Review on 🍓 Raspberry Pi Serial Expansion HAT with SC16IS752, 2-ch UART, 8 GPIOs, and I2C Interface, Stackable up to 16 Modules for 32-ch UART by Daniel Greene

I used this in the Nano project mainly as a test. The Waveshare Arduino library on github (MultiSerial-master.zip) has a feedback sketch that required a change to compile and run, limiting the search for the 4 addresses the code supports to only the first 3. Change 3 to 4 and the compilation errors go away and the test finds and verifies the board. The reverse loop worked well until 38km where I stopped. I saw an ad somewhere that the upper limit on standard divisors was about 76k, I don't need more than 38 anyway. I would prefer the DFRobot dual UART form factor, which is also cheaper, but that doesn't have an English translation of the manual, to enable the interrupt output, and their sample program contains no references to enable it. Also, it's made for the Pi, so the form factor is perfect for that purpose. This board supports interrupts, although they are connected to the 40-pin header on pin 15, which appears to be an open collector often found on interrupt outputs. I particularly enjoyed seeing the transmit and receive data flashing during the loopback test. It runs on 3.3V not 5V. It doesn't provide level shifting for +-12V RS232 so you might need converters.