Abaco ARINC429 Special Considerations

Description

This document aims to address the considerations that must be taken into account when using certain encoding type combinations and/or when defining signals' parameters for a given ARINC-429 message.

BNR Considerations

The "Start bit" parameter does not include the BNR sign bit, e.g., 29.
The ARINC-429 RT-LAB driver supports signed BNR and also unsigned BNR encoding:
- The signed BNR specifies a "Min/Max" range of "[ −Limit, +Limit [".
- The unsigned BNR specifies a range of "[ 0.0, +Limit [".
The "Size (bits)" parameter value for a given BNR signal does not includes the sign bit (both for single and mixed BNR signal types).
Notice that, per ARINC convention, the "Digits" value does not include the sign bit when the signal is a single BNR signal (not mixed).
For signed BNR, the +Limit value must be the absolute value of −Limit, minus one resolution value.

Example:

For a signed BNR signal

Range [−256.0 : 256 [ and a resolution of 0.25 we can calculate the following values:

Scale = 256.0
Resolution = 0.25
Digits = log₂(Scale / Resolution)
Max = −(Min) − Resolution

The configuration parameters for this signal would then be:

Start bit: 28
Size: 10
Min: −256.0
Max: +255.75

For a unsigned BNR signal

Range [ 0.0 : 512 [ and a resolution of 0.25 we can calculate the following values:

Scale = 512.0
Resolution = 0.25
Digits = log₂(Scale / Resolution)
Max = 512.0 − 0.25

The configuration parameters for this signal would then be:

Start bit: 28
Size: 11
Min: 0.0
Max: +511.75

BCD Considerations

BCD signals, by definition, are divided into groups of 4 bits, however, when the Size" parameter of the signal is not a multiple of 4, special considerations must be taken regarding the Most Significant Digit (MSD) value.

Example:
- For Size = 6: MSD max value = 3 (binary 011)
- For Size = 7: MSD max value = 7 (binary 111)