I have the ITech 8511+ as well, and use a Maynuo USB/TTL cable:
eBay auction: #112542793626All these types of cables will work, just be aware that they'll all probably have a counterfeit FTDI chip, and will require playing driver hokey-pokey depending on your host OS.
The 8511+ does support SCPI as well as its native FRAME protocol, so you might have luck with other applications. I just use the horrible ITech software.