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extreme importance in bus controller design that the bus controller be readily able to
accommodate terminals of differing protocol’s and status word bits used. Equipment
designed to MIL-STD-1553A will be in use for a considerable period of time; thus, bus
controllers must be capable of adjusting to their differing needs. It is also important to
remember that the bus controller will be the focal point for modification and growth within
the multiplex system, and thus the software must be written in such a manner as to permit
modification with relative ease.
10.3 Multiplex Selection Criteria
The selection of candidate signals for multiplexing is a function of the particular application
involved, and criteria will in general vary from system to system. Obviously, those signals
which have bandwidths of 400 Hz or less are prime candidates for inclusion on the bus. It is
also obvious that video, audio, and high speed parallel digital signals should be excluded.
The area of questionable application is usually between 400 Hz and 3 kHz bandwidth. The
transfer of these signals on the data bus will depend heavily upon the loading of the bus in a
particular application. The decision must be based on projected future bus needs as well as
the current loading. Another class of signals which in general are not suitable for
multiplexing are those which can be typified by a low rate (over a mission) but possessing a
high priority or urgency. Examples of such signals might be a nuclear event detector output
or a missile launch alarm from a warning receiver. Such signals are usually better left
hardwired, but they may be accommodated by the multiplex system if a direct connection to
the bus controller’s interrupt hardware is used to trigger a software action in response to the
signal.
10.4 High Reliability Requirements
The use of simple parity for error detection within the multiplex bus system was dictated by a
compromise between the need for reliable data transmission, system overhead, and remote
terminal simplicity. Theoretical and empirical evidence indicates that an undetected bit error
rate of 10
-12
can be expected from a practical multiplex system built to this standard. If a
particular signal requires a bit error rate which is better than that provided by the parity
checking, then it is incumbent upon the system designer to provide the reliability within the
constraints of the standard or to not include this signal within the multiplex bus system. A
possible approach in this case would be to have the signal source and sink provide appropriate
error detection and correction encoding/decoding and employ extra data words to transfer the
information. Another approach would be to partition the message, transmit a portion at a
time, and then verify (by interrogation) the proper transfer of each segment.
10.5 Stubbing
Stubbing is the method wherein a separate line is connected between the primary data bus line
and a terminal. The direct connection of a stub line causes a mismatch which appears on the
waveform. 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, as defined in 4.5.1.5.1. This method provides
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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, as
defined in 4.5.1.5.2 of this standard 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 a
circuit board) and the main bus junction will cause failure of that entire bus. It can be
expected that when the direct coupled stub length exceeds 1.6 feet, that it will begin to distort
the main bus waveform. Note that this length includes the cable runs internal to a given
subsystem.
10.6 Use of Broadcast Option
The use of a broadcast message as defined in 4.3.3.6.7 of this standard represents a significant
departure from the basic philosophy of this standard in that it is a message format which does
not provide positive closed-loop control of bus traffic. The system designer is strongly
encouraged to solve any design problems through the use of the three basic message formats
without resorting to use of the broadcast. If system designers do choose to use the broadcast
command, they should carefully consider the potential effects of a missed broadcast message,
and the subsequent implications for fault or error recovery design in the remote terminals and
bus controllers.
**20. Referenced Documents
Not Applicable.
**30. General Requirements
**30.1 Option Selection
This section of the appendix shall select those options required to further define portions of
the standard to enhance tri-service interoperability. References in parentheses are to
paragraphs in this standard which are affected.
**30.2 Application
Section 30 of this appendix shall apply to all dual standby redundant applications for the
Army, Navy, and Air Force. All Air Force aircraft internal avionics applications shall be dual
standby redundant, except where safety critical or flight critical requirements dictate a higher
level of redundancy.
**30.3 Unique Address (4.3.3.5.1.2)
All remote terminals shall be capable of being assigned any unique address from decimal
address 0 (00000) through decimal address 30 (11110). The address shall be established
through an external connector, which is part of the system wiring and connects to the remote
terminal. Changing the unique address of a remote terminal shall not require the physical
modification or manipulation of any part of the remote terminal. The remote terminal shall,
as a minimum, determine and validate its address during power-up conditions. No single
point failure shall cause a terminal to validate a false address. The remote terminal shall not
respond to any messages if it is has determined its unique address is not valid.
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**30.4 Mode Codes (4.3.3.5.1.7)
**30.4.1 Subaddress/Mode (4.3.3.5.1.4)
An RT shall have the capability to respond to mode codes with both subaddress/mode of
00000 and 11111. Bus controllers shall have the capability to issue mode commands with
both subaddress/mode of 00000 and 11111. The subaddress/mode of 00000 and 11111 shall
not convey different information.
**30.4.2 Required Mode Codes (4.3.3.5.1.7)
**30.4.2.1 Remote Terminal Required Mode Codes
An RT shall implement the following mode codes as a minimum:
Mode Code
Function
00010
Transmit status word
00100
Transmitter shutdown
00101
Override transmitter shutdown
01000
Reset remote terminal
**30.4.2.2 Bus Controller Required Mode Codes
The bus controller shall have the capability to implement all of the mode codes as defined in
4.3.3.5.1.7. For Air Force applications, the dynamic bus control mode command shall never
be issued by the bus controller.
**30.4.3 Reset Remote Terminal (4.3.3.5.1.7.9)
An RT receiving the reset remote terminal mode code shall respond with a status word as
specified in 4.3.3.5.1.7.9 and then reset. While the RT is being reset, the RT shall respond to
a valid command with any of the following:
a.
no response on either data bus,
b.
status word transmitted with the busy bit set, or
c.
normal response.
If any data is transmitted from the RT while it is being reset, the information content of the
data shall be valid. An RT receiving this mode code shall complete the reset function within
5.0 milliseconds following transmission of the status word specified in 4.3.3.5.1.7.9. The
time shall be measured from the mid-bit zero crossing of the parity bit of the status word to
the mid-sync zero crossing of the command word at point A on Figures 3-9 and 3-10.
**30.4.4 Initiate RT Self Test (4.3.3.5.1.7.4)
If the initiate self test mode command is implemented in the RT, then the RT receiving the
initiate self test mode code shall respond with a status word as specified in 4.3.3.5.1.7.4 and
then initiate the RT self test function. Subsequent valid commands may terminate the self test
function. While the RT self test is in progress, the RT shall respond to a valid command with
any of the following:
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a.
no response on either data bus,
b.
status word transmitted with the busy bit set, or
c.
normal response.
If any data is transmitted from the RT while it is in self test, the information content of the
data shall be valid. An RT receiving this mode code shall complete the self test function and
have the results of the self test available within 100.0 milliseconds following transmission of
the status word specified in 4.3.3.5.1.7.4. The time shall be measured from the mid-bit zero
crossing of the parity bit of the status word to the mid-sync zero crossing of the command
word at point A on Figures 3-9 and 3-10.
**30.5 Status Word Bits (4.3.3.5.3)
**30.5.1 Information Content
The status word transmitted by an RT shall contain valid information at all times, e.g.,
following RT power up, during initialization, and during normal operation.
**30.5.2 Status Bit Requirement (4.3.3.5.3)
An RT shall implement the status bits as follows:
Message error bit (4.3.3.5.3.3) - Required
Instrumentation bit (4.3.3.5.3.4) - Always logic zero
Service required bit (4.3.3.5.3.5) - Optional
Reserved status bits (4.3.3.5.3.6) - Always logic zero
Broadcast command received bit (4.3.3.5.3.7) - If the RT implements the broadcast
option, then this bit shall be required.
Busy bit (4.3.3.5.3.8) - As required by 30.5.3
Subsystem flag bit (4.3.3.5.3.9) - If an associated subsystem has the capability for self
test, then this bit shall be required.
Dynamic bus control acceptance bit (4.3.3.5.3.10) - If the RT implements the dynamic
bus control function, then this bit shall be required.
Terminal flag bit (4.3.3.5.3.11) - If an RT has the capability for self test, then this bit
shall be required.
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**30.5.3 Busy Bit (4.3.3.5.3.8)
The existence of busy conditions is discouraged. However, any busy condition, in the RT or
the subsystem interface that would affect communication over the bus shall be conveyed via
the busy bit. Busy conditions, and thus the setting of the busy bit, shall occur only as a result
of particular commands/messages sent to an RT. Thus for a non-failed RT, the bus controller
can, with prior knowledge of the remote terminal characteristics, determine when the remote
terminal can become busy and when it will not be busy. However, the RT may also set the
busy bit (in addition to setting the terminal flag bit or subsystem flag bit) as a result of
failure/fault conditions within the RT/subsystem.
**30.6 Broadcast (4.3.3.6.7)
The only broadcast commands allowed to be transmitted on the data bus by the bus controller
shall be the broadcast mode commands identified in Table 3-I. The broadcast option may be
implemented in remote terminal. However, if implemented, the RT shall be capable of
distinguishing between a broadcast and a non-broadcast message to the same subaddress for
non-mode command messages. The RT address of 11111 is still reserved for broadcast and
shall not be used for any other purpose.
**30.7 Data Wrap-Around (4.3.3.5.1.4)
Remote terminals shall provide a receive subaddress to which one to N data words of any bit
pattern can be received. Remote terminals shall provide a transmit subaddress from which a
minimum of N data words can be transmitted. N is equal to the maximum word count from
the set of all messages defined for the RT. A valid receive message to the data wrap-around
receive subaddress followed by a valid transmit command to the data wrap-around transmit
subaddress, with the same word count and without any intervening valid commands to that
RT, shall cause the RT to respond with each data word having the same bit pattern as the
corresponding received data word. A data wrap-around receive and transmit subaddress of 30
(11110) is desired.
**30.8 Message Formats (4.3.3.6)
Remote terminals shall, as a minimum, implement the following non-broadcast message
formats as defined in 4.3.3.6:
a.
RT to BC transfers
b.
BC to RT transfers
c.
RT to RT transfers (receive and transmit)
d.
mode command without data word transfers.
For non-broadcast messages, the RT shall not distinguish between data received during a BC
to RT transfer or data received during a RT to RT transfer (receive) to the same subaddress.
The RT shall not distinguish between data to be transmitted during an RT to BC transfer or
data to be transmitted during an RT to RT transfer (transmit) from the same subaddress. Bus
controllers shall have the capability to issue all message formats defined in 4.3.3.6.
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**30.9 RT to RT Validation (4.3.3.9)
For RT to RT transfers, in addition to the validation criteria specified in 4.4.3.6, if a valid
receive command is received by the RT and the first data word is received after 57.0 ± 3.0
microseconds, the RT shall consider the message invalid and respond as specified in 4.4.3.6.
The time shall be measured from the mid-bit zero crossing of the parity bit of the receive
command to the mid-sync zero crossing of the first expected data word at point A as shown
on Figures 3-9 and 3-10. It is recommended that the receiving RT of an RT to RT transfer
verify the proper occurrence of the transmit command word and status word as specified in
4.3.3.6.3.
**30.10 Electrical Characteristics (4.5)
**30.10.1 Cable Shielding (4.5.1.1)
The cable shield shall provide a minimum of 90.0% coverage.
**30.10.2 Shielding (4.5.1)
All cable to connector junctions, cable terminations, and bus-stub junctions shall have
continuous 360 degree shielding which shall provide a minimum of 75.0% coverage.
**30.10.3 Connector Polarity
For applications that use concentric connectors or inserts for each bus, the center pin of the
connector or insert shall be used for the high (positive) Manchester bi-phase signal. The
inner ring shall be used for the low (negative) Manchester bi-phase signal.
**30.10.4 Characteristic Impedance (4.5.1.2)
The actual (not nominal) characteristic impedance of the data bus cable shall be within the
range of 70.0 Ω to 85.0 Ω at a sinusoidal frequency of 1.0 MHz.
**30.10.5 Stub Coupling (4.5.1.5)
For Navy applications, each terminal shall have both transformer and direct coupled stub
connections externally available. For Navy systems using these terminals, either transformer
or direct coupled connections may be used. For Army and Air Force applications, each
terminal shall have transformer coupled stub connections, but may also have direct coupled
stub connections. For Army and Air Force systems, only transformer coupled stub
connections shall be used. Unused terminal connections shall have a minimum of 75%
shielding coverage.
**30.10.6 Power On/Off Noise
A terminal shall limit any spurious output during a power-up or power-down sequence. The
maximum allowable output noise amplitude shall be ±250 mV peak, line-to-line for
transformer coupled stubs and ±90mV peak, line-to-line for direct coupled stubs, measured at
point A of Figure 3-12.
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NOTES
Acronyms and Abbreviations
Ω
ohms
µs
microseconds
AEIS
Aircraft/Store Electrical Interconnection System
ARINC
Aeronautical Radio, Incorporated
ASI
Aircraft Station Interface
BC
Bus Controller
BIT
Built in Test
BM
Bus Monitor
CDS
Common Display System
COTS
Commercial-Off-The-Shelf
CSI
Carriage Store Interface
CSSI
Carriage Store Station Interface
dB
decibel
DC
direct current
DT/OT
Developmental Test/Operational Test
EMC
electromagnetic capability
ft
foot
GPS
Global Positioning System
HUD
Head-Up Display
I/O
input/output
kHz
kilohertz
LRU
Line Replaceable Unit
Mbps
Mega bit per second
MFD
Multi-Function Display
MHz
Mega hertz
MIL-HDBK
Military Handbook
MIL-STD
Military Standard
MSB
most significant bit
MSI
Mission Store Interface
mV
millivolts
NATO
North Atlantic Treaty Organization
NAV
Navigation system
NRZ
Non-return to zero
ns
nanoseconds
P
Parity
PC/AT
Personal Computer/Advanced Technology
PCI
Peripheral Component Interconnect
PCM
Pulse Code Modulation
PMC
Peripheral Component Interconnect Mezzanine Card
RMS
root-mean-square
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Acronyms and Abbreviations (Cont'd)
RT
Remote Terminal
SAE
Society for Automotive Engineers
STANAG
Standardization Agreement
T/F
Terminal/Flag
T/R
Transmit/Receive
TDM
Time Division Multiplexing
US
United States
V
volt
VME
Versa Module Eurocard
VXI
Vmebus Extensions for Instrumentation
Z
O
impedance
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APPENDIX A
MIL-STD-1553 Notice Overview
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