MIL-STD-1553B

MIL-STD-1553B has evolved since the original publication of MIL-STD-1553 in 1973. The standard has developed through 1553A standard issued in 1975 to the present 1553B standard issued in September 1978. The basic layout of a MIL-STD-1553B data bus is shown in Figure 2.11. The data bus comprises a screened twisted wire pair along which data combined with clock information are passed. The standard generally supports multiple-redundant operation with dual-multiple-redundant operation being by far the most common configuration actually used. This allows physical separation of the data buses within the aircraft, therby permitting a degree of battle damage resistance.

Control of the bus is performed by a bus controller (BC) which communicates with a number of remote terminals (RTs) (up to a maximum of 31) via the data bus. RTs only perform the data bus related functions and interface with the host (user) equipment they support. In early systems the RT comprised one or more circuit cards, whereas nowadays it is

O 4 8 12 16 20 24 28 32

Source Data Identifier

Parity Signal Status Matrix Label 'Data' - encoded depending

upon message type Binary Coded Decimal (BCD) Binary (BNR)

Discretes

Alphanumeric Formats etc Data Rate is 12 to 14 kHz

or 100 kHz (100kHz is more usual)

Figure 2.10 A429 data word format.

usually an embedded chip or hybrid module within the host equipment. Data are transmitted at 1 MHz using a self-clocked Manchester biphase digital format. The transmission of data in true and complement form down a twisted screened pair offers an error detection capability.

Words may be formatted as data words, command words or status words, as shown in Figure 2.12. Data words encompass a 16 bit digital word, while the command and status

Bus

Figure 2.11 MIL-STD-1553B data bus.

RT Address Sub-address

T/R Mode Data word count

/mode code

Figure 2.12 MIL-STD-1553B data bus word formats.

AEROSPACE-SPECIFIC DATA BUSES 65

words are associated with the data bus transmission protocol. Command and status words are compartmented to include various address, subaddress and control functions, as shown in Figure 2.12.

MIL-STD-1553B is a command–response system in which transmissions are conducted under the control of a single bus controller at any one time; although only one bus controller is shown in these examples, a practical system will employ two bus controllers to provide control redundancy.

Two typical transactions are shown in Figure 2.13. In a simple transfer of data from RT A to the BC, the BC sends a transmit command to RT A, which replies after a short interval known as the response time with a status word, followed immediately by one or more data words up to a maximum of 32 data words. In the example shown in the upper part of the figure, transfer of one data word from RT A to the BC will take approximately 70 ms (depending upon the exact value of the response time plus propagation time down the bus cable). For the direct transfer of data between two RTs as shown from RT A to RT B, the BC sends a receive command to RT B followed by a transmit command to RT A. RT A will send its status word plus the data (up to a maximum of 32 words) to RT B which then responds by sending its status word to the BC, thereby concluding the transaction. In the simple RT to RT transaction shown in Figure 2.13, the total elapsed time is around 120 ms for the transmission of a single data word, which appears to be rather expensive on account of the overhead of having to transmit two command words and two status words as well. However, if the maximum number of data words had been transmitted (32), the same overhead of two command and two status words would represent a much lower percentage of the overall message time. For further reading, see MIL-STD-1553B (1986).

Transmit

Remote Terminal A to Bus Controller Transfer

Remote Terminal A to Remote Terminal B Transfer

Figure 2.13 MIL-STD-1553B typical data transactions.

MIL-STD-1553B has proved to be a very reliable and robust data bus and is very well established as a legacy system. Attempts have been made to increase the data rate which is the only major shortcoming. A modification of 1553 called 1553 enhanced bit rate (EBR) running at 10 Mbit/s has been adopted for bomb carriage on the JSF/F-35 using the miniature munitions/store interface (MM/SI). Other vendors have run laboratory demonstrators at 100 Mbit/s and above, and a feasibility program has been initiated to demonstrate 1553 bit rates of 100 Mbit/s with the aim of extending data rates to 500 Mbit/s. This possible derivative is termed enhanced 1553 (EB-1553), and the US Air Force recently hosted a workshop to investigate the possibilities. In October 2003 the Society of Automotive Engineers (SAE) formed an ‘Enhanced Performance 1553’ task group to address the prospect of launching applications with throughputs of 200–500 Mbit/s using existing cables and couplers. A further standard using 1553 is MIL-STD-1760 – a standard weapons interface which is described in Chapter 9.

2.2.4 STANAG 3910

The evolution of STANAG 3910 was motivated by a desire to increase the data rate above the 1 Mbit/s rate provided by MIL-STD-1553B. The basic architecture is shown in Figure 2.14.

The high-speed fibre-optic data terminals pass data at 20 Mbit/s and are connected using a star coupler. Control is exercised by MIL-STD-1553B using electrical connections. The encoding method is Manchester biphase, as for 1553, and data transactions are controlled by means of a bus controller, as is also the case for 1553.

The use of fibre optics passing data at 20 Mbit/s offers a significant improvement over 1553. Furthermore, the ability to transfer messages of up to 132 blocks of 32 words (a total of 4096 data words) is a huge advance over the 32 word blocks permissible in 1553. A total of 31 nodes (terminals) may be addressed, which is the same as 1553.

There are four possible implementations of STANAG 3910 (Table 2.1). The standard also makes provision for the high-speed channel to be implemented as an optical transmissive star coupler, a reflective star coupler or a linear Tee coupled optical bus. Eurofighter Typhoon utilises the type A network with an optical reflective star coupler in a federated architecture.

Bus Controller

20 Mbit/sec Channel (Fibre-Optic)

Figure 2.14 STANAG 3910 architecture.

AEROSPACE-SPECIFIC DATA BUSES 67

The STANAG bus is used for the avionics bus and the attack bus while standard MIL-STD-1553B is used for the weapons bus. The Typhoon architecture is discussed in Chapter 9 of this book.

The similarity of transactions to 1553 may be seen by referring to the remote terminal (RT) to bus controller (BC) transaction on the low-speed bus as shown in Figure 2.15. The BC issues a standard 1553 command word followed by a high-speed (HS) action word (taking the place of a 1553 data word). After a suitable interval the RT issues a 1553 status word that completes the transaction. After an intermessage gap, the next transaction is initiated. This cues a high-speed message frame on the high-speed bus which enables the transmission of up to 132 blocks of 32 data words, up to a maximum of 4096 words, as has already been stated. A full description of all the STANAG data transactions may be found in Wooley (1999) which is also a useful source on many other data buses that may be used in avionics applications.

Table 2.1 STANG 3910 implementations

Type A Low-speed channel 1553B data bus High-speed channel Fibre-optic data bus

Type B High-speed channel Fibre-optic equivalent of 1553B Low-speed channel Physically separate fibre-optic data bus Type C High-speed channel Fibre-optic equivalent of 1553B

Low-speed channel Wavelength division multiplexed with low-speed channel

Type D High-speed channel 1553B data bus

Low-speed channel Physically separate wire data bus Note: Both the low-speed and high-speed buses use Manchester biphase encoding.

Command

3910 Remote Terminal A to Bus Controller Transfer (initiated after one RT to BC message)

HS Message Frame

[Maximum Transfer of 4096 16 bit words]

MIL-STD-1553

Figure 2.15 STANAG 3910 RT to BC data transaction.

The formats of the low-speed bus HS action word and HS status word are shown in Figure 2.16. It can be seen that the general format is similar to the 1553 words, except that these protocol words have positive-going synchronisation pulses as they are replacing the standard 1553 data word [which also has a positive-going synchronisation pulse (Figure 2.12)]. Also, the word content, instead of containing a 16 bit data word, contains message fields that relate to the high-speed bus message content or transmitter/receiver status. As for other 1553 words, the final bit (bit 20) is reserved for parity.

The format of the STANAG 3910 high-speed bus data structure is shown in Figure 2.17.

The data content is preceded by 56 bits associated with the message protocol and followed

HS

Reserved HS Transmitter TF P

Status

HS Receiver Status

Figure 2.16 STANAG 3910 HS word formats (low-speed bus).

PR SD FC PA DA WC

Protocol Data Units (PDU)

Protocol Data Units (PDU) SD

Figure 2.17 STANAG 3910 data structure.

AEROSPACE-SPECIFIC DATA BUSES 69

by a further 20 bits, giving a total overhead of 76 bits in all. Therefore, a maximum 4096 data word transfer will takeð56 þ 4096  16 þ 20Þ ¼ 65 612 bits. At a the high-speed bus data rate of 20 Mbit/s, the total elapsed time is 3280.6 ms.

The application of STANAG 3910 is confined to two European fighter aircraft programmes.

Eurofighter Typhoon uses a variant called the EFA bus, which is a type A implementation, while the French Rafale employs type D, using the electrical high-speed bus version.