MIL-STD-1553 is a 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 as an avionic data bus for use with military avionics, but has also become commonly used in spacecraft on-board data handling (OBDH) subsystems, both military and civil, including use on the James Webb space telescope. It features multiple (commonly dual) redundant balanced line physical layers, a (differential) network interface, time-division multiplexing, half-duplex command/response protocol, and can handle up to 31 Remote Terminals (devices); 32 is typically designated for broadcast messages. A version of MIL-STD-1553 using optical cabling in place of electrical is known as MIL-STD-1773. MIL-STD-1553 was first published as a U.S. Air Force standard in 1973, and first was used on the F-16 Falcon fighter aircraft. Other aircraft designs quickly followed, including the F/A-18 Hornet, AH-64 Apache, P-3C Orion, F-15 Eagle and F-20 Tigershark. It is widely used by all branches of the U.S. military and by NASA. Outside of the US it has been adopted by NATO as STANAG 3838 AVS. STANAG 3838, in the form of UK MoD Def-Stan 00-18 Part 2, is used on the Panavia Tornado; BAE Systems Hawk (Mk 100 and later); and extensively, together with STANAG 3910 "EFABus", on the Eurofighter Typhoon. Saab JAS 39 Gripen uses MIL-STD-1553B. The Russian made MiG-35 also uses MIL-STD-1553. MIL-STD-1553 is being replaced on some newer U.S. designs by IEEE 1394 (commonly known as FireWire).
Revisions MIL-STD-1553B, which superseded the earlier 1975 specification MIL-STD-1553A, was published in 1978.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. Six 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". MIL-STD-1553C is the last revision made in February 2018. Revision C is functionally equivalent to Revision B but contains updated graphics and tables to ease readability of the standard. The MIL-STD-1553 standard is maintained by both the U.S. Department of Defense and the Aerospace branch of the Society of Automotive Engineers.
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, optionally, a coupling transformer. This reduces the impact of a short circuit and ensures 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 per 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 has better resistance to electromagnetic interference, including EMP. This is known as MIL-STD-1773. NASA's "AS 1773" experiment has a dual rate of 1 Mbit/s or 20 Mbit/s – probably a predecessor of STANAG 3910.
Bus protocol
A MIL-STD-1553 multiplex data bus system consists of a Bus Controller (BC) controlling multiple Remote Terminals (RTs) all connected together by a data bus providing a single data path between the BC and all the associated RTs. There may also be one or more Bus Monitors (BMs). BMs are specifically not allowed to take part in data transfers, and are only used to capture or record data for analysis, etc. In redundant bus implementations, several data buses are used to provide more than one data path, i.e. dual redundant data bus, tri-redundant data bus, etc. All transmissions onto the data bus are accessible to the BC and all connected RTs. Messages consist of one or more 16-bit words (command, data, or status). The 16 bits comprising each word are transmitted using Manchester code, where each bit is transmitted as a 0.5 μs high and 0.5 μs low for a logical 1 or a low-high sequence for a logical 0. Each word is preceded by a 3 μs sync pulse (1.5 μs low plus 1.5 μs high for data words and the opposite for command and status words, which cannot occur in the Manchester code) and followed by an odd parity bit. Practically each word could be considered as a 20 bit word: 3 bit for sync, 16 bit for payload and 1 bit for odd parity control. The words within a message are transmitted contiguously and there has to be a minimum of a 4 μs gap between messages. However, this inter-message gap can be, and often is, much larger than 4 μs, even up to 1 ms with some older bus controllers. Devices are required to start transmitting their response to a valid command within 4–12 μs and are considered to not have received a command or message if no response has started within 14 μs.
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