[Bill Porter] has a tip for designing circuits that have multiple connections to a single microcontroller UART. This stemmed from a review of a friend’s circuit design that used the UART in the project, but also called for an FTDI chip in order to reprogram via USB and a bootloader. Unlike the schematic above, the circuit called for straight connections without any resistors. With that design, a conflict will occur if two devices are connected and attempting to communicate at the same time.
The fix is easy. [Bill] discusses how to prioritize the connection by adding the pair of current limiting resistors seen above. This helps to ensure that damage will not occur, and that the FTDI chip will take precedence. Now the external hardware will not preclude the FTDI chip from accessing and programming via the bootloader. The tutorial is intended for those rolling their own boards out of an Arduino-based prototype, but it will work in any situation where you need multiple connections to a single set of UART pins.