The built-in library, SERIAL_LCD_SPARKFUN, offers support for the serial backpack to a character LCD. My problem is the 4x20 module I want to use is only available in 5V. Wondering if the Photon’s TX pin is capable of direct interface to the RX pin of the serial backpack (PIC16F88) powered at 5V? Best I can determine the PIC has a weak pull-up on this pin, but will the TX output tolerate connection to 5V and is it sufficient? Would a smaller value resistor to 5V on the RX input be a better option?
The RX/TX pins on the Photon/Electron are 5V tolerant, so there is no danger in trying it out.
The thing we can’t be sure without your actual hardware is if the display does take the 3.3V TX HIGH signal as high - but I’d assume it will.
Otherwise you might need a level shifter to boost the TX signals.
I’d feel better tho’ if there were no pull-up on the display, but no big risk there either.