Negative number in ARINC-429
BNR labels that can be negative are represented in 2's complement form
2.3.1.1 Numeric Data

The Mark 33 DITS should accommodate numeric data
encoded in two digital languages, (i) BNR expressed in
twos complement fractional notation and (ii) BCD per the
numerical subset of ISO Alphabet Number 5

If your label is indicating 7 significant bits and an LSB of 1, your range must be +/-128. Bit 29 is the sign bit, and the data bits work down from there.

-1 = 1 1 1 | 1 1 1 1 | 1 0 0 0 | 0 0 0 0 | 0 0 0 0