My L293B was functioning a year the right way. Newly, when I connected it to pwm 9pin of Arduino and digi-pins 7 and 8, it did not respond to flipping HIGH / LOW (didn't change direction). It all the time turns in one direction as if it can't see the signal is switched.
To test it, I first checked the pins: connected pin 7 to an LED, then setting HIGH/LOW, switched the LED on/off correctly. Next, I took GND and +5v of Arduino to a breadboard, then sent Enabling signal from Arduino pwm pin 9 to L293, but for commanding, plugged the Input A, B of chip directly to GND and 5v of Arduino on breadboard. Result: chip changed direction as expected!
I also measured the output of chip and I read that when I use GND and 5v of Arduino, I get +11.5 and -11.5 when flipping wires, from my 12v battery, so all ok. When I command from digi-pins 7 and 8 of Arduino, the measurement says +11.5 but never changes to -11.5!
I changed the chip with L293E: this behavior didn't repeat and the system worked ok.
So, is it enough to conclude that the Chip must be changed?