DB_write Issue in Snap7

[INDRANI125]

While writing String from my Python application to TIA Portal, I am not getting the exact string in my TIA portal as well as in Python program as attached. Where as it is working fine in case of read operation. Can anyone help me out?It is getting random values.

Python Code-
import snap7
IP='192.168.0.1'
RACK=0
SLOT=1
DB_NUMBER=100
START_ADDRESS=1
SIZE=259

plc=snap7.client.Client()
plc.connect(IP,RACK,SLOT)
plc_info=plc.get_cpu_info()
print(f'Module Type: {plc_info.ModuleTypeName}')
print(f'State: {plc.get_cpu_state()}')
db=plc.db_read(DB_NUMBER,START_ADDRESS,SIZE)
plc.db_write(DB_NUMBER, START_ADDRESS, b'Python to TIA')
product_name=db[0:255].decode('UTF-8').strip('\x00')
print(f'Product Name: {product_name}')
product_value=int.from_bytes(db[255:257], byteorder='big')
print(f'Product Value: {product_value}')
product_status=bool(db[257])
print(f'Product Status: {product_status}')
Python Output-
Module Type: b'CPU 1510SP-1 PN'
State: S7CpuStatusRun
Product Name: �TIA Portal to Python // problem in writing this output
Product Value: 14
Product Status: True

[nikteliy]

The first two bytes in the 'String' data type contain additional information about the size of the string. You should use get_string() and set_string() functions to work with 'String' data type. I modified your code a bit:

import snap7
from snap7.util import set_string, get_string

IP = '192.168.0.1'
RACK = 0
SLOT = 1
DB_NUMBER = 100
START_ADDRESS = 0
SIZE = 259

plc = snap7.client.Client()
plc.connect(IP, RACK, SLOT)

plc_info = plc.get_cpu_info()
print(f'Module Type: {plc_info.ModuleTypeName}')
print(f'State: {plc.get_cpu_state()}')

string_to_db = 'Python to TIA'
# create a buffer and set buffer size equal to your string length plus 2 bytes for additional info 
buffer = bytearray(len(string_to_db) + 2)
# write string value to the buffer
set_string(buffer, 0, string_to_db, 254)
# now you are ready to write data to the plc
plc.db_write(DB_NUMBER, START_ADDRESS, buffer)

db = plc.db_read(DB_NUMBER,START_ADDRESS, SIZE)

# get the string value from the String data type
product_name = get_string(db[0:255], 0, 254)

print(f'Product Name: {product_name}')
product_value=int.from_bytes(db[255:257], byteorder='big')
print(f'Product Value: {product_value}')
product_status=bool(db[257])
print(f'Product Status: {product_status}')

[nikteliy]

All data types can be written to the PLC in the same way:

1. Convert a python data type to a PLC data type.
2. Write it to a block.

Helper functions are located in the util.py.
Examples can be found in the example directory.
Tests can also be useful as examples.

[INDRANI125]

Hi.Thank you but for these example python program.Do we have tia projects too?If yes, can you share that link?

[swamper123]

Hey @INDRANI125
better now than never to respond. ^^
It isn't a TIA Portal thing or that we need an example for that.

On TIA/S7 side, you need to declare how big your String is. Otherwise it is 255 chars long as standard, which results in those empty $0 symbols (because nothing/garbage is in there).

In TIA/S7, if you declare product_name to String[20] it would limit that your name is 20 chars long (and would reduce the zeros at least).

What I haven't tried myself atm. is to set a String and just say it has to be smaller than 255 and is the same length than my actual string.

[swamper123]

@INDRANI125 I made a fix #368 if this is still relevant. It was an error on our side.

[SkilledZero]

The first two bytes in the 'String' data type contain additional information about the size of the string. You should use get_string() and set_string() functions to work with 'String' data type. I modified your code a bit:

import snap7
from snap7.util import set_string, get_string

IP = '192.168.0.1'
RACK = 0
SLOT = 1
DB_NUMBER = 100
START_ADDRESS = 0
SIZE = 259

plc = snap7.client.Client()
plc.connect(IP, RACK, SLOT)

plc_info = plc.get_cpu_info()
print(f'Module Type: {plc_info.ModuleTypeName}')
print(f'State: {plc.get_cpu_state()}')

string_to_db = 'Python to TIA'
# create a buffer and set buffer size equal to your string length plus 2 bytes for additional info 
buffer = bytearray(len(string_to_db) + 2)
# write string value to the buffer
set_string(buffer, 0, string_to_db, 254)
# now you are ready to write data to the plc
plc.db_write(DB_NUMBER, START_ADDRESS, buffer)

db = plc.db_read(DB_NUMBER,START_ADDRESS, SIZE)

# get the string value from the String data type
product_name = get_string(db[0:255], 0, 254)

print(f'Product Name: {product_name}')
product_value=int.from_bytes(db[255:257], byteorder='big')
print(f'Product Value: {product_value}')
product_status=bool(db[257])
print(f'Product Status: {product_status}')

---- im new to github. is this code applicable to s7300? what will be the code if adding more product name like product name 1 product name 2 and product name 3?

[djosef39]

Please help me, I'm new to this topic:

When I execute the following instructions:

plc_info = plc.get_cpu_info()
print(f'Module Type: {plc_info.ModuleTypeName}')
print(f'State: {plc.get_cpu_state()}')

I get an error:

b'CPU : Item not available'
Traceback (most recent call last):
File "C:\Users\ymehda\PycharmProjects\pythonProject1\main.py", line 35, in
plc_info = plc.get_cpu_info()
File "C:\Users\ymehda\AppData\Local\Programs\Python\Python39\lib\site-packages\snap7\client.py", line 161, in get_cpu_info
check_error(result, context="client")
File "C:\Users\ymehda\AppData\Local\Programs\Python\Python39\lib\site-packages\snap7\common.py", line 92, in check_error
raise Snap7Exception(error)
snap7.exceptions.Snap7Exception: b'CPU : Item not available'

Does anyone have any idea about this problem?

[djosef39]

Hello, thanks for your reply,
Yes, PUT/GET in the tia portal for the plc is enable. There are some functions that work, and some of the functions do not work and give an error, as shown in the attached



pictures

[lautarodapin]

Some funciona dont work on PLCs 1200/1500, like the get_db
In the snap7 doc doesn't mention that this function won't work for those models.
Do you have another plc to test it? I don't have plcs at the reach of my hand anymore.
Optimized block access is enabled also right?

[swamper123]

From the original snap7 doc:

[djosef39]

thank you very much for your reply, Unfortunately, I don't have another PLC to test, Please send me the original snap7 doc, if you can. djosef39@mail.ru. thanks in advance.

[swamper123]

@djosef39 http://snap7.sourceforge.net/

[denissanga]

Hi all, I have the same problem. In particular I would use the client.full_upload("DB", 100) function with SIEMENS 1200 but the function seems not available

[swamper123]

Hey ho,

It seems, that this is not available for 1200 and 1500.

[denissanga]

Hi all, just one more questions. It is possible connect to plc remotely to plc with snap7 ? many thanks

[swamper123]

If you can establish a connection to the IPv4 network (via VPN as example), sure.