I recommend updating the firmware to the latest version available for your PLC.  If it already has the latest version, I suggest installing one version older, power cycling and then reinstall the latest firmware again.

The idealist in me says this is a complete waste of time.  The practical realist in me says try it simply because I have done it in the past and problems magically went away.

Anytime I see orange LEDs on an s7-1200, I remember a conversation with Siemens Support where they told me to update the firmware - essentially writing the flash memory again which seemed to strengthen the integrity of the bits in the flash.

Oddly enough, it has worked every time except the one time the s7-1200 was dead forever.  Lightning strikes can do that when the building ground rod is corroded, not to the proper depth, buried in a gravel field, and made of the wrong material - all were the case for that building (long since corrected).

engco

I reset factory and the red led stopped flashing and i am able now to download the program

Search the support site for firmware downloads and the s7-1500 system manual where it describes how to update the firmware.

I recommend enabling the built-in web server and using that to perform the firmware update.  It is easier. 

engco

How to update the firmware?besides, I tried to download the program more than one time and failed to find the plc. IP address somteimes. I this also due to the old firmware?