Hei guys,

I really have a sticky situation this time...

I have 2 x 315-2DP that are connected via Profibus (one is the Master and the other is the Slave). The communication is already working and they are exchanging data.

In one of the PLCs I have made several FBs that read and write from a Global DB. Each FB can individually turn on/off Bools in several Arrays that lie in this Global DB. The idea is that one FB turns a bit on, another see it on and executes a task and, when finished, turns it off thus signaling to the first that it has finished. This works quite well and has saved me a lot of coding. 

The second PLC contains exactly the same type of operating method, i.e. several FBs write/read from a Global DB.

Here’s the tricky part: I would like to make a clone of the DB that is on the second PLC in the first. When I need to “execute” something on the second PLC I can do it simply by turning a Bool on the first. When it is done it turns it off.

This seems very easy, simply copying the DB from one PLC to the other but the problem is that if it is always reading/writing it does not work. When using the Profibus communication one writes to the Peripheral Area (PIW,PQW)  and on the other side we read from it. This is my problem because it is always overwriting the data.

I know that there are ways to avoid this… things like writing a protocol with a timestamp or a counter and transmitting the number of block of date, etc . But I would like to really keep it simple and not having to write something that needs continuous supervision of the supervision that supervises the supervision.

Anyone with a good, simple idea?

Hi,

Have a look at this:

Client / Server Communication with (I) Slaves via S7 Basic Communication (I_PUT/ I_GET)

Cheers

Hi

What You need is to write on the edge at the source.

Could this be like:

You copy the DB to peripherial area and

write from PIW to DB on raising edge of bit in question.

Sounds complicated but should work.

The DB in first PLC would be exact copy of DB from target (this can be used as the Acknowledge

and to control when action was executed and its bit cleared).

WRITING PLC                                                   EXECUTING PLC

GLOBAL_DB ---------------------------------------> PIW ----(P)---->GLOBAL_DB (Write on EDGE)

GLOBAL_DB_ACK <----------------------------------------------------GLOBAL_DB (write at all times)

let's say we set GLOBAL_DB.DBX10.0 to True.

on raising edge slave writes it in its copy of GLOBAL_DB

Master sees it in ACK DB.

When value in ACK DB changes back to 0 command is executed in slave PLC

Hello guys,

thank you for your answers.

Piotr.M I cannot thank you enough. Excelent solution!

It worked only in one direction. I could write to the "slave" and he replied by setting the bit off. 

Later I needed to to the same in the other direction so both PLCs are now reading only when there is a flank up or down and always writing to the periphery.

I did a small block in SCL. Here is the idea:

//INPUT
FOR cnt := 0 TO 15 BY 1 DO
    
    IF  INPUT1[cnt] AND NOT m_INPUT1[cnt] THEN
           "DB_INPUT".ENTRYPOINT[cnt_e] := TRUE;
    END_IF;

    IF  NOT INPUT1[cnt] AND m_INPUT1[cnt] THEN
           "DB_INPUT".ENTRYPOINT[cnt_e] := FALSE;
    END_IF;

  m_INPUT1 [cnt] := INPUT1[cnt];

END_FOR;

Kind Regards,