Reading from PLC | db_read ArgumentError about python-snap7

[wilkpio]

@grawee python-snap7 is just a ctypes wrapper around snap7 c library.
The python function db_read() just calls Cli_DBRead() function from c library.
According to header file (snap7.h) the "start" augment is declared as integer.
See: https://github.com/SCADACS/snap7/blob/master/examples/cpp/snap7.h#L436

The types must match. Therefore you have to use integer as a start parameter in db_read() and read whole byte(s) from PLC.
You should be able to read individual bits from returned bytearray with snap7.util.get_bool().

See example:

reading = plc.db_read(31, 120, 1)  # read 1 byte from db 31 staring from byte 120
snap7.util.get_bool(reading, 0, 5)  # read a value of fifth bit

[gijzelaerr]

Thanks @wilkpio for answering this question.

[grawee]

Thanls for the answer @wilkpio. It's working fine. I have one more question or problem..
So far, I used the db_write(db_number, start, data) command to write to the PLC. The problem is the same with this function as well, when I try to set float offset value for start. So, I tried to use the set_bool(_bytearray, byte_index, bool_index, value), but I don't know what I need to set to "_bytearray". Can you show me an example for this function.

Maybe good to know, when I want to write in PLC, I read from MySQL database, so I get a bool from SQL what I want to read in PLC.

[spreeker]

Dear @grawee

The numbers are not FLOATS they look like floats but they are definatly NOT floats.

The smallest thing you can read from a PLC is a BYTE with snap7. One byte is 8 bits. 8 boolean values fit in a byte. on position 0 till position 7. BYTEINDEX.BITINDEX. =! FLOAT.
00000000 = 1 BYTE

1 byte can be used to store 8 boolean values. Packing so many values in a byte is done to so save precious PLC space and memory.

So how to change a bool value?

1. Read out the _bytearray your boolean values are in.
2. In this byte_array you need to find the byte (byteindex)
3. Find the bit in this byte (bitindex) so you can
4. set the correct value with utility function set_bool which does all the hard work.
   set_bool(_bytearray = data which you read out before.., byte_index, bool_index, value),
5. write back the changed bytearray back to the PLC.

A round trip wil take 5ms.

The example code

https://github.com/gijzelaerr/python-snap7/blob/master/example/

reading = plc.db_read(31, 120, 1)  # read 1 byte from db 31 staring from byte 120
snap7.util.set_bool(reading, 0, 5)   # set a value of fifth bit
plc.db_write(reading, 31, 120, 1)    # write back the bytearray and now the boolean value is changed 
                                                       #  in the PLC.

hope this helps :)

[KeshariGupta-ML]

Is there any way to write integer/string in PLC db..... Any link or help would be appreciated....

[Mutexman]

Good day!
Please tell me how to use snap7.util.get_bool () in C ++. Why can't I figure it out myself! And is there any documentation and where can I see it?

[swamper123]

@Mutexman

1. This isue is closed.
2. Before you start opening a new issue, pls. use Discussions for questions (so we can seperate it)
3. This repo is a python layer around snap7 ...I don't know what you mean with "how to use this in C++"???

EDIT: Where do you got problems with

python-snap7/snap7/util.py

Line 114 in b6481ec

def get_bool(bytearray_: bytearray, byte_index: int, bool_index: int) -> bool:

?

[KeshariGupta-ML]

I have done through python.. I didn't use c++ directly

[Mutexman]

Is there a similar C ++ layer around snap7?

[Mutexman]

I understand correctly, this was used
https://python-snap7.readthedocs.io/en/1.0/_modules/snap7/util.html#get_bool
?
It turns out that you can rewrite this code in C ++ and there will be a wrapper in C ++.