I can use cu by using a modem cable which only has tx,rx,gnd connected.
If I use my regular cable, I get the same errors.  The only reason I ever
used cu was the same reason you cite.  Good luck.

Dan

In message <Pine.OSF.3.91.970526225544.8602A-100000@yoyo.cc.monash.edu.au>, Joel Reicher writes:
>Chances are cu is trying to write to the modem while it's not asserting 
>carrier detect. Two solutions: configure your modem to assert this even 
>
>> cu -l tty00 in my attempt to find out why it isn't connecting, but I get
>> cu: write: Input/output error
>>