Originally Posted by Piper J3
For what its worth, I have Winds Aloft displayed on both the primary D-180 and D-100 on the right side of the panel. It appears that each IFIS does its own calculations for Winds Aloft as they display slightly different values at times. I would have thought that the math would be done in the D-180 and then shared over to the D-100 via RS232 link.
An EFIS uses GPS ground speed, GPS ground track, TAS, and magnetic heading to compute wind speed direction and crosswind. If any of these have error, the computation will have an error.
I am pretty sure your two screens operate interdependently (since each is capable of doing this computation them-self if they were the only unit in a panel.
Since they both receive the same GPS data, most likely the magnetometer or ASI calibrations are slightly different between the two.