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MIL-STD-1553 & 1773 DATA BUS
In recent years, the use of digital techniques in aircraft
equipment has greatly increased, as have the number of avionics subsystems and the volume of data processed by
Because analog point-to-point wire bundles are inefficient and cumbersome means of interconnecting the
sensors, computers, actuators, indicators, and other equipment onboard the modern military vehicle, a serial
digital multiplex data bus was developed. MIL-STD-1553 defines all aspects of the bus, therefore, many groups
working with the military tri-services have chosen to adopt it.
The 1553 multiplex data bus provides
integrated, centralized system control and a standard interface for all equipment connected to the bus. The bus
concept provides a means by which all bus traffic is available to be accessed with a single connection for testing
and interfacing with the system. The standard defines operation of a serial data bus that interconnects multiple
devices via a twisted, shielded pair of wires. The system implements a command-response format.
MIL-STD-1553, "Aircraft Internal Time-Division Command/Response Multiplex Data Bus," has been in use since 1973
and is widely applied. MIL-STD-1553 is referred to as "1553" with the appropriate revision letter (A or B) as a
suffix. The basic difference between the 1553A and the 1553B is that in the 1553B, 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 compatibility between designs by different
manufacturers could be electrically interchangeable.
The Department of Defense chose multiplexing because
of the following advantages:
- Weight reduction
Some 1553 applications utilize more than one data bus on a vehicle. This is often done, for example, to
isolate a Stores bus from a Communications bus or to construct a bus system capable of interconnecting more
terminals than a single bus could accommodate. When multiple buses are used, some terminals may connect to both
buses, allowing for communication between them.
Multiplexing facilitates the transmission of information along the data flow. It permits the transmission of
several signal sources through one communications system.
The bus is
made up of twisted-shielded pairs of wires to maintain message integrity. MIL-STD-1553 specifies that all devices
in the system will connect to a redundant pair of buses. This provides a second path for bus traffic should one of
the buses be damaged. Signals are only allowed to appear on one of the two buses at a time. If a message cannot be
completed on one bus, the bus controller may switch to the other bus. In some applications more than one 1553 bus
may be implemented on a given vehicle. Some terminals on the bus may actually connect to both buses.
There are only three functional modes of terminals allowed on the data bus: the bus
controller, the bus monitor, and the remote terminal. Devices may be capable of more than one function. Figure 1
illustrates a typical bus configuration.
Figure 1. 1553 Bus Structure
- Bus Controller - The bus controller (BC) is the terminal that initiates information transfers on the data
bus. It sends commands to the remote terminals which reply with a response. The bus will support multiple
controllers, but only one may be active at a time. Other requirements, according to 1553, are: (1) it is "the
key part of the data bus system," and (2) "the sole control of information transmission on the bus shall reside
with the bus controller."
- Bus Monitor - 1553 defines the bus monitor as "the terminal assigned the task of receiving bus traffic and
extracting selected information to be used at a later time." Bus monitors are frequently used for
- Remote Terminal - Any terminal not operating in either the bus controller or bus monitor mode is
operating in the remote terminal (RT) mode. Remote terminals are the largest group of bus components.
The signal is transferred over the data bus using serial digital
pulse code modulation.
The type of data encoding used
by 1553 is Manchester II biphase.
- A logic one (1) is transmitted as a bipolar coded signal 1/0 (in other
words, a positive pulse followed by a negative pulse).
- A logic zero (0) is a bipolar coded signal 0/1 (i.e., a negative pulse followed by a positive pulse).
A transition through zero occurs at the midpoint of each bit, whether the rate is a logic one or a logic zero.
Figure 2 compares a commonly used Non Return to Zero (NRZ) code with the Manchester II biphase level code, in
conjunction with a 1 MHz clock.
BIT TRANSMISSION RATE
transmission rate on the bus is 1.0 megabit per second with a combined accuracy and long-term
stability of +/-
0.1%. The short-term stability is less than 0.01%.
There are 20 1.0-microsecond bit times allocated for
each word. All words include a 3 bit-time sync pattern, a 16-bit data field that is specified differently for each
word type, and 1 parity check bit.
Bus traffic or communications travels along the bus in words.
A word in MIL-STD-1553 is a sequence of 20 bit times consisting of a 3 bit-time sync wave form, 16 bits of data,
and 1 parity check 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, with the
most significant bit (MSB) first. There are three types of words: command, status, and data. A packet is defined
to have no intermessage gaps. The time between the last word of a controller message and the return of the
terminal status byte is 4-12 microseconds. The time between status byte and the next controller message is
undefined. Figure 3 illustrates these three formats.
3. 1553 Word Formats
Command words are transmitted only by the bus controller
and always consist of:
- 3 bit-time sync pattern
- 5 bit RT address field
- 1 Transmit/Receive (T/R) field
- 5 bit subaddress/mode field
- 5 bit word count/mode code field
- 1 parity check bit.
Data words are transmitted either
by the BC or by the RT in response to a BC request. The standard allows a maximum of 32 data words to be sent in
a packet with a command word before a status response must be returned. Data words always consist of:
- 3 bit-time sync pattern (opposite in polarity from command and status words)
- 16 bit data field
- 1 parity check bit.
Status words are transmitted by
the RT in response to command messages from the BC and consist of:
- 3 bit-time sync pattern (same as for a command word)
- 5 bit address of the responding RT
- 11 bit status field
- 1 parity check bit.
The 11 bits in the status field are used to notify the BC of the operating
condition of the RT and subsystem.
basic types of information transfers are defined by 1553:
- Bus Controller to Remote Terminal transfers
- Remote Terminal to Bus Controller transfers
- Remote Terminal to Remote Terminal transfers
These transfers are related to the data flow and are referred to as messages. The basic formats of these
messages are shown in Figure 4.
Figure 4. 1553 Data Message Formats
The normal command/response operation involves the transmission of a command from the BC to a selected RT
address. The RT either accepts or transmits data depending on the type (receive/transmit) of command issued by the
BC. A status word is transmitted by the RT in response to the BC command if the transmission is received without
error and is not illegal.
Figure 5 illustrates the 1553B Bus Architecture in a typical aircraft.
Figure 5. Typical Bus Architecture
MIL-STD-1773 contains the requirements for utilizing a
fiber optic "cabling" system as a transmission
medium for the MIL-STD-1553B bus protocol. As such, the standard
repeats MIL-STD-1553 nearly word-for-word. The standard does not specify power levels, noise levels, spectral
characteristics, optical wavelength, electrical/optical isolation or means of distributing optical power. These
must be contained in separate specifications for each intended use.
Data encoding and word format are
identical to MIL-STD-1553, with the exception that pulses are defined as transitions between 0 (off) and 1 (on)
rather than between + and - voltage transitions since light cannot have a negative value.
standard applies to cabling only, the bus operates at the same speed as it would utilizing wire. Additionally,
data error rate requirements are unchanged.
Different environmental considerations must be given to fiber
optic systems. Altitude, humidity, temperature, and age affects fiber optics differently than wire conductors.
Power is divided evenly at junctions which branch and connectors have losses just as wire connectors do.
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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