As I think you were implying, the varying resistor values in the transistor circuit really shouldn't matter much because those are there just to limit base current. The transistor should be all the way on or off, and in particular, we need them to turn all the way off when trying to pass SDCARD data. It is unclear to me why the cross-coupling between DTR and RTS is required rather than just operating EN and IO0 independently (with the transistor emitters going to ground). I can see the difference in the logic table where DTR and RTS both at 1 leaves EN and IO0 both at 1 rather than pulling them to 0, but is there a reason why we need to let both DTR and RTS go to 1? Or is that for the "open" condition when the USB serial is not connected? -- Steve