When I used a switchbox with my Epson 800, I tried two standard
serial print cables- the printer would not work.  I called Belkin, and
they said I needed to use the special Belkin "throughput" cable model
number ?????-???-?????, from my mac to the switchbox.  I bought the cable,
and although it looked the same as a print cable, my printer and modem
worked.  You can find the correct cabling, or at least the Belkin hotline
on the box.

Hope this helps,
Graham

Graham Johnson
Medical Illustrator
The Salk Institute
La Jolla, CA