Hi Jay - sorry for not responding to you earlier; I was preparing for my own class today!
I have a vague recollection that not all 3rd-party TMCC implementations respond to absolute commands. The CAB-1 only sent relative commands, not absolute ones, so most people wouldn't notice until Legacy came along and enabled the CAB-2 to send out TMCC absolute speed commands.
If you're not wedded to the command prompt I like CoolTerm as a serial monitor; easy to use in hex or ascii mode.