Using a quad half-H chip

From HBRC WIki

A simple way for beginners to drive a pair of motors is to use a quadruple half-H driver chip, such as the SN754410. However, the data sheet for this chip can be confusing for beginners, and really only makes sense once you already understand what it's trying to say.

The diagram at right shows the pinout and logic table for the 754410 chip. There are four "half-H" circuits, identified as 1, 2, 3, and 4. Each one has an output ("Y") which can be held at high or low voltage, according to the function table shown.

Each motor will use two half-H circuits together to form a full H bridge. Typical usage would be to use half-H circuits 1 and 2 (on the "left" side of the chip) for one motor, and circuits 3 and 4 (on the other side of the chip) for the other motor. Note that circuits 1 and 2 together are enabled by pin 1, and circuits 3 and 4 are enabled together by pin 9. For the sake of clarity, let's first consider how one would connect and control just one motor, using circuits 1 and 2.

The motor should be connected across the two "Y" outputs — that is, the two leads from the motor would connect to pins 3 (1Y) and 6 (2Y) on the driver chip. Now, when these two outputs are both high or both low, there is no voltage difference between them, and so no voltage across the motor — the motor is unpowered. When output 1Y is high and 2Y is low, there is a voltage (equal to the supply voltage, Vcc2 on pin 8) across the motor, and the motor runs. When 1Y is low and 2Y is high, the same voltage difference appears but with the opposite sign, and the motor runs the other way.

So, by controlling the signals to pin 1 (to enable the outputs), and pins 2 (1A) and 7 (2A), you can set the voltage across the motor (between 1Y and 2Y) to -Vcc2, 0, or +Vcc2.

The table below summarizes what is going on:

Enable	Input 1	Input 2	Motor Behvior
L	X	X	Free wheel
H	L	L	Braking
H	H	L	Forward
H	L	H	Backward
H	H	H	Braking

The table below show where to pulse (P) a line to get a pulse width modulated H-bridge signal. There are three ways each to drive the motor forward or backward; one of these ways allows the wheel to rotate freely in between pulses, while the other two actively brake the motor between pulses.

Enable	Input 1	Input 2	Motor Behvior
P	H	L	Forward (free wheel)
P	L	H	Backward (free wheel)
H	P	L	Forward (braking)
H	P	H	Backward (braking)
H	H	P	Forward (braking)
H	L	P	Backward (braking)