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MX Foundation 4
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MX Foundation 4
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ARINC 708 is a data transfer method using transformer-coupled, Manchester encoded signals, similar to the MIL-STD-1553 protocol. Termination of the bus is essential for good signal quality. Furthermore, the bitstream is continuous and requires good re-synchronization. Data frames are 1600 bits long with the header portion of the frame consisting of parameters such as range, tilt, gain, status, etc. The data portion is organized into 512 range-bits per scan angle value. Each range bit contains a color value to indicate the intensity at that position. Settings for the ARINC 708 system are typically controlled using an ARINC 429 interface
ARINC 708 can be used with an airborne, pulse Doppler Weather Radar system which is intended for installation in commercial transport type aircraft. Its primary purposes are weather and forward looking wind shear detection, ranging and analysis. Its secondary purpose is ground mapping to facilitate navigation by display of significant land contours. Its tertiary purposes are detecting weather events with turbulence and displaying auxiliary information from external sources such as ACARS and TCAS.
Modulation
The signal should be transferred over the data bus in serial digital pulse code modulation form.
Data Code
The data code should be Manchester bi-phase level. A logic “one” should be transmitted as a bi-polar code signal 1/0 (i.e., a positive pulse followed by a negative pulse). A logic “zero” should be bi-polar coded signal 0/1 (i.e., a negative pulse followed by a positive pulse). A transition through zero occurs at the midpoint of each bit time.
Transmission Rate
The transmission rate on the bus should be 1.0 megabits per second with a long term stability of +0.1% (i.e., c-3 ±1000 Hz). The short term stability, measured over a 1.0 second interval should be at least +0.01% (i.e., +100 Hz).
Each data consists of the following fields (transmission sequence left to the right).
| Beginning of Word sync | Data | End of Word Sync |
|---|---|---|
| 3 bits wide | Transmission : 1600 bits | 3 bits wide |
Transmission interval
Time elapsed between the beginning of one frame and the beginning of the next one : 5.00 to 7.82 milliseconds.
This section describes the digital transfer system elements considered to be principally related to the electrical aspects of the signal circuit.
Transmission System Interconnect
A data source should be connected to the data sink(s) by means of a single twisted and shielded pair of wires.
Cable Characteristics
The cable length of any main bus may be up to 300 feet. The wire-to-wire distributed capacitance should not exceed 50.0 picofarads per foot. The cable should be formed with not less than one twist per inch and the should provide a minimum of 80% coverage. At a frequency of 1.0 MHz, the cable power loss should not be greater than 1dB/100 feet. The characteristic impedance of the cable should be 70 ohms +10% as measured at a sinusoidal frequency of 1.0 MHz.
Cable Terminations
The main bus cable should be terminated at both ends with a low inductance resistor of 70 +5% ohms. The c-3 cable should be coupled to the receiver or transmitter. A short stub is defined as any stub less than one foot in length. The length of long stubs should not exceed 20 feet (the use of long stubs is discouraged). If long stubs are required in a particular installation, resistor and transformer stub coupling is required at the point where the stub connects to the main bus. Stub match/isolation resistors within the equipment should be mounted as close as possible to the connector.
In this way the stub length is held to a minimum thus reducing line mismatch problems. The cable shield should be connected to air vehicle ground at every break point with low impedance ground leads. The ground lead should not exceed three inches in length. The use of splices with the shield continued through the splice, should a shield not be considered a shield break. The shields should be connected to ground in the airframe at each equipment connector. The cable in all cases should be routed and terminated to reduce noise pickup from other systems.
