# Should I use given resolution or calculate resolution based on the max range and number of sig bits?

In ARINC429, Should I use given resolution or calculate resolution based on the max range and number of sig bits given?

ForeExample, to calculate Ambient Static Pressure (Label-203, Equipment ID - 10A),

Given Range (Min to Max): 1.5 to 20

Given No of SIG. bits: 11

Given Resolution: 0.016

Calculated Resolution:

MAX RANGE / 2 ^ (No of SIG. bits)
20 / (2^11)
20 / 2048
= 0.009765625.


Question 1: What will be the Ambient Static Pressure value if all the 11 bits are 0.

Question 2: Should I use given resolution or calculated resolution for my LSB value?

## 1 Answer

It's important to recognize the the resolution provided in the ARINC 429 spec is an approximation so you can't necessarily use that exactly as it is.

From the commentary in section 2.1.6 Data Standards

For convenience, all binary word ranges in Attachment 2 are shown as whole binary numbers rather than such numbers less one least significant bit value. Also, the resolutions shown are approximate only. Accurate resolutions can be determined, if required, by reference to the range values and numbers of significant bits for the words of interest.

In order to provide a range of 1.5 to 20, the label will have an actual range of 0-32, which with 11 significant bits, will provide an actual resolution of $$\frac{32}{2048} = 0.015625$$ (approximated to 0.016 in the ARINC 429 spec)