R2S Plus USB-C Debug UART

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Trying to make use of the USB-C : onboard USB to Debug UART converter, 1500000bps bauds.

When I plug it into a MacBook Pro... nothing seems to be recognized. I'm looking in /dev/* (tty, cu, etc)

Running the standard latest friendlywrt build. Any tricks to this? something I need to enable?

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I found that the cable i was using had some issues.
Swapped it out and got a little further..?

to the point of just getting garbage output. on every UART one could think of to try.

screen /dev/cu.usbserial-1440 1500000
screen /dev/cu.usbserial-1440 115200

1500000, 115200, 57600, 38400, 19200 ,9600, 4800 ....

no luck. Tried passing cs8 and cstopb.

any guidance?