AET Data Bus Systems and Protocols — Questions and Answers
Question 1: What is the primary architectural characteristic of the ARINC 429 data bus used in commercial transport avionics?
- Bidirectional multi-drop bus supporting up to 32 transmitters
- Unidirectional simplex bus with one transmitter and up to 20 receivers (Correct answer)
- Ring topology using token-passing for bus access control
- Full-duplex serial bus operating at 10 Mbps
Correct answer: Unidirectional simplex bus with one transmitter and up to 20 receivers
ARINC 429 is a simplex (one-way) bus: a single transmitter sends data to up to 20 receivers on the same wire pair. If two-way communication is needed, a second separate ARINC 429 bus must be used in the opposite direction.
Question 2: What is the maximum data transfer rate of the MIL-STD-1553B data bus commonly used in military aircraft?
- 100 kbps
- 1 Mbps (Correct answer)
- 10 Mbps
- 100 Mbps
Correct answer: 1 Mbps
MIL-STD-1553B operates at a fixed 1 Megabit per second (1 Mbps). It uses a time-division multiplexing scheme and a command/response protocol managed by the Bus Controller.
Question 3: In a MIL-STD-1553B network, which device is responsible for managing and initiating all data transfers on the bus?
- Remote Terminal (RT)
- Bus Monitor (BM)
- Bus Controller (BC) (Correct answer)
- Redundancy Manager (RM)
Correct answer: Bus Controller (BC)
The Bus Controller (BC) initiates all transactions on a MIL-STD-1553B bus. Remote Terminals (RTs) respond only when commanded by the BC. The Bus Monitor (BM) passively records traffic without participating in transfers.
Question 4: How many bits are contained in each ARINC 429 data word?
- 8 bits
- 16 bits
- 32 bits (Correct answer)
- 64 bits
Correct answer: 32 bits
Each ARINC 429 transmission consists of a 32-bit word, divided into a label field (bits 1–8), a source/destination identifier (bits 9–10), data field (bits 11–28), sign/status matrix (bits 29–30), and a parity bit (bit 32).
Question 5: What is the primary advantage of ARINC 664 (AFDX — Avionics Full-Duplex Switched Ethernet) over ARINC 429 in modern aircraft?
- Lower installation cost due to use of standard CAT5 cable
- Vastly higher bandwidth and bidirectional data flow on a single connection (Correct answer)
- Better immunity to electromagnetic interference than twisted-pair buses
- Direct compatibility with consumer Ethernet switches and routers
Correct answer: Vastly higher bandwidth and bidirectional data flow on a single connection
ARINC 664/AFDX provides 10/100 Mbps full-duplex bandwidth, supporting bidirectional communication over a single connection — a dramatic improvement over ARINC 429's 12.5 kbps or 100 kbps unidirectional bus. It is used in aircraft such as the Airbus A380 and Boeing 787.
Question 6: Why is fiber optic cable preferred over copper wiring for certain aircraft data bus applications?
- Fiber optic is less expensive and easier to splice in the field
- Fiber optic provides immunity to electromagnetic interference and is unaffected by lightning-induced transients (Correct answer)
- Fiber optic cables can carry both electrical power and optical data simultaneously
- Fiber optic connectors are more standardized than copper connector types
Correct answer: Fiber optic provides immunity to electromagnetic interference and is unaffected by lightning-induced transients
Optical fiber transmits data as light pulses and has no metallic conductors, making it completely immune to EMI, RFI, and lightning-induced voltage transients — a significant advantage in the electromagnetically demanding environment of an aircraft.
AET Data Bus Systems and Protocols
What is the primary architectural characteristic of the ARINC 429 data bus used in commercial transport avionics?