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,Avionic Systems Standardisation Committee, Avionic Data Transmission Interface Systems Part 2 : Serial, Time Division Command/Response Multiplex Data Bus Standard, Def Stan 00-18, Issue 2, 28 September 1990 is used on the Panavia Tornado; BAE Systems Hawk (Mk 100 and later); and extensively, together with STANAG 3910 "EFABus", on the Eurofighter Typhoon.George Marsh, Typhoon: Europe’s Finest, Avionics Today, June 1st 2003. 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)."The Electric Jet." Philips, E. H. Aviation Week & Space Technology. 2007-02-05.
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MIL-STD-1553B, which superseded the earlier 1975 specification MIL-STD-1553A, was published in 1978.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.ASSIST-QuickSearch - Basic Profile . 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.MIL-STD-1553B vs MIL-STD-1553C change summary, Electra IC
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.MIL-STD-1773: Fiber Optics Mechanization of an Aircraft Internal Time Division Command/Response Multiplex Data Bus NASA's "AS 1773" experiment has a dual rate of 1 Mbit/s or 20 Mbit/s – probably a predecessor of STANAG 3910.Christian Seidleck. MPTB AS-1773 Fiber Optics Data Bus (FODB) Lessons Learned
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 and all connected s. 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 high and low for a logical 1 or a low-high sequence for a logical 0. Each word is preceded by a sync pulse ( low plus 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 word: for sync, for payload and for odd parity control. The words within a message are transmitted contiguously and there has to be a minimum of a gap between messages. However, this inter-message gap can be, and often is, much larger than , even up to with some older bus controllers. Devices are required to start transmitting their response to a valid command within and are considered to not have received a command or message if no response has started within .
Command sequence All communication on the bus is under the control of the using commands from the BC to the s to receive or transmit. The sequence of words is in the form of and is a notation similar to Communicating Sequential Processes (CSP). For transfer of data from the BC to a RT the sequence is:
master.command(terminal) terminal.status(master) master.data(terminal) master.command(terminal) terminal.status(master)
For RT to RT communication it 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)
This means that during a transfer, all communication is started by the , and a terminal device cannot start a data transfer on its own.
The sequences ensure that the terminal is functioning and able to receive data. The status word 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.
However, the standard does not specify any particular timing for any particular transfer — that's up to the system designers.
Whilst s cannot start a transfer directly on their own, the standard does include a method for when an RT needs to transmit data that is not automatically scheduled by the BC.
Six types of transactions are allowed between the BC and a specific RT or between the Bus Controller and a pair of RTs:
MIL-STD-1553B MIL-STD-1553B introduced the concept of optional broadcast transfers, in which data is sent to all s that implement the option, but to which no RTs respond, as this would cause conflicts on the bus. These can be used where the same data is sent to multiple RTs, to reduce the number of transactions and thus reduce the loading on the data bus. However, the lack of explicit responses by the RTs receiving these broadcasts means that these transfers cannot be automatically re-tried in the event of an error in the transaction.
Four types of broadcast transactions are allowed between the and all capable s. All are sent to the broadcast address (31):
Command word The command word is built as follows. The most significant 5 bits are the RT address. Bit 10 is 0 for receive or 1 for transmit. The next 5 bits indicate the location (sub-address) to hold or get data on the terminal. The last 5 bits indicate the number of words to expect. In the case of a mode code, these bits indicate the mode code number (e.g., initiate self test and transmit BIT word).
Status word The status word decodes as follows. The first 5 bits are the address of the RT that is responding. The rest of the word is single bit condition codes, with bits 5-7 reserved and set to zero. A flag set to one indicates the condition is true. More than one condition may be true at the same time.
Example The image below exemplifies many of the protocol and physical layer concepts explained above. For example, the command word in binary is 000 the address contained in the command word has a value of 0x3 (in range of 0 to 31). The sixth bit is 1, indicating a transmit from the RT. The sub-address is 0x01. The last 5 bits indicate the number of words requested, which has a value of 1. The RT responds with a status word (binary 000) containing its address (0x3, as the BC requested), and the reserved bits zeroed and all status flags set to false, followed by the single word of data requested (which had a value of 2).
Conceptual description
Figure 1 shows a sample MIL-STD-1553B system that consists of: Redundant MIL-STD-1553B buses. A Bus Controller. A Backup Bus Controller. A Bus Monitor. A standalone Remote Terminal with one or more subsystems communicating with it. A subsystem with an embedded Remote Terminal.
Bus controller There is only one Bus Controller (BC) at a time on any MIL-STD-1553 bus. It initiates all message communication over the bus. The 1553B spec dictates that all devices in the system be connected to a redundant pair of buses to provide an alternate data path in the event of damage or failure of the primary bus. Bus messages only travel on one bus at a time, determined by the .
Backup Bus Controller While there may be only one BC on the bus at any one time, the standard provides a mechanism for handover to a Backup Bus Controller (BBC), using flags in the status word and Mode Codes. This may be used in normal operation where handover occurs because of some specific function, e.g. handover to or from a BC that is external to the aircraft, but connected to the bus. Procedures for handover in fault and failure conditions generally involve discrete connections between the main and s, and the backup monitoring the actions of the main BC during operation. For example, if there is a prolonged quiescence on the bus indicating that the active BC has failed, the next highest priority BBC, indicated by the discrete connections, will take over and begin operating as the active BC.
Bus monitor