Hi, I have a problem with L293D. On Vcc2 is 5V, but on JP1, JP2 is only 3V.
I am not positive, but, it would seem to me that with HIGH being less that VCC2, and, LOW being more than 0V. Your motors will never see full VCC2. I would guess that you will only see the swing from VOH to VOL. So, your reading was only telling half the story.
And with SN754410 it will be the same? I know about the 0.7V on diode and 5V - 0.7V = 4.3V , but on out was only 3V, something what I did not expect :D .
Check the datasheet for the SN754410. I think you will find it is about the same. They are both transistor based h-bridge circuits.
The P-N junction of a diode or transistor is about 0.7V. Since the circuit drops through two transistors to run the motor, that's about 1.4V, in theory. The actual drop can be higher.
you will see your output will be between VCC2 - 1.4 and VCC2 - 1.8. Your 3V out is pretty close to the low side, and, therefore expected.
You will probably also loose 0.7V on the input diode.
Good catch, Geir. I didn't notice the input diode on VCC2.
That will drop about 0.7V, plus the drop of the h-bridge itself easily explains why he is only getting 3V to the motors.
Try a higher voltage for VCC2. Maybe a 7.2V NiMH battery.