Circuit transmits ARINC 429 data

  
The physical transmission medium for the 429 standard is 78Ω shielded, twisted-pair cable that uses a complementary, differential bipolar RZ (return-to-zero) waveform (Figure 2). The voltages are the net differentials that the biphase drive develops: For example, the differential is 10V when you drive the Data A signal in Figure 1 to 5V and the Data B signal to –5V. In addition to the signal levels, a 429 system must closely control the rise and fall times to conform to the specification. This control limits both intra-cable signal crosstalk and EMI radiation that might interfere with sensitive aircraft communication and navigation systems. The operation of the transmitter in Figure 1 centers around IC1, a 4-bit×16-word FIFO memory. The serial ARINC bit stream comes from the microcontroller's synchronous serial-peripheral interface (SPI); the C input of IC1 buffers the data stream. In addition, the D input of IC1 serves as a buffered ARINC-enabled bit (the microcontroller's J port, bit 2). When low, this bit disables the ARINC transmitter logic and permits other system peripherals to use the SPI hardware.
Circuit transmits ARINC 429 data - schematic

Bits A and B of IC1 go unused. The 100-kHz ARINC high-speed baud rate comes from the 1-MHz reference supplied to the IC3A divide-by-10 circuit. The 100-kHz signal drives IC1's shift-out (SO) pin and the IC2 pulse gate. The presence of bits in the IC1 FIFO (indicated by QR=1) resets the ARINC RDY bit in IC3B and enables IC3A. If IC1's D bit (Pin 10) is also high, it gates the 100-kHz square wave to the IC4 multiplexer. This action causes the sequential gating of –5, 0, and 5V onto the A/B data-output signals in ARINC-compatible waveforms. The LRC network at the output ensures compliance with the 429 requirements for rise and fall times. The circuit must process five 8-bit SPI bytes to generate each 32-bit, 429-compliant output word. Table 1 shows the format of the SPI bits. The first four bytes in Table 1 combine to form a 32-bit ARINC 429 word. The 32-bit word, reading from right to left, starts with byte 1 (again, reading from right to left), then tacks on byte 2, and so on.




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