Polling multiple data blocks at a time with Snap7

[bouba]

Hello everybody,

I use snap7 to poll 4 data blocks of my Siemens S7 1200 PLC. It's works(with help of David) at the beginning. I declare 4 S7 Client and I store buffer into 4 arrays of bytes. After I parse buffer and store each variable. I use Thread to do this job with 20ms of delay.
However, one of S7 Client disconnects after few minutes and I don't have true values.

I want to know why one of client disconnects after approximately 30 minutes.

Thanks
Best regards
B.M

[Davide Nardella]

With S7 protocol you are using get/put :

https://support.automation.siemens.com/WW/llisapi.dll?func=cslib.csinfo&lang=en&objid=38051505&caller=view

[bouba]

OK.How can I use snap7 for pooling 4 DB at same moment? Are there function or methods for do this?
Thanks

[Davide Nardella]

How much is the total size that you need to transfer ?

[bouba]

for DB1 ---> 4 bytes
DB2 ---> 4 bytes
DB3 ----> 140 bytes
DB4 -----> 40 bytes

At total 188 bytes

[Davide Nardella]

In this case you can use Cli_ReadMultiVars() that can transfer up to 20 vars in the same telegram.
The drawback (it's a S7 protocol limit) is that the whole size must fit in a single PDU including the header.

[bouba]

OK
What do you mean about 20 vars (how many bytes?).What is the max PDU size for Siemens S71200 CPU 1214 PLC.Therefore i must use ReadMultiVars if i want to poll 4 DB simultaneously?

THX

[Davide Nardella]

A var is a Tag, i.e. a block of byte/word/dword, the same that you transfer via Cli_ReadArea() function.

The maximum payload is:

ReadMultiVars:
MaxPayloadSize=PDULength-(14+4*NumItems)

WriteMultiVars:
MaxPayloadSize=PDULength-(12+16*NumItems)

Where PDULength is the size negotiated during the connection, you can obtain it via Cli_GetPduLength() (ref man pag.163) and NumItems is the number of blocks read/wrote.

You can read up to 20 different DB at the same time if the total size is less or equal to the above value.

[bouba]

hello

I use ReadMultiVars function like this in C#:

            byte[] DataDB305= new byte[4];
            byte[] DataDB306=new byte[4];
            byte[] DataDB300=new byte[40];
            byte[] DataDB405=new byte[140];

            S7Client.S7DataItem[] items = new S7Client.S7DataItem[3];
            items[0].Area = S7Client.S7AreaDB;
            items[0].WordLen = S7Client.S7WLByte;
            items[0].DBNumber = 305;
            items[0].Start = 0;
            items[0].Amount = 4;
            items[0].pData = &DataDB305 ;      //error

I have an error with items[0].pData, I pass address but I have an error

THX

[bouba]

I use GCHandle for passing byte array address but I have another problem with myclient.ReadMultiVars(items, 4).It only take reference as argument.

            items[0].Area = S7Client.S7AreaDB;
            items[0].WordLen = S7Client.S7WLByte;
            items[0].DBNumber = 305;
            items[0].Start = 0;
            items[0].Amount = 4;
            GCHandle myGcHandle1 = GCHandle.Alloc(DataDB305, GCHandleType.Pinned);
            IntPtr DataDB305Ptr = myGcHandle1.AddrOfPinnedObject(); 
            items[0].pData = DataDB305Ptr;
            myGcHandle1.Free();
            myclient.ReadMultiVars(items, 4);         //error

[LanceL]

I had to modify ReadMultiVars in snap7.net.cs to to get it to work.
Instead of taking a ref to S7DataItem it takes an IntPtr to the first element of an array (that has been marshalled into unmanaged memory).

[DllImport(S7Consts.Snap7LibName)]
protected static extern int Cli_ReadMultiVars(IntPtr Client, IntPtr Item, int ItemsCount);
public int ReadMultiVars(IntPtr Item, int ItemsCount)
{
    return Cli_ReadMultiVars(Client, Item, ItemsCount);
}

I also added this method to marshal the array back and forth.

public int ReadMultiVars(ref S7DataItem[] Items)
{
    int iCount = Items.Length;
    int iSize = Marshal.SizeOf(typeof(Snap7.S7Client.S7DataItem));

    IntPtr pItems = Marshal.AllocHGlobal(iSize * iCount);

    for (int i = 0; i < iCount; i++)
    {
        var pItem_i = new IntPtr(pItems.ToInt64() + (i * iSize));
        Marshal.StructureToPtr(Items[i], pItem_i, false);
    }

    var result = this.ReadMultiVars(pItems, iCount);

    for (int i = 0; i < iCount; i++)
    {
        var pItem_i = new IntPtr(pItems.ToInt64() + (i * iSize));
        Items[i] = (S7DataItem)(Marshal.PtrToStructure(pItems, typeof(S7DataItem)));

    }

    Marshal.FreeHGlobal(pItems);

    return result;
}

This is the first time I have ever done any work with unmanaged code so please review these changes for yourself before using them.

With this code I can read one int and two reals at a rate of 190 to 200 times per second from S7-319 CPU. This is reading from a Data Block.

[Davide Nardella]

Hi LanceL,
thanks for the contribute ;)
please, could you show a little example of calling this ?
Dave

[compactview]

1) Modify ReadMultiVars and WriteMultiVars in snap7.net.cs:

    [DllImport(S7Consts.Snap7LibName)]
    protected static extern int Cli_ReadMultiVars(IntPtr Client, S7DataItem[] Items, int ItemsCount);
    public int ReadMultiVars(S7DataItem[] Items, int ItemsCount)
    {
        return Cli_ReadMultiVars(Client, Items, ItemsCount);
    }

    [DllImport(S7Consts.Snap7LibName)]
    protected static extern int Cli_WriteMultiVars(IntPtr Client, S7DataItem[] Items, int ItemsCount);
    public int WriteMultiVars(S7DataItem[] Items, int ItemsCount)
    {
        return Cli_WriteMultiVars(Client, Items, ItemsCount);
    }

2) Modify S7DataItem in snap7.net.cs:

    [StructLayout(LayoutKind.Sequential, Pack = 1)]
    public struct S7DataItem
    {
        public Int32 Area;
        public Int32 WordLen;
        public Int32 Result;
        public Int32 DBNumber;
        public Int32 Start;
        public Int32 Amount;
        public IntPtr pData;

        public void Set<T>(Int32 Area, Int32 WordLen, Int32 DBNumber, Int32 Start, Int32 Amount, ref T[] Buffer)
        {
            Set(Area, WordLen, DBNumber, Start, Amount, ref Buffer, 0);
        }

        public void Set<T>(Int32 Area, Int32 WordLen, Int32 DBNumber, Int32 Start, Int32 Amount, ref T[] Buffer, Int32 Offset)
        {
            this.Area = Area;
            this.WordLen = WordLen;
            this.Result = 0;
            this.DBNumber = DBNumber;
            this.Start = Start;
            this.Amount = Amount;
            GCHandle handle = GCHandle.Alloc(Buffer, GCHandleType.Pinned);
            this.pData = handle.AddrOfPinnedObject() + Offset * Marshal.SizeOf(typeof(T));
            handle.Free();
        }
    }

3) Calling example:

        using Snap7;

        S7Client client = new S7Client();
        int error = client.ConnectTo("192.168.0.10", 0, 2);
        if (client.Connected())
        {
            byte[] E = new byte[200];
            S7Client.S7DataItem[] items = new S7Client.S7DataItem[2];
            items[0].Set(S7Client.S7AreaPE, S7Client.S7WLByte, 0, 0, 1, ref E);
            items[1].Set(S7Client.S7AreaPE, S7Client.S7WLByte, 0, 20, 1, ref E, 20);

            client.ReadMultiVars(items, 2);
            client.Disconnect();

            MessageBox.Show("EB0: " + E[0].ToString() + ", EB20: " + E[20].ToString());
        }

[Davide Nardella]

Thank you very much ;)
I will update the library asap.