#25 write data on PLC using snap7

[Sushil Dhote]

How can i get i responce from db_write() or write_area() in snap7 ....that daata is written on PLC...please help ...thanks

[Sushil Dhote]

@davenardella can you please help

[Davide Nardella]

what wrapper (Hi level language) are you using ?

Every snap7 function returns an integer, if it is zero, it means that the PLC, in accord to the protocol, sent a message saying that the operation was accomplished. This is valid also for functions that don't return values (like dbwrite and writearea).

So simply test the result.

[Sushil Dhote]

@davenardella i am using python language and using python-snap7 wrapper ...

[Sushil Dhote]

https://github.com/gijzelaerr/python-snap7 this is github link that i am using

in my code i tried
temp = plc.db_write(db_number=db, start=start, data=result)
print("write...success:",temp)

        but it is getting "None" as response

[Davide Nardella]

From the question you asked me, I imagined this answer.
You should speak with the python-snap7 programmer.

I don't know python at all, because I don't like it at all.
Sounds very strange to me that a function does not returns a value and that an exception must be raised also if a return value is not zero.

Snap7 as dll (or .so) does not raise exceptions, it returns an integer that must be checked.

[Sushil Dhote]

yes , i am trying to connect with him ,i thought u may have some idea..OK thanks for rply

[Davide Nardella]

As far I can see in write_area (but I'm not an expert):

return self.library.Cli_WriteArea(self.pointer, area, dbnumber, start,
size, wordlen, byref(cdata))

you should see the value returned by the library function

[Sushil Dhote]

yes right , but i am getting "None" in temp varible as i showned above my above code.
in both the scenirio ....when it success or fail

[Davide Nardella]

Since you don't have a result, what do you mean for "fail" ?
Do you don't find the values into the DB ?

1 - Try to write the same values using a non-existent DB as db_number param, what happens ?
2 - Are you completely sure that in your user program you don't overwrite that values ?

[Sushil Dhote]

1."fail" means ....when i try to write on PLC 2 ,it is not getting write on that particular Dbaddress
2. when i try to write on PLC 1 with same code it it success ,i can see values on that Dbaddress

3.yes ,after you ask to write same values on non-existent Db number ,it get's Exception that "Address out of range"
4.yes tried lots of different values and diffrent datatype values also,,,,,writing string,int,Dint,....getting "None" for all.....pretty sure not overwrite

[Davide Nardella]

Cannot happen that a PLC answers "Ok I performed" (and so Result==0) but it didn't write the values.

Into PLC 2 create a new DB similar to previous one , and write into that, or write consecutively into the old DB and into the new one.

I bet that you will find the data into the new DB.
Let me know ;)

[Sushil Dhote]

1.yes, i checked ,Cli_DBWrite () return 0 if it get successufullly write vvalues on PLC.
2.Cli_DBWrite() return the error if it not able to write.
3.the problem i found is when i code below like this

temp = plc.db_write(db_number=db, start=start, data=result)
print(temp)

"temp" varible not able to get that 0 value,it act as None

when i tried
if (plc.db_write(db_number=db, start=start, data=result));
pass
else:
print("success")

it is working properly.

now the problem is when it not able to write it return error but i also need 1

[Davide Nardella]

Sorry I cannot help you on Phyton side...

[Benjamin Wischer]

Its because the function is actually wrapped.
the function error_wrap returns the executed function without any return Value.
The function check_error raises an exception in case of any errors.

#this is what the call actually looks like:
plc.error_wrap(db_write(db_number=db, start=start, data=result))


  @error_wrap
   def db_write(self, db_number, start, data):
    dosomething()

    def error_wrap(func):
    """Parses a s7 error code returned the decorated function."""

        def f(*args, **kw):
            code = func(*args, **kw)
            check_error(code, context="client") #this function raises an exception

        return f

Possible Solution:

try:
  plc.db_write(db_number=db, start=start, data=result)
  # continue with programm everything is fine

 except Snap7Exception as s7ex:
 # Fehlerfall
     print(s7ex.message)