The old version of NCU will show "b" instead of "6" , i check you picture of NCU --It looks very good.

you system lose communication between NCU to HMI, the related factors :

#1--NCU should be good, after booting showing b or 6 and PR Led and 5V (new version (NCOK) LED on---now you are good 

#2--Connection cable -- or call Profibus /MPI cable is connected and in good condition, bus terminal switch should in correct position--(check 840D 611D startup guide for detail)

#3--HMI special for PCU50 (old MMC 103) communication setting --should be good, --(check 840D 611D startup guide for detail)

#1 is good, #2 you check by yourself

#3 you can go to Start-up--> MMC--> Operator Panel--> check or take screenshot, I may help you to check

did you check the cable ?

and please post picture of connection cable on the NCU , for your screenshoot setting (1.5Mb) you need connect to the top lift 

Valo1923

Thank you very much for your answer.

But I checked this parameters. Start-Up-> MMC->Operator panel. 

See attachment

I tried to set commuication speed to 187k, the result was same.

so I can't guess where to look next?

I checked the cables first.

 

 I attach picture -the 2 page from Siemens 840D start-up manual, you can search online to download save it in the future use, Now follow my instruction , if you think you checked everything good, I suggest you make new cable with new connectors. 

By The way you or someone know this machine (long time ago?) has worked good, communication from NCU to HMI was good?

Valo1923

I checked cables again.Its were good.

Here you are the photos of NCU.

Everything is connected according to the machine's electrical diagrams.

I don't understand what's wrong yet

Thanks for your attention.

 Here attached manual you need to keep 

and you need do what you can do or I instruct to you and If you get help can you please feedback

for our free support--If you like my comments please
use "Thank" bottom left or Rating Top right

Valo1923

Hello to all experts,

Maybe someone else can suggest something else?

Maybe there are some other ideas?

With best Regards.