Okay, a few more questions:

1. Is the amp line connected to DTR. I see it is connected to pin 4 on the serial, but does the Kernel think it is DTR?

2. Is the amp line high or low when the amp is powered up? ISTR it is low when powered up and high when off.

3. I am guessing that the Kernel considers switching these lines as its responsibility, that is why it is returning EINVAL. Correct?