MIL-STD-1553

MIL-STD-1553 is a United States Department of Defense military standard which defines the mechanical, electrical and functional Characteristics of a serial data bus originally designed for use with military avionics. In recent years it has also become commonly used in spacecraft On Board Data Handling (OBDH) subsytems, 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 fighter aircraft. It is widely used by all branches of the U.S. military and as been adopted by NATO as STANAG 3838 AVS.

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.

A special trait of the copper form of this bus is that branches and attachments between bus sections are made by transformers with no direct electrical connection. This is made possible by the use of a Non-return-to-zero modulation thus eliminating any DC component in the data stream. This specific provision prevents single-point shorts of the bus, and assures that the bus does not conduct current through the aircraft.

The bus is Dual or triply-redundant, and all devices are connected to all buses. 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.

The standard message sent and received consists of one or more 16-bit words (command, data or status). Each word is preceded by a 3 bit time sync pulse and followed by a parity bit. Parity is always odd, and words are transmitted contiguously. The format of the word varies depending on the device to which it is addressed. There is always a bit error to indicate that the remote device thinks it's malfunctioning. The bit transmission rate on the bus is 1.0 megabit per second using Manchester II bi-phase code. Manchester II code is a Self-clocking signal and thus overcomes the problems of clock synchronization mentioned under Non-return-to-zero modulation. The combined accuracy and long-term stability of the bit rate is ±0.1%. The short-term stability is less than 0.01%.

Generally, words are generated by a flight control computer which is dual or triply redundant and acts as a bus control computer. Although redundancy is employed only one bus controller is in use at any time. The bus controller is connected to all busses in the MIL-STD-1553 network. 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.

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 tranmission 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.

Flight control words are generated by a program in the flight control computer that implements differential equations. These "flight control equations" accept pilot commands from the manual controls and sensor data from the aircraft and calculate control surface positions that will stabilize the plane and carry out the intentions of the pilot.

MIL-STD-1553

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