I want all the programs in a project to use the same device comments file so that all programs
show the same comments.

Can I do this?
How?

Use the Comment file and check tools options(see attachment)

Just tried that and It doesn't seem to make any differenece.
I have one comment file "MAIN". If I try to edit a comment in one of the programs other than MAIN I get
a "the comment data to be referenced does not exist" error.

I've missed something obvious, I'm sure.

Your global Comment file should be named COMMENT

Thanks, that sent me in the right direction.

Device comment, right click-> New -> Data type = common comment.