"So for 1, 2, and 3, writing a 1 to the latch causes not Q to be low, turning off FET and presenting a high to external source? So external souce must be able to sink IIL referenced current to ground to pull port low and so "0 = on state" because if port is 0 to external souce, port cannot be pulled high by external souce due to FET grounding? External source shouldn't source port high but rather should float pin (very high impedance) to avoid excessive current?"

you have to decide whether you are goin to use the port pin as an input pin or as out put, if u write "1" to the LATCH it becomes an input pin with internal pullups on(port 1,2,3) presenting "1" to the pin and you can pull down the pin by external source to read the pin as logic "0". if you dont do so the latch output (which is "1") will be read as logic 1(note that even you are not trying to input a "1" a one is read). when you write a "0" to port it pulls all the "pullups" down to zero. if you try to pull the pin up, it damages the device. refer to datasheet how much current pin can bear.

port0 is lil differant as it has open drain o/ps. when you write "1" it floats.


abhishek