Module 5.4, 5.9, 5.10 Data buses, multiplexing and fibre optics

Knowledge levels

Sub-module	A	B3	B1	B2 / B2L
5.4 Data buses (ARINC and other specifications, aircraft network/Ethernet)	—	—	2	2
5.9 Multiplexing	—	—	—	2
5.10 Fibre optics	—	—	1	2

Multiplexing is examined only for B2/B2L.

Why data buses?

Instead of a separate wire pair for every signal, digital systems send many parameters as serial binary words over a shared bus. This saves weight, improves accuracy and allows many receivers to use the same data.

ARINC 429

• The most common civil avionics bus. Unidirectional: one transmitter, up to 20 receivers.
• Twisted shielded pair; bipolar return-to-zero (BPRZ) signalling: +10 V = logic 1, −10 V = logic 0, 0 V between bits (differential signal, about ±10 V between the two wires).
• Speeds: high speed 100 kbit/s, low speed 12-14.5 kbit/s.
• 32-bit word:

Bits	Field
1-8	Label (octal) - identifies the parameter, e.g. altitude, airspeed
9-10	SDI (source/destination identifier)
11-28	Data (BNR binary or BCD)
29	sign (in BNR words)
30-31	SSM (sign/status matrix): normal, no computed data, functional test, failure warning
32	Parity (odd)

• A gap of at least 4 bit times separates words; receivers synchronise on the null between bits (self-clocking).

Key fact: ARINC 429 = one transmitter, many receivers (max 20), 32-bit words, odd parity, 100 kbit/s high speed.