THE WORLD of AVIONICS IN YOUR HAND
The ARINC 708 is a protocol developed specifically for use with airborne weather radar systems. It is normally used to display the output from the radar on the radar weather display.
ARINC 708 is designed for use with airborne pulse Doppler weather radar display systems. ARINC 708 data is transmitted over the display data bus from the radar transmitter/receiver to the radar weather display units for the pilot and copilot.
ARINC 708 DATA FRAME
The data words are 1600 bits long and are composed of a single 64-bit status word plus 512 3-bit data words (1600 bits total) ARINC 708 uses a transformer-coupled Manchester encoded signal with a 1MHz bit-rate
In an ARINC 708 weather display system, all data frames (words) are 1600 bits
The bus uses 2-wires, is a Manchester encoded, and runs at a one-megabit data rate.
Each frame contains both header information and data information. Each frame starts with a unique 3us sync pattern (1.5us high, 1.5us low) and ends with another 3us sync pattern (1.5us low, 1.5us high). Therefore, a complete frame takes 1606us on the bus.
Each 1600-bit frame writes one line of pixels on the display screen on successive lines. Each group of frames, therefore, displays a color picture of the current weather pattern in the vicinity of the radar sensor. Between successive messages as 4 ms gap is inserted All devices must start transmitting a response to a command within 4 to 12ms. If they do not start transmitting within 14 ms, they are considered to have not received the command message.
ARINC 708 couples and cables
The ARINC 708 definition of a data bus is “a twisted-shielded pair transmission line, a 70-85 ohm coaxial wire, made up of a main bus and a number of attached stubs”. Shielding limits signal interference from outside sources and the twisted pair maintains message integrity through noise canceling.
ARINC 708 allows two methods of coupling terminal devices to the main bus:
a. Direct coupling
Direct Coupling Direct coupling connections are wired directly to the bus cabling. The isolation resistors and transformer are internal to the terminal device, not requiring additional coupling hardware. Direct coupling connections can only be used with stub lengths of less than 1 foot. Isolation resistors provide some protection for the main bus in the event of a stub or terminal short, but ARINC 708 cautions the use of direct coupling because a terminal short could disable the entire bus. Output Voltage signals pick to pick are +-27 Volts.
b. Transformer coupling.
Transformer Coupling a second isolation transformer, located external to the terminal device, in its own housing with the isolation resistors. Transformer coupling extends the stub length to 20 feet and provides electrical isolation, better impedance matching and higher noise rejection characteristics than direct coupling. The electrical isolation prevents a terminal fault or stub impedance mismatch from affecting bus performance. Output Voltage signals pick to pick are +-`19 Volts.