Hello John A couple of M5Stack modules (like GPS) already use a serial connection via GPIO16 and GPIO17. However GPIOs only provide logical voltage levels (0V / +3.3V). Assuming your source is a standard RS232 device the voltage levels can go down to -15V and up to +15V so you probably need a level converter like this one: https://www.amazon.com/Serial-Converter-Equipment-Upgrades-Doupont/dp/B01N42QW8X/ref=sr_1_6?crid=2OZ5U3Q7GY5OS&dchild=1&keywords=ttl+rs232+converter&qid=1597992249&sprefix=ttl+rs2%2Caps%2C276&sr=8-6 Cheers Felix

Thanks for the info. Solves the issue. So looking at the schematic I would connect to pin 28 for 5V and pin 1,3 or 5 for GND. Cheers.

@jnicol actually , our screen supplier test data, the screen, allow max brightness 853nit. maybe you could modifying the program, the display effect in the sun will be improved