protecting input pins of 74LS165 shift register

[danjel]

I have used schottky diodes tied to +5V and GND in order to protect the input pins of a microcontroller from over voltage and negative voltage.

However, I am not sure if I need to use the same methods if I am feeding a 74LS165 shift register? In the datasheet it does not show a max voltage rating but it does show a minium voltage (which I assume is voltage threshold for the logic).

Am I safe with just a current limiting input resistor or should I be adding more protection?

The max voltage that would be accidently connected in this system is +/-12V

thanks!

 

[danjel]

p.s. I am not hard locked to this IC. I could also use a 74HC165 CMOS IC.... basically any 8bit parallel to serial shift register ic that is widely abundant.

 

[ericgibbs]

p.s. I am not hard locked to this IC. I could also use a 74HC165 CMOS IC.... basically any 8bit parallel to serial shift register ic that is widely abundant.

hi,
Depending upon the logic driving source impedance, the internal clamp diodes of the PIC will give protection if the source is greater than say 1K.

If the source impedance is low and if the voltage exceeds the PIC +Vs, say 12V, you could add a 470R in series with the PIC input pin and a 5V6 zener from the pin to 0V.

Why do you think there is a risk of 12Volts.???