Here’s an update on the mysterious RS232 marking on the 5" Intelligent display.
Before doing anything, I measured the open voltage on the RX and TX pins from this display. RX was 3.6V and TX was 4.9V. This surprised me! I’ve always used Enhanced displays in my designs and the Nextion data sheets for these indicate they are compatible with 3.3V logic. Apparently they changed the Intelligent displays to run 5V TTL levels, which means (at least to me) that they are not safe to interface directly to 3.3V CPUs since many are not 5V tolerant on their inputs. My remote display connects to a Teensy 4.1 development board and I use MAX563 dual RS232 IC’s on both ends so I can extend the display away from the CPU. The MAX563 is a 3.3V part and the specs say max input levels are -.3V to VCC-.3V so 5V is not acceptable. It must tolerate the higher voltage because it’s still working but I’m not taking any chances
Next, I removed R50, the 0 ohm resistor shown in the photo and again checked the RX and TX pins on the display. RX was still at 3.6V and TX dropped to 0V. TX should have gone below -3.5V if RS232 was supported. Question answered. I re-installed R50.
In the past I always used 5V to 3.3V level converters to my designs using Nextion displays with 3.3V CPUs. I removed them once I reviewed the Enhanced display specs. Looks like I’ll be adding them back in if I want to use the Intelligent displays.