MIL-STD-1553

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MIL-STD-1553 ( MIL-Bus for short ) is a field bus that the US Air Force introduced in 1973.

properties

Data is transmitted serially at a speed of up to 1 Mbit / s . Transmitted words consist of 20 bits : 16 bits for information, three leading bits for synchronization and a trailing control bit for the (odd) parity. They are transmitted in half-duplex mode and belong either to the data , command or status category .

The abbreviation MIL stands for military and STD for standard . The MIL bus is widely used in military aircraft construction and in space travel to control system components, while the CAN bus has become established in automotive construction . The MIL bus can be found in Ariane 5 , the ISS and was used in the space shuttles . However, the SpaceWire is being used for high data rates in space travel . Civil aviation, on the other hand, uses ARINC buses, mainly the ARINC 429 bus, which can be found in most commercial aircraft models today.

structure

Bus structure

The MIL-STD-1553 structure consists of a bus controller (BC) and up to 31 remote terminals (RT).

Bus participants:

  • The bus controller organizes the flow of data on the fieldbus . The BC represents an independent computer in the aircraft in the system.
  • Each remote terminal represents an interface for a device in the aircraft. The device can participate in the bus via this interface.
  • The bus monitor is a test device with which the bus is monitored. This test facility can record all data on the bus or reproduce them directly at runtime, which enables errors in the transmission to be tracked down.

Communication flow

Example of data transmission on the bus, one data word

There are basically three different directions of communication. BC to RT, RT to BC and RT to RT. In addition, a broadcast functionality is supported in which one bus participant (BC or RT) sends to all other connected RTs. All communication over the bus - BC to RT, RT to BC and RT to RT - is initiated by the bus controller.

Electrical construction

In order to increase the immunity to interference, the system components are galvanically isolated from the mains . In the simplest case, an inductive transmitter couples the signal. This is possible because the signal levels are free of direct voltage due to the Manchester coding. There are various options for coupling: Transformer Coupled or Direct Coupled . Transformer Coupled means that the bus is terminated with a resistor network to the transformer. "Direct" means that the RT or BC is terminated with resistors on its transformer side, there are no resistors in the bus, therefore also direct.

There is a primary and a secondary channel (these are redundant , so it is never allowed to send on the primary and secondary channel at the same time). Each channel has two signal lines on which the data is transmitted: signal one (“true”) and signal two as its complement. The signals are measured differentially, which means that they are less susceptible to interference. It is a symmetrical signal transmission or more precisely a "differential transmission."

Web links

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