Hi.

One step back please..... why Modbus RTU?

Is there any reason why you are not building an Ethernet network and have the 3 devices exchange devices with each other with S7 type link?

w

HINT:
* Instructions GET and PUT for PLC to PLC linking via Profinet.
* HMI can link to both PLCs at the same time.

Hi.

Thank you, I understand your situation better now.

It seems as though you are faced with a synchronization problem. There is an article about something like that (I did not work through it yet)....
https://support.industry.siemens.com/cs/ww/en/view/40556214

I would have 5 sets of data if I designed this..
(a) Link to HMI
(b) Buffer for (a) to identify change
(c) Link from Master
(d) Buffer to (c) to identify change
(e) Link to Master

Coding is then needed to evaluate changes made and decide how to make all equal again.

If you take it a step further, (c) and (d) only carry values that needs to be synchronised. Not handle a block of data, only that what needs changing. Very much the same idea as Modbus already does.

So what specifically can the Forum help you with?
w

sms_mstr

Modbus works at 1200m. I'm right?

As an after thought... do be careful about that promise of the distance.
It very much depends on the
    type cabling, routing environment and (slow) communication speed.

The throughput may be bad...
so be very careful on choosing the amount of data flowing.
ALSO.. be sure you do not need real time / time-critical info to swap.

w

sms_mstr

thank you very much for your answers and for the link to article about data synchronization. I will read that, maybe it helps me.

I thought that maybe I just do not know of a simple ordinary resolution of my situation, because

I have not worked before with modbus for s7-1200

perhaps I should make 2nd PLC (which has a connection to the HMI) in master mode ? But then 1st PLC cant control equipment if there is no connection between 1st and 2nd PLC.

could you explain in a little bit more detail your way to solve? That a,b,c,d,e should be in Slave PLC (PLC near HMI)? 

Good to hear from you.

The synchronisation problem is unrelated to what communication you will be using. You need to work out how the information is going to flow and which device has priority... your application will dictate that.

I have no thoughts beyond what I have written to you.
The problem is too general to make suggestions.
For me to go in deeper will mean doing a project that you are paid for.
Siemens does not pay me for sharing my thoughts here.

Spend some time and effort
to work out how the data flow should look like.
Then return with specific question like...
"how do I use this instruction to do this and that."

Have a look at the link too.

Good luck.
w