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Electronic Warfare and Radar Systems Engineering Handbook
- Data Busses -
[Go to TOC]
The avionics systems on aircraft frequently contain general purpose computer
components which perform certain processing functions, then relay this information to other systems. Some common
examples are the mission computers, the radar processors, RWRs, and Jammers. Each system is frequently laid out as
shown in Figure 1.
The Input/Output (I/O) modules will vary in function, but all serve the same purpose -
to translate the electrical signals from one protocol to one of another in order to exchange information. I/O
modules are used similarly in general purpose computers in laboratories to test equipment and/or tie computers
together via a local area network (LAN) to exchange information. Some of the methodologies include a star, ring,
or bus type network (see terminology at the end of this section). The high speed data
on avionics/computers do not operate as fast as the CPU clock speed, but they are much faster than the interface
busses they connect to. There are a number of interface busses which are widely used by aircraft, avionics systems
and test equipment. The most common include the RS-232, the RS-422, the RS-485, the IEEE-488 (GP-IB/HP-IB) and the
MIL-STD-1553A/B. The MIL-STD-1773 bus is a fiber optic implementation of the 1553 bus and may be used in the
future when technology requires it to reduce susceptibility to emissions or other reasons. A summary follows in
Table 1, then a brief description of each follows immediately, while a section covering each in more detail is
Table 1. Summary of Bus Characteristics
See notes below
||100 feet max
50 ft at 20k BPS
||150 - 19,200
baud per sec
||5- to 8- bit
||see figure in
||20 k BPS
NOTES FROM TABLE:
(1) Max Number of Terminals does not include the bus
(2) Including ground/shield
(3) Tb = time duration of the unit interval at the applicable data signaling rate (pulse
(4) Length is function of data signaling rate influenced by the tolerable signal
distortion, amount of longitudinally coupled noise and ground potential difference introduced between the
controller and terminal circuit grounds as well as by cable balance. See RS-422 section for graph.
(5) Physical arrangement of multiple receivers involves consideration of stub line lengths, fail-safe networks,
location of termination resistors, data rate, grounding, etc.
(6) Rate can go
up to 1 MHz if special conventions are followed.
(7) Max Number of Terminals
includes terminal reserved for broadcast commands.
ADDRESS: A unique designation for the location of data or the identity of an intelligent device; allows each
device on a single communications line to respond to its own message.
ASCII (American Standard Code for
Information Interchange): Pronounced asky. A seven-bit-plus-parity code established by ANSI to achieve
compatibility between data services.
ASYNCHRONOUS OPERATION: 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.
BAUD: Unit of signaling speed. The speed in baud is the
number of discrete events per second. If each event represents one bit condition, baud rate equals bits per second
(BPS). When each event represents more than one bit, baud rate does not equal BPS.
BIT: Contraction of
binary digit: may be either zero or one. 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.
BIT RATE: The number of
bits transmitted per second.
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
BUS CONTROLLER: The terminal assigned the task of initiating information transfers on the data
BUS MONITOR: The terminal assigned the task of receiving bus traffic and extracting selected
information to be used at a later time.
BYTE: A binary element string functioning as a unit, usually
shorter than a computer "word." Eight-bits per byte are most common. Also called a "character".
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.
CRC: Cyclic Redundancy Check; a basic error-checking
mechanism for link-level data transmissions; a characteristic link-level feature of (typically) bit-oriented data
communications protocols. The data integrity of a received frame or packet is checked via a polynomial algorithm
based on the content of the frame and then matched with the result that is performed by a sender and included in a
(most often, 16-bit) field appended to the frame.
DATA BUS: Whenever a data bus or bus is referred to in
MIL-STD-1553B, 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
DCE (Data Communications Equipment): Devices that provide the functions required to
establish, maintain, and terminate a data-transmission connection; e.g., a modem.
DTE (Data Terminal
Equipment): Devices acting as data source, data sink, or both.
DYNAMIC BUS CONTROL: The operation of a data bus system in which designated terminals are offered control of
the data bus.
EIA (Electronic Industries Association): A standards organization in the U.S.A. specializing
in the electrical and functional characteristics of interface equipment.
Multiplexor: A device that divides the available transmission frequency range into narrower banks, each of which
is used for a separate channel.
FDX (Full Duplex): Simultaneous, two-way, independent transmission in both
GPIB: General Purpose Interface Bus (see section 9-5)
HALF DUPLEX: Operation of a data transfer system
in either direction over a single line, but not in both directions on that line simultaneously.
HANDSHAKING: Exchange of predetermined signals between two devices establishing a connection. Usually part of a
HPIB / HPIL: Hewlett-Packard Interface Bus / Hewlett-Packard Interface Loop
IEEE (Institute of Electrical and Electronic Engineers): An international professional society that issues its own
standards and is a member of ANSI and ISO.
MANCHESTER ENCODING: Digital encoding technique (specified for
the IEEE 802.3 Ethernet baseband network standard) in which each bit period is divided into two complementary
halves; a negative-to-positive (voltage) transition in the middle of the bit period designates a binary "1," while
a positive-to-negative transition represents a "0". The encoding technique also allows the receiving device to
recover the transmitted clock from the incoming data stream (self-clocking).
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.
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.
MODEM (Modulator-Demodulator): A device used to
convert serial digital data from a transmitting terminal to a signal suitable for transmission over a telephone
channel, or to reconvert the transmitted signal to serial digital data for acceptance by a receiving terminal.
MULTIPLEXOR (also Multiplexer): A device used for division of a transmission into two or more subchannels, either
by splitting the frequency band into narrower bands (frequency division) or by allotting a common channel to
several different transmitting devices one at a time (time division).
NETWORK: An interconnected group of
nodes; a series of points, nodes, or stations connected by communications channels; the assembly of equipment
through which connections are made between data stations.
NODE: A point of interconnection to a network. Normally, a point at which a number of terminals or tail
circuits attach to the network.
PARALLEL TRANSMISSION: Transmission mode that sends a number of bits
simultaneously over separate lines (e.g., eight bits over eight lines) to a printer. Usually unidirectional.
PHASE MODULATION: One of three ways of modifying a sine wave signal to make it "carry" information. The sine wave
or "carrier" has its phase changed in accordance with the information to be transmitted.
POLLING: A means
of controlling devices on a multipoint line.
PROTOCOL: A formal set of conventions governing the formatting
and relative timing of message exchange between two communicating systems.
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.
REMOTE TERMINAL (RT): All
terminals not operating as the bus controller or as a bus monitor.
SERIAL TRANSMISSION: The most common
transmission mode; in serial, information bits are sent sequentially on a single data channel.
Stubbing is the method wherein a separate line is connected between the primary data bus line and a terminal. The
direct connection of stub line causes a mismatch which appears on the waveforms. This mismatch can be reduced by
filtering at the receiver and by using bi-phase modulation. Stubs are often employed not only as a convenience in
bus layout but as a means of coupling a unit to the line in such a manner that a fault on the stub or terminal
will not greatly affect the transmission line operation. In this case, a network is employed in the stub line to
provide isolation from the fault. These networks are also used for stubs that are of such length that the mismatch
and reflection degrades bus operation. The preferred method of stubbing is to use transformer coupled stubs. The
method provides the benefits of DC isolation, increased common mode protection, a doubling of effective stub
impedance, and fault isolation for the entire stub and terminal. Direct coupled stubs should be avoided if at all
possible. Direct coupled stubs provide no DC isolation or common mode rejection for the terminal external to its
subsystem. Further, any shorting fault between the subsystems' internal isolation resistors (usually on the
circuit board) and the main bus junction will cause failure of that entire bus. It can be expected that when the
direct stub length exceeds 1.6 feet, that it will begin to distort the main bus waveforms. Note that this length
includes the cable runs internal to a given subsystem.
SUBSYSTEM: The device or functional unit receiving
data transfer service from the data bus.
SYNCHRONOUS TRANSMISSION: Transmission in which data bits are sent at
a fixed rate, with the transmitter and receiver synchronized. Synchronized transmission eliminates the need for
start and stop bits.
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 units or be contained within the elements of
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
WORD: A set of bits or bytes comprising the smallest unit of addressable memory. In MIL-STD-1553B, a word is a
sequence of 16 bits plus sync and parity.
Table of Contents for Electronics Warfare and Radar Engineering Handbook
Abbreviations | Decibel | Duty
Cycle | Doppler Shift | Radar Horizon / Line
of Sight | Propagation Time / Resolution | Modulation
| Transforms / Wavelets | Antenna Introduction
/ Basics | Polarization | Radiation Patterns |
Frequency / Phase Effects of Antennas |
Antenna Near Field | Radiation Hazards |
Power Density | One-Way Radar Equation / RF Propagation
| Two-Way Radar Equation (Monostatic) |
Alternate Two-Way Radar Equation |
Two-Way Radar Equation (Bistatic) |
Jamming to Signal (J/S) Ratio - Constant Power [Saturated] Jamming
| Support Jamming | Radar Cross Section (RCS) |
Emission Control (EMCON) | RF Atmospheric
Absorption / Ducting | Receiver Sensitivity / Noise |
Receiver Types and Characteristics |
General Radar Display Types |
IFF - Identification - Friend or Foe | Receiver
Tests | Signal Sorting Methods and Direction Finding |
Voltage Standing Wave Ratio (VSWR) / Reflection Coefficient / Return
Loss / Mismatch Loss | Microwave Coaxial Connectors |
Power Dividers/Combiner and Directional Couplers |
Attenuators / Filters / DC Blocks |
Terminations / Dummy Loads | Circulators
and Diplexers | Mixers and Frequency Discriminators |
Detectors | Microwave Measurements |
Microwave Waveguides and Coaxial Cable |
Electro-Optics | Laser Safety |
Mach Number and Airspeed vs. Altitude Mach Number |
EMP/ Aircraft Dimensions | Data Busses | RS-232 Interface
| RS-422 Balanced Voltage Interface | RS-485 Interface |
IEEE-488 Interface Bus (HP-IB/GP-IB) | MIL-STD-1553 &
1773 Data Bus |
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