What are some ARINC 429 electrical characteristics?
An ARINC 429 data bus uses two signal wires to transmit 32 bit words. Transmission of sequential words is separated by at least 4 bit times of NULL (zero voltage). This eliminates the need for a separate clock signal wire. Thatâs why this signal is known as a self-clocking signal. The nominal transmission voltage is 10 Â±1 volts between wires (differential), with either a positive or negative polarity. Therefore, each signal leg ranges between +5V and -5V. If one leg is +5V, the other is -5V and vice versa. One wire is called the "A" (or "+" or "HI") side and the other is the "B" (or "-" or "LO") side. This is known as bipolar return-tozero (BPRZ) modulation. The composite signal state may be one of three levels:
- HI which should measure between 7.25 and 11 volts between the two wires (A to B).
- NULL which should be between 0.5 and -0.5 (A to B).
- LO which should be between -7.25 and -11 volts (A to B).
The received voltage depends on line length and the number of receivers connected to the bus. No more than 20 receivers should be connected to a single bus. Since each bus is unidirectional, a system needs to have its own transmit bus if it is required to respond or to send messages.
The transmitting and receiving circuits must be designed to reliably send and detect the null transition between high and low states. The parameters vary with the type of operation.
The following table summarizes ARINC 429 characteristics:
Electrical Characteristic Value Voltage Levels, each leg +5V, 0V, -5V with respect to ground Voltage Levels, Leg A +10V, 0V, -10V with respect to Leg B Bit Encoding Bipolar Return to Zero Word size 32 bits Bit Rates 100K or 12.5K bits per second High Speed Slew Rate 1.5 +/- 0.5 Âµsec Low Speed Slew Rate 10 +/- 5 Âµsec
ARINC 429 is a very simple, point-to-point protocol. There can be only one transmitter on a wire pair. The transmitter is always transmitting either 32-bit data words or the NULL state. There is at least one receiver on a wire pair; there may be up to 20.
In most cases, an ARINC message consists of a single data word. The label field of the word defines the type of data that is contained in the rest of the word.
The slew rate refers to the rise and fall time of the ARINC waveform. Specifically, it refers to the amount of time it takes the ARINC signal to rise from the 10% to the 90% voltage amplitude points on the leading and trailing edges of the pulse.
The following table summarized ARINC Bit Characteristics:
Parameter High Speed Low Speed Bit Rate 100K bits/second 12.5K-14.5K bits/second Time Y (one bit) 10 Âµsec Â± 2.5% 1Ã·(bit rate) Âµsec Â± 2.5% Time X 5 Âµsec Â± 5% Y/2 Âµsec Â± 5% Pulse Rise Time 1.5 Â± 0.5 Âµsec 10 Â± 5 Âµsec Pulse Fall Time 1.5 Â± 0.5 Âµsec 10 Â± 5 Âµsec
Please read our ARNINC Tutorial for more information about ARINC 429 electrical characteristics.