The limit length between Pixhawk and Teraranger?

As below log analysis, when Arducopter(3.4.6) raised up over about 2m Teraranger is death.

I was afraid the cable length ( about 50cm ) between Pixahwk and Teraranger was causing this problem so

I made 30cm cable between them and it works.

Is there any clear guide on this issue? I couldn’t find sth from Teraranger site.