Mid is the "geometric center of  the spans of  all of the pins in a 
component.. (not the body outline.)
Ref is the origin of the component when the component was created in the 
Pad is Pad1 of the component...

Most of the time the component is and I believe should be built with the 
origin (Ref) at the center of PIN 1
so that when placement is performed you have the best chance to align all 
or the most number of pins to a
testable /routing grid..  Since Protel does such a great job of calculating 
the centroid for the mid of the component
anyway.  Secretly the object of the game is to get as many pins as possible 
on an even increment of the routing grid.
Which should be an even increment of the placement and tester grid..

Most Assembly houses want  the centroid.. which Protel considers to be the 
for surfacemount components... However they want pin1 for axials etc.
You can easily provide that by providing the PIK file..

The X,Y coordinates are based from where the origin of the workspace is 
defined at the time
in which you generate the PIK file..  so if you move the origin (set) it to 
a fiducial or tooling hole
prior to generating the PIK file  then all coordinates (X,Y) will emanate 
from that point..  Also doubly cool
from a manufacturing perspective...

Rotation is based upon the orientation of the component outline at the time 
it was originally created in the library..
meaning.. if one DIP16 was created in the vertical that would be considered 
"0" rotation..
  if another DIP20 was created in the horizontal  in the library it too 
would be considered
"0" also , so when both are place in a workspace or database.. so that they 
looked vertical .. one would have an rotation of
"0"  and the other "90".. due to their different orientation at the time of 
their respective creations.

Hope this helps...


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