http://www.8052.com/forum/read.phtml?id=129641

It sounds to me like all he needs is an automatic A-B switch (or "T-switch")?

ie, an automatic one of these:

A----+------0
     |
     |
     |
     |           +--------------- ASCOM paging system
     |          /
     |         /
     |        / ^
B----|--+---0   :
     |  |       :
     |  |       :
     |  |  +----+----+
     |  |  |         |
     |  +--+         |
     |     |   micro |
     +-----+         |
           |         |
           +---------+

All the micro needs to do is detect activity on either the 'A' or the 'B' line, and then control the switch accordingly...