FT857 CAT interface.
 

I am trying to connect my PC via a home made interface (with opto
couplers) to the CAT interface on the radio. When trying to connect no
response is received. After doing some measuring i find that the output
voltage at the TX lead of the FT857 is 5V without anything connected.
Is this normal, shouldn't it be 0V while not transmitting?
Does anyone have experience with this?