Hello,
I had the same issue when using LabWindows CVI. Hooked up a scope to verify the Tx waveform. Exact same; Arduino Serial Monitor versus CVI. While idle, the Enable pin on the APC220 measured high, however, when the CVI program invoked the USB port via the VCP, the enable pin went low and remained low during byte transmission. I removed the enable pin from the APC220, and all is working, as the node is held up on it's resident PCB. I'd rather control the pin versus make a mechanical fix, but that will take some time to implement. Hope this helps.
