motor shield questions

I'm working on my first arduino project, and I need to control a couple of motors. I'm not using the motor shield for various and sundry reasons, but as it turns out, the circuit I'm making is quite similar. I found a circuit diagram for the shield, and a few things are puzzling me. Forgive me if these are obvious questions, I'm coming to this from the software side, and don't have much experience with the building circuits (this is the first time I've built one with anything more complicated than a 555 timer).

The most confusing thing is that the direction signal passes through the 74HC00N (quad NAND gate) twice to reach 2A and 3A on the SN754410NE (quad half H-Bridge), when it seems that the removing the first pass would work just as well (the directionality would just be reversed). Is there some reason that the signal should go through the 74HC00N at least once before reaching the SN754410NE? Does the NAND gate serve to clean the signal or something?

I'm also unclear as to the purpose of the 100Kohm resistors connecting each of the arduino's outputs to ground.

It wouldn't let me link to the schematic in my first post, here it is:

The inverters are there to save microprocessor pins: if you're driving a DC motor or a bipolar stepper, you always want the 1A and 2A pins to be the opposite of each other. Since the extra gates need to be either used, or have their inputs tied to some constant signal, you might as well use them as buffers.

The pull-down resistors are there to make sure that the L293 doesn't turn itself on when the microprocessor isn't actively driving it: when micros are reset, they usually set their I/O pins to be inputs by default. Without the resistors, the L293 could pick up electrical noise, and start turning motors off and on at random.

With the resistors, the motor drivers are forced off until the CPU initializes itself, sets the controlling pins to be outputs, and (with good luck and/or good management ;) ) sets them to safe values.


Oh, so you can't leave any of the NAND gates NC? Good to know, thanks.