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
3.3 Pulse Code Modulation (PCM)
3.4 Time Division Multiplexing (TDM)
3.5 Half Duplex
3.9 Data Bus
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
3.12 Bus Monitor
3.13 Remote Terminal (RT)
3.14 Asynchronous Operation
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
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 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.
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
The mode code definition was added to 1553B because of the definition of several mode code operations in paragraph 22.214.171.124.1.7. 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.