Re: Raspberry Pi as a storage device for a PET.
Yes, I suppose if you think about it, there will be a brief moment, as the logic voltage transitions from 0V to 3V3 or vice-versa, when it is both low enough to turn the PNP transistor on and high enough to turn the NPN transistor on, so that resistor may be there to prevent 'shoot-through' during the transition period.
As to why the complementary pair instead of a single transistor and pullup resistor, maybe the aim was to have the output both turn on and turn off faster by having active drive high as well as active drive low.
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