MIL-STD-1553

MIL-STD-1553

MIL-STD-1553 is a military fieldbus standard developed in the USA. Earlier, it was used mainly in the US Air Force, but today it is used in all military sectors. It is also used in astronautics and in civil aeronautics.

Historical Background

The otherwise popular systems of point-to-point connection in the 1970s were not suited to aeronautics and the aircraft industry, as the wiring work and the mass of the system were too large and too unwieldy for air traffic. For this reason, a committee was established in 1968, which made it its task to find a solution for this problem. This committee named itself SAE (Society of Automotive Engineers), as it was composed of experts from the industrial sector as well as the military sector.

SAE-A2K – the first Step towards MIL-STD-1553

The first design for a bus system was known by the abbreviation SAE-A2K (Multiplexing for Aircraft Committee). With this, industrial as well as military requirements were made for the bus and were realized. This design formed the basis for numerous bus systems in this field, among others MIL-STD-1553. MIL-STD-1553 was installed for the first time in the F-16 fighter. Between 1973 and 1975, the MIL-STD-1553 was improved further and was standardized for the Air Force and the Navy. Requirements of the industry in particular were incorporated into these improvement suggestions. The standard MIL-STD-1553A was finally published in April 1975.

First weaknesses of the new innovation become visible

Multiple use of the new fieldbus system quickly showed obvious faults and weaknesses: Similar devices of different manufacturers could not be used together. Now the hardware and the software had to be adapted individually for each system. A new team of the SAE was formed and worked on a suggested solution.

Birth of the MIL-STD-1553B

The problem could be solved by a very precise definition of the interface. The official result was named MIL-STD-1553B and was presented to the public in September 1978. In the following years, further expansions were made to the standard, among other things, the needs of foreign states also were included, as the interest in this MIL-STD-1553 was very strong.

Areas of Application

As mentioned above, the largest field of operation for the MIL-STD-1553 is the military. As fast, error-free, and stable data transmission is demanded for this area, the bus system soon found greater importance also in other areas. NASA puts its trust into the high level of stability of the bus and uses it in satellites and space stations. The bus is also used in civil applications such as subways.

The different Bus components

The MIL-STD-1553 functions via message transfer. Access to the bus is possible via three components, the so- function modes:

Bus Controller

The bus controller is the most important part in the system, as it is in charge of data exchange. Commands are sent to the remote terminals, which then return the requested data. The bus supports several bus controllers in the system, but always only one master bus controller may be active at a time. In case of failure, an inactive bus controller takes over the task of communication.

Bus Monitor

The task of the bus monitor is recording of the bus activities, without it being a part of the communication processes. The data to be recorded can be set, which is an important instrument for troubleshooting.

Remote Terminal

The remaining components operate in the remote terminal mode. They form the largest group and can operate either as an interface or as a bridge. Accordingly, either a connection to a partial system or a connection between two MIL-STD-1553 buses is made.

Technological Principle of the MIL-STD-1553

1. Physical Layer
The bus itself is composed of a service pair with a resistance of 75 Ω. Transmitter and receiver are connected via the transmitter at the bus exit, the so-called stub. However, the coupling is considered effective only after a further transmitter and two resistors are used for isolation. This guarantees that short-circuits occur less frequently and that no current is directed through the aircraft. The bit rate for transmission is at 1 Mbps, and the voltage of the transmitter is between 18 and 27 V. Every device is coupled with every bus, but the signals are not transferred on all buses, but only on one. In case of failure of one bus, switching to the next bus is carried out.

2. Bus Protocol
A transferred message is composed of 16-bit words, which can occur in three different forms:
a. Command Word
b. Data Word
c. Status Word

3. Information Transfer
MIL-STD-1553 has three different methods for information transfer:
a. Bus Controller to Remote Terminal
b. Remote Terminal to Bus Controller
c. Remote Terminal to Remote Terminal

The master bus controller controls the entire communication, and it is started with a command word of the bus controller to a remote terminal. Before each data transfer, the master bus controller polls the status of the remote terminal that should receive the data, so that secure data transfer is guaranteed. A remote terminal can only send or receive when the master gives the command for this. The remote terminal also is polled after each data transfer to check the success of the data transfer