Circuit transmits ARINC 429 data
Posted on Jan 26, 2013 8777
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.