I have KTP700 basic pn HMI connected to S7 1214 PLC. Button on hmi is set as setbitwhilekeypressed function. It works fine till the communication between plc and hmi is ok.

           As I disconnect the communication cable between hmi and plc while hmi button is pressed, the bit in plc does not gets off, it remains in ON state.

          What should be done to make the bit off in PLC if the communication is lost ?

You can query the panel's life bit.

SIMATIC WinCC WinCC Advanced V14 SP1 - Communication

If the life bit stays high or low for more than 2 seconds, the connection to the panel is broken.

Yes, by using the area pointer "Coordination".  In this word is the bit "Life bit". The bit changes his state every second.

Is this bit more then 2 seconds high or low then is the communication between HMI and PLC broken.