You need a RJ-45 modular cable.
communication cable between Levinson amp/pre-amp?
I just bought a Mark Levinson 331 amp and 380 pre-amp and they have ports to hook up a cable between them. I assumed this was a standard CAT6 cable, but the port on the pre-amp marked "master" is not a CAT6 input. Is this a Levinson only cable I need to get from them?