I am building an application where I intend that two CM3 units will use the serial lines to communicate between each other. I thought it would be a good start to have two processes on the same CM3 talking to each other. I was able to get /dev/serial0 and /dev/serial1 both working individually, This was verified by connecting TX to RX of the same UART and using minicom to see the loopback echo.
But now when I try to connect serial0 TX to serial1 RX and vice versa, minicom shows garbage characters no matter what I type.
I also tried connecting the CM3 to the UART pins of a pi3, closer to my intended application, making sure to keep a common ground, and I have the exact same problem there.
So is there something I am missing here? What other setup is needed to have two UARTs talk to each other?