BTR / Fanuc tape drive interfaces

I'm looking for a cheap BTR (behind tape reader) for a Fanuc 7m tape
drive (less than $200). But If I can't find one to buy I'd like to
build one. The interface to the serial or parallel port of a PC is
well know, but it's the pin out and interface to the Fanuc 7m tape
drive that I'm missing. The unit I have has the 50 pin connector.
I'm looking for pin out on the tape drive, signal names and timing
information. I'm also intrest in the pin outs on anyother tape drive.
I'd also like to know if there is any interest from other people for
an inexpensive BTR for Fanuc and other older controllers. If there is
interest, I'd like to try to market an open source BTR interface cable
and software in the $150 to $200 price range.