Yes an auto-guider output to the mount is a "low". I tested my old Gotostar by shorting from earth to one of the guide inputs and could see the appropriate command like "Ra+" appear on my LCD screen. Don't know if other mounts can display this?
Most guide cameras have an RJ11, 6 pin...and unfortunately the configuration of these pins is and can be different between mfgrs and designers see
here (for the most common config) and
here for an alternate and there are lots.
Lx200 protocol is a set of
serial command instructions (written long long ago that became a Pseudo standard) that is usually used in ASCOM to control a scope mount. It has things like Mw (move west), Me (move east), Ms, (move south), Qn (quit north), Qe (quit east)...you get the idea.
So you can hook up your mount to your computer and control the mount as long as it conforms to the command set. You can hook up your computer to mount via serial and use hyper terminal to see how it responds with the above commands.
Hope this helps.