Hello..Soumitra,
Interrogation Voltage may have different meanings to understand but in our region....
INTERROGATION VOLTAGE IS A TERM USED IN RELATION WITH DIGITAL INPUTS TO PLC. NORMALLY WHEN SOMETHING HAPPENS IN A FIELD,i.e ACTUATION OF A HARDWARE CONTACT TO GET CLOSED OR ACTUATION OF A CLOSE CONTACT TO BECOME OPEN. THESE CONTACT MAY BE LOCATED IN FIELD, MOTOR CONTROL CENTRES, LIMIT SWITCHES ETC...
NOW THE POWER SUPPLY UNIT INTERROGATES THE CONDITION OF THESE HARDWARE CONTACTS (CLOSE/OPEN) BY SENDING A INTERROGATION VOLTAGE & TRYING TO RECEIVE IT BACK.....SO PLC COMES TO KNOW WHAT HAPPENED IN THE FIELD BY MEANS OF THIS INTERROGATION VOLTAGE & I/O CARD.
I hope i am clear....
shailesh