or end of the message or in the event of an error. The subsytem is responsible for processing any errors and interpreting the status word contents, to decided what the next message should be, and then issuing it.

Single-Word Terminals
In a system with a single-word terminal, the subsystem must process each word in each message individually. That is, a single-word terminal requires subsystem intervention or action for every word. After all the words have been received, the subsytem processor must determine the validity of the message and construct the proper response. The response must then be transferred to the terminal and transmitted one word at a time.

Software Development Kit (SDK)
A set of development tools that allows for the creation of applications for a certain software package, software framework, hardware platform, computer system, operating system, or similar platform. SDKs may have attached licenses that make them unsuitable for building software intended to be developed under an incompatible license.  SDKs maybe something as simple as an application programming interface (API) in the form of some files to interface to a particular programming languare, or include sophisticated hardware to communicate with a certain embedded system. Commin tools include debugging aids and other utilities often presented in an integrated development environment (IDE). They also frequently include sample code and supporting technical notes or other supporting documentation to help clarify points from the primary reference material.

A document prepared specifically to support procurement that clearly and accurately describes the essential technical requirements for purchased material. Also, included are procedures necessary to determine that the requirements have been met for the purchased material covered by the document.

A military standard is a document that establishes engineering and technical requirements for processes, procedures, practices, and methods that have been adopted as standard.

Status Flags
Specific one bit fields in the status word generated by the RT to indicate its status.

The connection of a terminal to the main bus, usually kept as short as possible to minimize distortion. May be either director transformer coupled.

Stub Coupling
The method of coupling an RT to the bus. These are generally transformer coupled, but some Navy applications require both transformer and direct coupled stubs.

The device or functional unit receiving data transfer service from the data bus. It is the hardware interfacing with the nonbus side of the terminal.

System Flag Bit
A status word bit that indicates that there is some fault condition in the subsystem associated with the RT.

The interacting assembly of hardware, software, data, personnel, and facilities capable of performing a designated function with specified results.

System Architecture
Includes the externally visible parts of a multiplex system, the internal partitioning of multiplex interfacing elements, and the hardware and software used for data transport and transport control. A multiplex system architecture consists of two major parts: system topology and system control.

System Block Diagram
A graphic presentation of the partitions among functions. The blocks may represent actual parts or maybe schematic representations. In a real system functions may be partitioned among the parts used to implement the design.

System Configuration
A 1553 bus system includes a BC (and possibly one of more backup BCs), one or more RTs (up to a maximum of 31), and zero or more bus monitors (typically not more than one, but could be any number). The BC has control of the system. It initates messages that transfer data to or from an RT or control the operation of an RT. Each RT receives data sent to it by the BC, transmits to the BC or to another RT the data requested by the BC, or performs the commanded control fucntion. A bus monitor listens to the traffic on the bus and extracts whatever information it has been programmed to extract.

System Control
The part of the architecture that implements the dynamic functioning of the multiplex system. System control methodology is used to implement the protocol required for data transfers, the rules used in achieving media control and the procedures for initialization or startup, normal data bus transfer operations (such as, time synchronization, data security, and data integrity), system error and fault management techniques, and bus control mechanization.


An unclassigied short name referring to investigations and studies or compromising emanations. It is sometimes used synonymously for the term "compromising emanations" (such as TEMPEST tests, TEMPEST inspections, TEMPEST control plan).

The electronic module necessary to interface the data bus with the subsytem and the subsystem with the data bus. Terminals may exist as seperate LRUs or be contained within the elements of the subsytem. The unit connects to the end of a stub, which may be a transmitter or receiver. This definition allows a terminal to be a totally seperate LRU, a circuit card, or a small portion of a circuit card; there is no restriction on the physical partitioning of the system. However, the current trend in technology is for smaller size and fewer parts. A terminal is either a Bus Controller (BC), a Remote Terminal (RT), or a Bus Monitor. Nothing in 1553B precludes a terminal from including the capability of performing the functions of more than one of these three types of terminals, but a terminal may perform only one function at any time.

Terminal Flag  Bit
This bit indicates that there is some fault condition in the RT. Remember that the RT is only that portion of the LRU necessary to communicate with the 1553 bus.

Terminal Paritioning
Terminals may have various proportions of their design in hardware, firmware, and software. Typically, an older design, simple terminal would be almost all hardware, while a newer design, complex terminal would consist largely of software or firmware.

The electrical termination of a signal involves providing a terminator at the end of a wire or cable to prevent an RT signal from being reflected back from the end, causing interference. The terimato is placed at the end of a transmission line or daisy chain bus (such as in SCSI), designed to match impedance and hence minimize signal reflection. There are three tupes of terminator: passive—consisting of a simple resistor with limited usage for highspeed applications, active—consisting of a voltage regulator that keeps the voltage used for the terminating resistor(s) at a contast level, and forced perfect termination (FPT)—which can be used on single ended buses where diodes remove over and undershoot conditions.

Time Division Multiplexing (TDM)
The transmission of information from several signal sources through one communication system with different signal samples staggered in time to form a composite pulse train.

Time to Next Message
A delay value calculated from Mid-Parity crossing of the first Command Word crossing to the Mid-Parity crossing of the next Messages Command word.

Telemetry Attributes Transfer Standard is Chapter 9 of the IRIG-106-93 publication. The standard was designed to provide a format for the transfer of information between the user and a test range or between test ranges. This format minimizes the  'station unique' activities that are necessary to support any test item. In addition, it is inteded to relieve the labor intensive process that was required to reformat the information by providing the inforamtion on computer compatible media, therefore reducing errors and requiring less preperation time for test support.

The attributes defined by TMATS are those parameters required by the receiving/processing system to acquire, process, and display the telemetry data received from the test item or source. Each attribute is represented by a unique conde name and associated data. The attributes are divided into these groupings: General Information, Transmission Attributes, Tape Source Attributes, Multiplex/Modulation Attributes, PCM Format Attributes, PCM Measurement Attributes, 1553 Bus Data  Attributes, PAM Attributes, Data Conversion Attributes,a nd Airborne Hardware Attributes.

The interconnectivity of the data bus(es) and their associated elements (terminals and controllers) to accomplish the desired data path required by the integration.

Receivers and transmitters are generally packaged together as transceivers.

A device which provides isolation for the electronic signals. The isolation transformer is another major analog component. To meet the terminals characteristics that are specified in 1553B—especially the common-mode rejection ration (CMRR) requirement—an isolation transformer is the most appropriate design choice.

Transformer Coupled
A method of connecting a stub to the 1553 data bus that uses a transformer and isolation resistors.

The element in the terminal that outputs waveforms to the bus. It accepts as its input the digital signal from the encoder (typically differential TTL) and produces a signal on the 1553 bus that meets the requirements of 1553. It typically contains two drivers, one for each side of the differential 1553 bus. Each is designed to control the rise and fall times and the waveshape of the outputs. A transmitter also typically contains an inhibit input by which it may be disabled.

Twisted Shielded Pair
A twisted, shielded pair of wires is used to interconnect the elements of a network (transformers, resistors, and connectors). It is the primary constituent of the networ and is commonly referred to as cable.

Transmission Coefficient
The ratio of the voltage wave transmitted beyond an impedance discontinuity over the incident voltage wave (referred to as CT in this document). It is the number between 0 and 2.


A computer operating system originally developed in the 1960s and 1970s by a group of AT&T employees at Bell Labs. Today's Unix systems are split into various branches, developed over time by AT&T as well as various commercial vendors and non-profit organizations.

Unix operating systems are widely used in both servers and workstations. The Unix environment and the cliner-server program model were essential elements in the development of the Internet and te reshaping of computing as centered in networks rather than in individual computers.

Unix was designed to be portable, multi-tasking, and multi-user in a time-sharing configuration. Unix systems are characterized by various concepts: the use of plain text for storing data; a hierarchical file system; trating devices and certain types of inter-process communication (IPC) as files; and the use of a large number of small programs that can be strung together through a command line interperter using popes as opposed to using a single monolithic program that includes all of the same functionality.

Under Unix, the "operating system" consists of many of these utilities along with the master control program, the kernel. The kernel provides services to start and stop programs, handle the file system and other common "high level" tasks that most programs share, and, perhaps most importantly, schedules access to hardware to avoid conflicts if two programs try to access the same resource or device simultaneously. To mediate such access, the kernel was given special rights on the system and led to the division between user-space and kernel-space.

Universal Serial Bus (USB) is one of the most popular methods of peripheral connection in PCs today. It consists of a host controller, and multiple daisy chained devices, often connected via hubs. Its popularity stems from its ease of use, hot swap, and plug and play capabilities. The lates version, USB 2.0 supports data rates up to 480Mbit/s.


Versa Module Eurocard. An 8-, 16- and 32-bit parallel-bus computer architecture that can implement single and multiprocessor systems, developed by the VMEbus Manufacturers Group (now called VITA). The bus includes four sub-buses, Data Transfer Bus, Priority Interrupt Bus, Arbitration Bus, and Utility Bus.

VME EXtensions for Instrumentation. A pheripheral bus specializing in data acquisition and realitme control systems, Introduced in 1987, VXI uses all Eurocard form factors and adds trigger lines, a local bus, and other functions suited for measurement applications.

A Real-Time Operating System (RTOS) made and sold by Wind River Systems. Like most RTOSs, it includes a multi-tasking kernel with pre-emptive scheduling and fast interrupt response, extensive intertask communications, and synchronization facilities.

VxWorks is designed for use in embedded systems. Unlike "self-hosting" systems such as Unix, VxWorks development is done  on a "host" machine running Linux, Unix, or Microsoft Windows, cross-compiling target software to run on various "target" CPU architectures.


Waveform Quality
A phrase which addresses the amount of distortion with which a waveform arrives at its destination.

Windows Operating System
Microsoft Windows is a family of Operating Systems which can run on different types of platforms such as embedded controllers, servers, and most typically personal computers. Mircosoft added their poular graphical user interface (GUI) to MS-DOS in the eaerly 90s, and have gone on to become the most popular operating system for PCs in the world. Its popularity is due to its ease of use, and intuitive interface.

A 1553 word is a sequence of 20 bit times consisting of a 3 bit-time sync, 16 bits of data, and 1 parity bit. This is the word as it is transmitted on the bus; 1553 terminals add the sync and parity before transmission and remove them during reception. Therefore, the nominal word size is 16 bits; most significant bit first. There are three types of words: command, status, and data.