Those look like they might be RFC 2217 remote serial port configuration. Try specifying the COM option to the IP address argument to the ASYN port configuration command.
I have read the manual for the MPC that you sent. Indeed it should never sent the unprintable characters like \377\372. I see that you are using a TCP connection to the MPC, which means you are using its internal Lantronix terminal server. I think you may have some setting on the Lantronix wrong, for example the start bits, stop bits, or parity. Can you open a Web page to the Lantronix and see what settings you are using?