Negative number in ARINC-429

Question

I'm a little bit confused by representation of negative number in ARINC-429.
Let's consider the following example:

1. Significant bits is equal to 7 (it does not count sign bit),

2. Resolution is equal to 1,

3. DATA section has to represent -1 value.
   Initially I thought that DATA section is represented as follows:

   1 1 1 | 1 1 1 1 | 1 0 0 0 | 0 0 0 0 | 0 0 0 0
   

But I'm confused by sensor documentation. There sign bit is not in DATA section and I started to think that maybe bits 29-11 should look like that:

1 0 0 | 0 0 0 0 | 1 0 0 0 | 0 0 0 0 | 0 0 0 0

Which way of thinking is correct?

Answer 1

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