Antenna tracker problem where the computer cannot be connected to the aircraft

I would like to ask a question. I use APM2.5 on both the aircraft and the antenna tracker. I use the ULRS data transmission between the aircraft and the antenna tracking. The antenna tracker and the PC are connected by a USB cable. Now the problem is that I use the mission planner to connect to the antenna tracker. Everything works fine, but when I connect to the plane, I can only see the attitude information, I can’t get the parameters and write the parameters. What is the reason? If I change the antenna tracker Into pix then the problem is solved, but a new problem with pix is that output channels 1 and 2 have no output.
ps:Please forgive a poor English-speaking Chinese who uses Google Translate, thank you

I just did a separate test. I used a USB connection between the APM and the computer. When I disconnected the APM serial port RX pin, the problem occurred as I mentioned above. The ground terminal can receive it. To the flight control attitude information, but can not write parameters, I guess this is a one-way connection, and did not establish mutual communication, so I wonder if the antenna tracker also has the same problem, the antenna tracking end and There is no two-way link on the aircraft end. It is only the antenna tracking segment that receives the information from the aircraft and cannot send it out.