I spent quite a bit of time trying to figure out why the Diagnostic disk wasn't listing the 8087 Math Coprocessor in one of my 5150s, and I think Ive got it.
At first I thought maybe I had a unique 5150 where a switch was built or wired incorrectly (I even removed the mobo to check continuity on the switch itself) but I checked three 5150s, and its the same thing with all of them.
I double checked the switch settings listed in the IBM Technical Reference (page 6-3) which indicates that Switch 1, Position 2, should be:
On if coprocessor is installed (must of OFF if coprocessor is not installed)
Turns out the problem was the Technical Reference its wrong !
I have a copy of the Option Instructions for the IBM Math Coprocessor Option, and on page 18 of those instructions, it states that if a coprocessor is installed, switch position 2 needs to be in the OFF position (ie: not the ON position that the Tech Ref dictates).
Hopefully, anyone else who comes across this anomaly will find this post, and not have to waste their time trying to figure it out.
PS: has anyone else noticed a similar thing on their 5150s, or am I the lucky first one to have gone through this?