Prevent bit-bang data-in line from back powering device?
Add a series resistor such as 10K to the input signal line only, which will limit the current without slowing down the signal too much. 

Unrelated, but you may wish to add a 100K pullup to establish a known logic level when the FTDI cable is unplugged. 







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There are certain logic chips that do not have the same kind of internal protection network and can be connected directly to an input voltage even when unpowered, but it's not necessary here. 

Another approach is to simply insert a BJT with a 10K base resistor, which inverts the signal. If you put an 1N4148 from base to emitter, it can withstand huge transients or ESD on the input without frying anything. 







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