What is the difference between ARINC 429 labels 025 and 102 for Selected Altitude?
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According to A429, the difference is the type of data coding. Label 025 is BCD. Label 102 is BNR.
Label 025 by being a Binary Coded Decimal codes each character of the altitude value in binary. With a range of 0 to 50000 feet, the first digit uses 3 bits and the remaining use 4 bits each for total of 19 data bits.
Label 102 is a Binary number. The range is 0 to 65535. There are 16 data bits (2^16 = 65536).
Both are transmitted. Receivers will use the version most appropriate for the application. BCD is normally used for display. BNR is normally used in computations.
