SEN0642 UV sensor can not get it working at 9600 baud

Hey William, The SEN0642 actually ships with its default baud rate set to 4800, not 9600 — that's almost certainly why nothing's responding at 9600. Drop your master side to 4800 baud, 8N1 (8 data bits, no parity, 1 stop bit), and you should start seeing Modbus responses right away. The default slave address is 0x01, by the way, so if your code was hardcoding something different that's another common gotcha.

If you want to change it to 9600 permanently so it matches the rest of your bus, you can send a Modbus write (function code 0x06) to the baud rate register — the exact register address and procedure are in the communication protocol doc on the product page.

Full docs and Arduino example are on the English wiki at https://wiki.dfrobot.com/sen0642/ if you want to double-check anything, and the product page is here: https://www.dfrobot.com/product-2912.html. Good luck, hope 4800 gets you talking!