Using server class for emulating PLC, data not available

[colingoodman]

I am attempting to stand up a fake PLC server for development purposes. I am having some trouble likely due to my lack of understanding with PLC devices generally.

I can stand up a server locally and connect to it fine, but attempting to populate it with some data fails.

server = snap7.server.Server()
server.start(tcpport=102)

db_data = (c_uint8 * 100)()
server.register_area(0, 1, db_data)

client = snap7.client.Client()
client.connect('127.0.0.1',0,1)
assert client.get_connected()
client.read_area(snap7.types.Areas.DB, 0, 100, 2)

It appears that my client has connected fine, though I get RuntimeError: b'CPU : Item not available' when trying to read the data I set via register_area(). I do not wish to use the default data provided just by running python -m snap7.server, I would like to specify my own.

Thanks in advance.

[colingoodman]

I've realized that the server class has a mainloop() function that appears to do almost what I want, I just need to change the values specified in _init_standard_values() to efficiently populate with whatever data I want.

[colingoodman]

I used the following to register some data at DB block 100 on the fake PLC server:

db_data = ctypes.c_int8 * len(bytearray(1000)
db_data = db_data.from_buffer(some_bytearray)

server.register_area(
    area_code=srvAreaDB,
    index=100,
    userdata=db_data
)

However, after a successful connection via my client, the following command
client.read_area(snap7.types.Areas.DB, 100, 0, 1)
yields
RuntimeError: b'CPU : Item not available'

Does anyone know why the first line fails to properly register anything?

[nikteliy]

Hi @colingoodman
This code works for me:

import snap7
import ctypes, time

db_data = ctypes.c_int8 * len(bytearray(1000))
db_data = db_data.from_buffer(bytearray(b"\xff" * 1000))
server = snap7.server.Server()
server.register_area(area_code=snap7.types.srvAreaDB, index=100, userdata=db_data)
server.start(tcpport=1102)
while True:
    while True:
        event = server.pick_event()
        if event:
            print(server.event_text(event))
        else:
            break
    time.sleep(1)

Here is the result of running this code in console:

I hope this helps you find the error

[colingoodman]

Thanks for your reply, your verifying that it should work helped me out. I actually found that the issue was quite silly. I was using an environment variable to set the index parameter and forgot to convert the value to an integer. It may be worth noting that feeding index a string appears to silently cause issues.