It usually means one of several things:

0. Use the online access from portal's project tree to perform a factory reset.

1. TIA Portal version mismatch. You can use "Online Access" with V15 to see the portal version used to load that PLC and then install and connect with that version of portal.

2. IP address / subnet problem with the chosen network adapter requires admin rights to change and for whatever reason, portal does not have those rights.  Launching tia portal "run as administrator" can get around this.  Repair of the portal install can correct the problem if portal was installed correctly in the first place.

3. Some recent Microsoft update has caused some connectivity issue, some permission issue, or updated a common library (DLL for example) that portal uses and suddenly a bug in the update is preventing portal from getting connected.  Uninstalling updates installed since the last successful connection to this plc can correct this problem.

4. Bad network cable.

5. PLC network jack pins are bent.  Trying the 2nd port can sometimes fix this.  Shining a bright light and looking at the pins for damage can tell you if damage has occurred. Toothpicks (plastic or wood - no metal ones) can correct the bent pins if you are patient and go slow and easy.

--- and quite a few more issues.

You can reload the firmware using a compatible simatic memory card.

nikivuk

HI, I have a problem with 1516-3 PN/DP PLC, since TIA portal V15 detects the PLC as an accessible device and its address, but will not connect with it. I simply get notification connection with PLC is terminated. 

Does anybody have an idea how to solve this? 

On the laptop that works the way you want, check your settings (options -> settings) for what it does when it gets connected.  Like, does it ask you every time if it can create a temporary IP, does it ask which adapter to use every time, does it ask you what connection method to use?

The simplest solution is to export the settings from the laptop that works well (for both v12 and v16), and then import them on the laptop having problems.

Then restart portal in the order you were having problems.

Should go away if it was a settings related problem.

ed finley

Same Here! Portal used to work until I installed V16 on my M4 PG (Windows 7 64 bit)

I can 'see' the PLCs attached when doing an available devices search, but when I try to connect to download, upload or monitor, it tells me target not reachable. Happens in every version from V12 to V16 SP4.

If I use different laptop, same OS, same cables, it works fine.

Tried everything I could think of over 2 days, including many suggestions from Siemens Automation Support.