MIL-STD-1553

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MIL-STD-1553 is military standard published by the United States Department of Defense that defines the mechanical, electrical and functional characteristics of a serial data bus. It was originally designed for use with military avionics, but has also become commonly used in spacecraft on-board data handling (OBDH) subsystems, both military and civil. It features a dual redundant balanced line physical layer, a (differential) network interface, time division multiplexing, half-duplex command/response protocol and up to 31 remote terminals (devices). It was first published as a U.S. Air Force standard in 1973, and was first used on the F-16 Fighting Falcon fighter aircraft. It is widely used now by all branches of the U.S. military and has been adopted by NATO as STANAG 3838 AVS. MIL-STD-1553 is being replaced on newer U.S. designs by MIL-STD-1394B.^[1]

1 Revisions
2 Physical layer
3 Bus protocol
4 Conceptual description
5 References
6 External links
7 See also

[edit] 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MIL-STD-1553B, which superseded the earlier 1975 specification MIL-STD-1553A, was published in 1978.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 compatibility between designs by different manufacturers could be electrically interchangeable.

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".

The MIL-STD-1553 standard is now maintained by both the US DOD and the Aerospace branch of the Society of Automotive Engineers.

[edit] Physical layer

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.

[edit] 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

master.command(terminal) → terminal.status(master) → master.data(terminal) → master.command(terminal) → terminal.status(master)

and for terminal to terminal communication is

master.command(terminal_1) → terminal_1.status(master) → master.command(terminal_2) → terminal_2.status(master) → master.command(terminal_1) → terminal_1.data(terminal_2) → master.command(terminal_2) → terminal_2.status(master)

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.

[edit] Conceptual description

Figure 1 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

[edit] The Bus Controller

There is only one Bus Controller in any Mil-Std-1553B-based system. 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

[edit] 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.

[edit] 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.