Dear Forum

I am currently working on some industrial control equipment. I would like to control my inputs with 24 V DC signals. As we all know, a '51 input pin can't stand more than Vcc - typically 5V. As we also know, using a '51 pin as input, acually means grounding it. So what about isolating the micro with a diode: anode to i/o pin, a pull up resistor to cathode. Grounding this pin will force current to flow through the diode, and take the i/o pin low. Is this a safe way to do it? Should I go for optocouplers?
Any comments apreciated.

Thax

Per Guldmann