Heim system data recorders can record and
play back this data format in its entirety. All original data is written to the
media in the Heim DATaRec proprietary format and replayed in real time or at a
faster speed if required.
Revisions
MIL-STD-1553B, which superseded the earlier 1975 specification MIL-STD-1553A,
was published in 1978. The basic difference between the 1553A and 1553B
revisions is that in the latter, 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
electrical compatibility between designs by different manufacturers could be
assured.
Five change notices to the standard have been published since 1978. For
example, change notice 2 in 1986 changed the title of the document from
"Aircraft internal time division command/response multiplex data bus" to
"Digital time division command/response multiplex data bus".
A single bus consists of a wire pair with 70–85 Ω impedance at 1 MHz. Where a
circular connector is used, its center pin is used for the high (positive)
Manchester bi-phase signal. Transmitters and receivers couple to the bus via
isolation transformers, and stub connections branch off using a pair of
isolation resistors and a coupling transformer. This reduces the impact of a
short
circuit and assures that the bus does not conduct current through the
aircraft. A
Manchester code is used to present both clock and data on the same wire pair
and to eliminate any
DC
component in the signal (which cannot pass the transformers). The
bit rate is
1.0
megabit per second (1 bit = 1 μs). The combined accuracy and long-term
stability of the bit rate is only specified to be within ±0.1%; the short-term
clock stability must be within ±0.01%. The peak-to-peak output voltage of a
transmitter is 18–27 V.
The bus can be made
dual or triply-redundant by using several independent wire pairs, and then
all devices are connected to all buses. There is provision to designate a new
bus control computer in the event of a failure by the current master controller.
Usually, the auxiliary flight control computer(s) monitor the master computer
and aircraft sensors via the main data bus. A different version of the bus uses
optical fiber which weighs less, and better resists electromagnetic
interference, including
EMP. This is known as MIL-STD-1773.
Bus protocol
Messages consist of one or more 16-bit words (command, data or status). Each
word is preceded by a 3 μs sync pulse (1.5 μs low plus 1.5 high, which cannot
occur in the Manchester code) and followed by an odd
parity bit.
The words within a message are transmitted contiguously and there is a 4 μs gap
between messages. Devices have to start transmitting their response to a valid
command within 4–12 μs and are considered to not have received a message if no
response has started within 14 μs.
All communication on the bus is under the control of the master bus
controller and is on the basis of a command from the master controller to a
terminal to receive or transmit. The sequence of words, (the form of the
notation is <originator>.<word_type(destination)> and is a notation similar to
CSP), for transfer of data from the master controller to a terminal is
The sequences ensure that the terminal is functioning and able to receive
data. The status request at the end of a data transfer sequence ensures that the
data has been received and that the result of the data transfer is acceptable.
It is this sequence that gives MIL-STD-1553 its high integrity. The above
sequences are simplified and do not show the actions to be taken in the case of
an error or other fault.
A terminal device cannot originate a data transfer of itself. Requests for
transmission from terminal devices are handled by the master controller
polling the
terminals. Higher-priority functions (for example, commands to the aircraft
control surfaces) are polled more frequently. Lower-priority commands are polled
less frequently. However, the standard does not specify any particular timing
for any particular word -- that's up to the system designers. The absence of a
response when a device is polled indicates a fault.
Five types of transactions exist between the Bus Controller (BC) and Remote
Terminals (RT).
Receive data. The Bus Controller sends one 16 bit command word,
immediately followed by 1 to 32, 16 bit data words. The selected Remote
Terminal then sends a single 16 bit Status Response word back to the Bus
Controller.
Transmit data. The Bus Controller sends one command word to
Remote Terminal. The Remote Terminal then sends a single Status Response
word, immediately followed by 1 to 32 words to the Bus Controller.
Broadcast data. New for 1553B. The Bus Controller sends one
command word with a Terminal address of 31 signifying a broadcast type
command, proceeded by 1 to 32 words. All Remote Terminals will accept the
data but none will respond. This can be used for system wide updates, such
as time of day.
Mode Code. The Bus Controller sends one command word with an
Subaddress of 0 or 31 signifying a Mode Code type command. This command may
or may not be followed by a single word depending on which code is used. The
Remote Terminal responds with a Status Response word which may or may not be
followed by a single data word.
RT to RT Transfer. The Bus Controller sends out a Receive data
command followed by a Transmit data command. The transmitting Terminal sends
a Status Word followed by 1 to 32 data words to the Receiving RT. The
receiving Terminal then sends it's Status Word.
The Command Word is built as follows. The first 5 bits are the Remote
Terminal address (0 ~ 31). The sixth bit is zero for Receive or one for
Transmit. Next 5 bits are the location (subaddress) to hold / get data on
Terminal (1 ~ 30). Notice 0 and 31 are reserved for Mode Codes. Last 5 bits are
the number of words to expect (1 ~ 32). All zero bits indicates 32 words. In the
case of a Mode Code, these bits are the Mode Code number; Initiate Self Test and
Transmit BIT Word are examples.
The Status Word decodes as follows. The first 5 bits are the address of the
Remote Terminal that is responding. The rest of the word is single bit condition
codes. Some bits are reserved. A 'one' state indicates condition is true;
Message Error and Service Request are examples. More than one condition may be
true at the same time.
Conceptual description
Figure 1. A conceptual model of a MIL-STD-1553B bus
Figure 1 at right shows a typical MIL-STD-1553B system.
It consists of:
a dual-redundant MIL-STD-1553B bus
a Bus Controller
three Remote Terminals
a Bus Monitor
The Bus Controller
There is only one Bus Controller at a time on any MIL-STD-1553 bus. It
initiates all message communication over the bus.
Figure 1 shows 1553 data bus details:
operates according to a command list stored in its local memory
commands the various Remote Terminals to send or receive messages
services any requests that it receives from the Remote Terminals
detects and recovers from errors
keeps a history of errors
The Remote Terminals
A Remote Terminal can be used to provide:
an interface between the MIL-STD-1553B data bus and an attached
subsystem
a bridge between a MIL-STD-1553B bus and another MIL-STD-1553B bus.
For example, in a tracked vehicle, a Remote Terminal might acquire data from
an inertial navigational subsystem, and send that data over a 1553 data bus to
another Remote Terminal, for display on a crew instrument. Simpler examples of
Remote Terminals might be interfaces that switch on the headlights, the landing
lights, or the annunciators in an aircraft.
Test Plans for Remote Terminals:
The RT Validation Test Plan is intended for design verification of
remote terminals designed to meet the requirements of AS 15531 and MIL-STD-1553B
with Notice 2. This test plan was initially defined in MIL-HDBK-1553,
Appendix A. It was updated in MIL-HDBK-1553A, Section 100. The test
plan is now maintained by the SAE AS-1A Avionic Networks Subcommittee as
AS4111.
The RT Production Test Plan is a simplified subset of the validation
test plan and is intended for production testing of remote terminals. This test
plan is maintained by the SAE AS-1A Avionic Networks Subcommittee as AS4112.
Bus monitor
A Bus Monitor cannot transmit messages over the data bus. Its primary role is
to monitor and record bus transactions, without interfering with the operation
of the Bus Controller or the Remote Terminals. These recorded bus transactions
can then be stored, for later off-line analysis.
Ideally, a Bus Monitor captures and records all messages sent over the 1553
data bus. However recording all of the transactions on a busy data bus might be
impractical, so a Bus Monitor is often configured to record a subset of the
transactions, based on some criteria provided by the application program.
Alternatively, a Bus Monitor is used in conjunction with a back-up Bus
Controller. This allows the back-up Bus Controller to “hit the ground running”,
if it is called upon to become the active Bus Controller.
