Raspberry Pi 3 UART speed workaround « Adafruit Industries – Makers, hackers, artists, designers and engineers!
Setting Up UART Serial Communication between Raspberry Pis
Amazon.com: DB9M RS232 Serial Hat Port Expansion PCB Board for Raspberry Pi A+/ B+/ 2/3/ 4 /Zero/ZeroW, UART Port to Male DB9M RS232 Socket, Compact GPIO TX/RX/GND Solder Points : Electronics
Raspberry PI3 PI4 Solution to UART PermissionDenied - YouTube