binary serial interface

["D. Peter Siddons" <siddons@bnl.gov>]

Hi all,
   I need to interface a USB motor controller which has a protocol 
which consists of sequences of binary (i.e. non-ASCII) byte string 
commands and responses. No end-of-string character is used, but the 
length of the response is coded in one of the first six response bytes. 
Some events (limit switches, e.g.) cause responses which are not a 
direct response to a command. What is the best tool to use for such a beast?
Thanks,
Pete.