Each input pin has two (Protector) diodes, one connected to VCC pin and another connected to GND pin.
The VCC to signal pin diode is Reversed biased.
Similarly the signal pin to GND pin is also Reversed biased.
Open and Short Circuit Tests in IC is done by Forcing Current and measuring voltage across the required diode.
To check first diode keep current source across First diode and change the polarity such that the diode becomes FWD biased.
When Current is applied all other signal pins, VCC and GND pins are grounded.
