In message , at 23:32:21
on Wed, 6 Nov 2013, Stephen Furley remarked:
My Oyster card will not read if I have a PATH Smartlink card next to
it. However, at least four years ago when I last used it, the Smartlink
card will work quite happily on the readers on the PATH turnstiles when
next to an Oyster card, so it can't be too difficult to make it ignore
a 'foreign' card not valid on that system.

Actually, there's no reason for it to be symmetrical. What if, when
energised, the Smartlink card produces a much stronger signal than an
Oyster so that when used on PATH it swamps the Oyster signal and is
recognised, and on a TfL gate it swamps the Oyster signal and the gate
can't see the Oyster.
--
Roland Perry