Robin2:
I don't have any HC12s so I can't help with that part. However if it was my project I would check that the communication works at the baud rate you want when the two Arduinos are connected by wires rather than using the HC12s....R
I checked the HC-12 first: communications work well at 1200 baud (testing with software serial). There is a delay of almost 100ms between the first bit being transferred to the transmitting HC-12 and the time when that first bit arrives on the receiving HC-12 output.
The total message length for what I want to transmit is about 200 bytes, so when using serial.flush(); I still have to take into account the time needed for the HC-12 to transmit all that, even after serial.flush() did it's job. What I don't know is how fast the Arduino serial output buffer is transferring to the HC-12. I did some testing without serial.flush, and anything less than 100ms cut-off (power down) prevents full message transmission. The message being about 200ms long it is safe to assume serial.flush() takes care of the last 100ms.
My conclusion is that at 2400 baud a 100ms delay is needed to allow the HC-12 to do its job before power can be cut to the HC-12 when using serial.flush();.
Next "to-do" is Robin2's suggestion to test without HC12 but with straight hardwared serial connections. I have to figure out a way to that without interfering with the hardware setup that cuts power in the sleep mode by opening the ground wire (instead of cutting VCC).
