I had a similar experience. >
A Mega using serial 1,2, & 3 into a custom board using a Xport , a Max232, and a DB9. The Mega was wired to the DB9, and I was monitoring the Xport from Telnet.
Basically I was coming into the wrong end of the MAX232. By pulling it out and wiring directly to the Xport it worked fine.
I think your problem is around the other way to mine. The MT8000 most likely has a proper UART com port running a high level input. You need a buffer in-between like a MAX232 to go from LOW to High levels.
I didn't find a lot of posts about serial problems like this one, so I thought I would add my 2 cents to this post. hope it helps someone.