How to encode an altitude in ARINC429?
From ARINC 429, Attachment 6:


  Label 025, Selected Altitude (BCD)
  Five binary coded decimals.  Range is up to 50000.  Resolution is 1
  foot.
  
  Bit 29 (MSC) ------------------------------ Bit 11 (LSC)
  4 2 1 | 8 4 2 1 | 8 4 2 1 | 8 4 2 1 | 8 4 2 1
  
  1 0 0 | 0 0 0 1 | 0 0 0 0 | 0 0 0 0 | 0 0 0 0
  The above line codes to 41000 ft.
  Note: Negative sign is annunciated in the sign/status matrix.
  
  Label 102, Selected Altitude (BNR)
  16 bits of data. Range of 65536. Resolution (value of LSB) is 1 foot.
  
  Bit 28 (MSB)------------------- Bit 13 (LSB) (Bits 12-11 are set to 0)
     1 0 1 0 0 0 0 0 0 0 1 0 1 0 0 0
  
  The above line codes to 41000 ft.
  
  Note: Negative values are encoded as the two’s complements of positive
  values and the negative sign is annunciated in the sign/status matrix.