MIL-STD-1553

3.0 Definitions

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3.1 Bit

Contraction of binary digit: may be either zero or one. In information theory a binary digit is equal to one binary decision or the designation of one or two possible values of states of anything used to store or convey information.

3.2 Bit Rate

The number of bits transmitted per second.

3.3 Pulse Code Modulation (PCM)

The form of modulation 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.

3.4 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.

3.5 Half Duplex

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

3.6 Word

In this document a word is a sequence of 16 bits plus sync and parity. There are three types of words: command, status and data.

3.7 Message

A single message is the transmission of a command word, status word, and data words if they are specified. For the case of a remote terminal to remote terminal (RT to RT) transmission, the message shall include the two command words, the two status words, and data words.

3.8 Subsystem

The device or functional unit receiving data transfer service from the data bus.

3.9 Data Bus

Whenever a data bus or bus is referred to in this document it shall imply all the hardware including twisted shielded pair cables, isolation resistors, transformers, etc., required to provide a single data path between the bus controller and all the associated remote terminals.

3.10 Terminal

The electronic module necessary to interface the data bus with the subsystem and the subsystem with the data bus. Terminals may exist as separate replaceable units (LRU's) or be contained within the elements of the subsystem.

This definition of terminal is intentionally broad. Terminals in 1553 have common operational characteristics, as well as assigned roles in data bus operation. The three allowable roles are defined in 3.11, 3.12, and 3.13. Common operational requirements of terminals are given in 1553B, paragraph 4.4.1. Note that the definition gives designers complete freedom of functional partitioning of the operating parts of a terminal, and that there is also no restriction of physical partitioning.

3.11 Bus Controller

The terminal assigned the task of initiating information transfers on the data bus.

3.12 Bus Monitor

The terminal assigned the task of receiving bus traffic and extracting selected information to be used at a later time.

3.13 Remote Terminal (RT)

All terminals not operating as the bus controller or as a bus monitor.

3.14 Asynchronous Operation

For the purpose of this standard, asynchronous operation is the use of an independent clock source in each terminal for message transmission. Decoding is achieved in receiving terminals using clock information derived from the message.

This definition refers to the electrical characteristic by which the timing of message bits in a word are decoded. This use of "asynchronous operation" should not be confused with an asynchronous message that may interrupt or suspend the transmission of synchronous (i.e., periodic) messages in an avionic system.

3.15 Dynamic Bus Control

The operation of a data bus system in which designated terminals are offered control of the data bus.

3.16 Command/Response

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

In the case of the definitions for message, bus controller, remote terminal, asynchronous operation, dynamic bus control, and command/ response, the change from 1553A to 1553B was developed to produce a more general definition. However, in the definition of data bus, 1553B encompasses more equipment. Instead of including only the wire, the data bus couplers are also included. Two definitions were added for clarity: subsystem and terminal. The others (bus monitor, redundant data bus, broadcast, and mode codes) were added to define the additional requirements stated in 1553B. The function of a bus monitor is to monitor the data bus and record specified bus activity. The objective of defining a bus monitor function is new to 1553B. Two basic capabilities have been identified for the monitor in paragraph 4.4.4 of 1553B: (1) an off-line application including a flight test recording, maintenance recording, or mission analysis, and (2) a unique data bus terminal, which provides an internal backup bus controller function, with sufficient information to take over as the active bus controller in the even of a switch­over or a failure of the active bus controller. In these two roles, the bus monitor hardware may have the performance capability of a terminal (unique address) or may be attached to the data bus without the knowledge of the other bus users (including the bus controller). In this second approach, no bus communication from or to the bus monitor by the bus controller). In this second approach, no bus communication from or to the bus monitor by the bus controller is possible. The bus monitor acts as a passive listener to the specified traffic it is assigned to record. Obviously, the performance of a bus monitor requires the monitoring of the data bus for command words, status words, and data words. From this monitoring, the specific message collection process can occur during normal and abnormal (bus error and recovery) bus traffic. To aid in accomplishing the detection of these words (command and status), the optional instrumentation bits (bit 10 in the status word) and the associated bit in the command word (bit 10) can be set to a logic 1 and a logic 0, respectively.

3.17 Redundant Data Bus

The use of more than one data bus to provide more than one data path between the subsystem, i.e., dual redundant data bus, tri-redundant data bus, etc.

The redundant data bus definition was added to 1553B to identify a particular approach for obtaining multiple data paths to improve message arrival probability. Paragraph 4.6 of 1553B discusses the use of a dual-redundant data bus where the operation is identified as dual standby. In this mode, only one bus is active at any given time, except when superseding commands are sent on the standby bus. Under this condition, the terminal responds to the most recent command.

3.18 Broadcast

Operation of a data bus system such that information transmitted by the bus controller or a remote terminal is addressed to more than one of the remote terminals connected to the data bus.
The broadcast definition has been added to 1553B to describe a new protocol option. The use of this protocol allows a bus controller or a remote terminal to address more than one terminal connected to the system. This is accomplished by transmitting a dedicated terminal address (11111) and each recover withholding the normal status word response.

3.19 Mode Code

A means by which the bus controller can communicate with the multiplex bus related hardware, in order to assist in the management of information flow.

The mode code definition was added to 1553B because of the definition of several mode code operations in paragraph 4.3.3.5.1.7. These optional mode codes are used to manage the information transfer system. The basic philosophy of the data bus system is that it is a "transparent data communication link." This means that its operation and management does not involve the use of the sensor data that it is transmitting or receiving. However, overhead is required to manage such a data link. Therefore, command words, status words, and message gaps are required to provide this capability. The combination of command word, mode codes, and responses to these mode codes provide the basis for managing the multiplex system.