Guys 

Please help me as I have a WinCC flexible installed in Siemens HMI, It shows ### for all readings

I ping the HMI, it is ok

I checked the IP address is ok

what next I should check pleeeeease- furstrated 

SaedAlhabbashi

Thank you very much for your reply. I was able to sort the problem partially as I had the CPU_A now run, while the CPU B is unreachable - I could not establish a connection. I suspect the Ram is defective - I am happy as the project can be run with one CPU

More problematic is the WinCC is showing ##### for all parameters 

 If Wincc shows crosses it means communication problem between CPU and HMI. is that PLC in stop mode??? 

which communication protocol used between those  PLC? 

it is ok that you changed with same type AI card, but are you sure you adjusted  jumper on side correctly? ( I mean analog input signal type configuration plugs) 

Thank you very much for your reply.

Updates; 

I have both CPUs are in run mode, the one in duty is A now. I changed over between them no problem

Details:

Yes, they are both connected to HMI ( Siemens 6AV6 644-0AC01-2AX1) via ethernet switch )

the problem now sometimes shows reading, sometimes show #### 

I suspect the ethernet switch but when I connect my laptop in the same switch and ping the HMI, it was ok. 

I found the cause;

it is when I plug the PC adapter in the one of the CPUs. the HMI readings show ###

So it is when I am programing! 

It is CPU315 2DP

I have checked it, it is as follows

PG comm. 1

OP comm. 1

S7 basic Comm. 0

max. no. of connection resources is 16

please note that I checked them offline , on the saved back up file