Aviation Multiplexing

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Questions and Answers

What was the primary nature of data signals, inputs and outputs from sending and receiving systems in the early days of avionics?

Analogue (correct)

Digital

Mixed signal

Radio frequency

What was the limitation of analogue data transfer in avionics systems?

High power consumption

Bidirectional transfer

Unidirectional transfer (correct)

Noisy signals

What is the primary function of a Bus Controller in avionics systems?

To transmit data to indicators

To control all communications over the multiplexing bus (correct)

To receive data from mission computers

To process navigation sensor data

What is the main advantage of multiplexing in avionics systems?

Reduced wiring required for data transmission Signup and view all the answers

What is the primary benefit of digital application in avionics systems?

Reduced number of analogue signals and the need for their conversion Signup and view all the answers

What type of transmission is used in digital avionics systems to reduce interconnections and receiver/driver circuitry?

Serial transmission Signup and view all the answers

What is the primary component that controls data transmitted over the multiplexer busses in an avionics system?

Mission computers Signup and view all the answers

What is a crucial requirement for a fully integrated digital avionics system?

A digital data bus Signup and view all the answers

What is the purpose of the status word transmitted by the Remote Terminal to the Bus Controller?

To validate the receipt of the message by the Remote Terminal Signup and view all the answers

What is the maximum number of data words that can be sent in a BC-RT message?

32 Signup and view all the answers

What is the term used to describe the message transmitted from the Bus Controller to the Remote Terminal?

Receive Command Signup and view all the answers

What information is contained in the command word transmitted by the Bus Controller?

The sub-address and number of data words Signup and view all the answers

What is the purpose of the command word transmitted by the Bus Controller?

To define the sub-address and number of data words Signup and view all the answers

What happens immediately after the Bus Controller transmits the command word?

The Bus Controller transmits the data words Signup and view all the answers

What is the term used to describe the message transmitted from the Remote Terminal to the Bus Controller?

Transmit Command Signup and view all the answers

What was the primary issue with integrating new systems or functions in analogue aircraft?

The need for additional hardware to output signals Signup and view all the answers

What was the purpose of Output Multiplier Boxes (OMBs) in analogue aircraft?

To replicate signals for output to multiple systems Signup and view all the answers

What is the primary principle behind the information transfer formats?

Command/response philosophy Signup and view all the answers

Why would it be impractical to design an aircraft with analogue wiring in modern times?

Because too much information is transmitted for routine operation of avionics systems Signup and view all the answers

What advantage did digital computers offer over analogue systems in avionics?

Easy growth and increased computational capability Signup and view all the answers

What was the impact of Output Multiplier Boxes on aircraft design?

They added to aircraft weight and space problems Signup and view all the answers

What was the primary function of analogue signals in avionics systems?

To transmit information from one box to another Signup and view all the answers

Why did point-to-point wiring pose a challenge in analogue aircraft systems?

Because it was difficult to modify the system to provide additional signals Signup and view all the answers

What was the consequence of adding new systems or functions in analogue aircraft?

It created an integration nightmare with potential system impacts Signup and view all the answers

What is one of the benefits of multiplexing in aviation?

Significant reduction in the weight of wire bundles Signup and view all the answers

What is the basic principle of multiplexing?

Two rotary switches are synchronised in their switching as they rotate around a series of contacts Signup and view all the answers

How are analogue signals typically handled in an aircraft multiplexing system?

They are converted to digital and then transmitted over the multiplexer network Signup and view all the answers

What is the role of the Bus Controller (BC) in an aircraft multiplexing system?

It receives all the inputs and distributes outputs and processed data Signup and view all the answers

What is the function of the sequenced gate outputs in a multiplexing system?

They are transmitted serially over a single transmission line Signup and view all the answers

What is the purpose of the demultiplexer in an aircraft multiplexing system?

To separate the serially transmitted signals into individual outputs Signup and view all the answers

What is the effect of transmitting eight separate inputs through a single transmission line using multiplexing?

Eight separate inputs are transmitted over the same transmission line Signup and view all the answers

What is the role of logic gates in a multiplexing system?

They respond in sequence to clock pulse signals to sample and transmit the input signals Signup and view all the answers

What was the year when the tri-service version, MIL-STD-1553A, was released?

1975 Signup and view all the answers

Which of the following organizations spent three years to produce MIL-STD-1553B?

The Society of Automotive Engineers (SAE) Signup and view all the answers

What is the primary function of the first 3 bits in a MIL-STD-1553 data word?

To allow device clocks to re-sync Signup and view all the answers

What is the length of a MIL-STD-1553 data word?

20 bits Signup and view all the answers

What is the purpose of the parity bit in a MIL-STD-1553 data word?

To ensure odd parity Signup and view all the answers

What type of word in a MIL-STD-1553 data word contains a terminal address?

Command word Signup and view all the answers

What was the reason for the government to 'freeze' the standard at the B level?

To allow component manufacturers to develop products and gain real-world experience Signup and view all the answers

How many distinct word types are defined by the MIL-STD-1553 standard?

3 Signup and view all the answers

Study Notes

Benefits of Multiplexing

Significant reduction in weight of wire bundles
Improved circuit reliability

Principle of Multiplexing

Two rotary switches are synchronised in their switching as they rotate around a series of contacts
Synchronised rotating contacts connect matching input and output lines in sequence
Data is transmitted over the common transmission line

Time Domain Multiplexing

Done by logic gates responding in sequence to clock pulse signals
Signal on each input line is sampled and passed to the common transmission line when the inputs AND gate is clocked ON
Sequenced gate outputs are serially transmitted to the demultiplexer
Eight separate inputs are transmitted through to eight separate outputs over the same transmission line

Multiplexing in Aircraft

Analogue signals must be converted to digital before being multiplexed
Bus Controller (BC) replaces sequencing controller and distributes outputs and processed data to systems requiring the information
Functions as a central hub, distributing digitised data to systems such as multifunction displays and thrust computers

Drawbacks of Traditional Aircraft Systems

Point-to-point wiring leads to added weight and complexity
Additional connections can have potential system impacts
Output Multiplier Boxes (OMB) are required to replicate signals, adding weight and complexity

Digital Aircraft System

Advent of digital technology led to increased computational capability and easy growth
Digital computers replaced analogue systems, reducing the need for A/D and D/A converters
Microprocessors further increased digitisation, reducing analogue signals and allowing bidirectional data transfer

Avionics Data Bus Layout

Bus Controller (BC) manages all communications over the multiplexing bus
BC works similarly to a base station radio operator
Serial transmission of data reduces number of interconnections and receiver/driver circuitry

MIL-STD-1553

A digital data bus standard for aircraft systems
Released in 1975, with improvements made in 1978 (1553B)
Standardised data bus layout and communication protocol for aircraft systems

MIL-STD-1553 Data Words

Three distinct word types: Command, Data, and Status words
Each word is 20 bits in length, with a unique format
Synchronisation field, information field, and parity bit make up each word

Command Words

Contain terminal address, telling remote terminals which component the command is addressed to
Part of the command/response philosophy, with error-free transmissions followed by a status word from the terminal to the BC

Bus Controller to Remote Terminal (BC-RT) Message

The Bus Controller outputs a command word defining the sub-address of the data and the number of data words
Data words are sent immediately, followed by a status word from the Remote Terminal indicating receipt and validation of the message

Remote Terminal to Bus Controller (RT-BC) Message

The Remote Terminal transmits its status word followed by the number of data words requested by the command word
Part of the command/response philosophy, with the Bus Controller issuing a transmit command word to the Remote Terminal

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Description

Learn about the benefits and principles of multiplexing in aviation, including weight reduction and improved circuit reliability.