Event driven reading

Download: 'Event driven reading'

If values from a PLC or NC are to be displayed continuously on a user interface, then it is very inefficient to use AdsSyncReadReq(), since this function must be called cyclically. By defining what are known as notifications (messages), a TwinCAT server can be made to transmit values via ADS to another ADS device. A distinction is drawn between whether the TwinCAT server is to transmit the values cyclically, or only when the values change.

A notification is begun with the AdsSyncAddDeviceNotificationReq() function. After this, the callback function is automatically invoked by TwinCAT. AdsSyncDelDeviceNotificationReq() is used to halt the notification again. Since the number of notifications is limited, you should ensure the notifications no longer required by your program are deleted. You will find further information under the description of the AdsNotificationAttrib structure.

The following program starts a notification on a variable handle in the PLC. Each time the PLC variable changes, the callback function is invoked. The callback function receives a variable of type AdsNotificationHeader() as one of its parameters. This structure contains all the necessary information (value, time stamp, ...).

Event driven reading 1:

Efficient Usage

    • Don't use time intensive executions in callbacks.
    • Remind to sync your callback and your mainthread, if you access each other (e.g. critical sections, mutex, events).
    #include <iostream>
    #include <conio.h>
    #include <windows.h>
    #include <winbase.h>

    // ADS headers for TwinCAT 3
    #include "C:\TwinCAT\AdsApi\TcAdsDll\Include\TcAdsDef.h"
    #include "C:\TwinCAT\AdsApi\TcAdsDll\Include\TcAdsAPI.h"

    using namespace std;

    void _stdcall Callback(AmsAddr*, AdsNotificationHeader*, unsigned long);

    void main()
    { 
    long         nErr, nPort; 
    AmsAddr        Addr; 
    PAmsAddr     pAddr = &Addr; 
    ULONG         hNotification, hUser; 
    AdsNotificationAttrib adsNotificationAttrib;
    char                 szVar []={"MAIN.PLCVar"};

    // open communication port on the ADS router
    nPort = AdsPortOpen();
    nErr = AdsGetLocalAddress(pAddr);
    if (nErr) cerr << "Error: AdsGetLocalAddress: " << nErr << '\n';

    // TwinCAT 3 RTS1 Port = 851
    pAddr->port = 851;

    // set the attributes of the notification
    adsNotificationAttrib.cbLength = 4;
    adsNotificationAttrib.nTransMode = ADSTRANS_SERVERONCHA;
    adsNotificationAttrib.nMaxDelay = 0;
    adsNotificationAttrib.nCycleTime = 10000000; // 1sec 

    // get handle
    nErr = AdsSyncReadWriteReq(pAddr, ADSIGRP_SYM_HNDBYNAME, 0x0, sizeof(hUser), &hUser, sizeof(szVar), szVar);
    if (nErr) cerr << "Error: AdsSyncReadWriteReq: " << nErr << '\n';

    // initiate the transmission of the PLC-variable 
    nErr = AdsSyncAddDeviceNotificationReq(pAddr, ADSIGRP_SYM_VALBYHND, hUser, &adsNotificationAttrib, Callback, hUser, &hNotification);

    if (nErr) cerr << "Error: AdsSyncAddDeviceNotificationReq: " << nErr << '\n';
    cout << "Notification: " << hNotification << "\n\n";
    cout.flush();

    // wait for user intraction (keystroke)
    getch();

    // finish the transmission of the PLC-variable 
    nErr = AdsSyncDelDeviceNotificationReq(pAddr, hNotification);
    if (nErr) cerr << "Error: AdsSyncDelDeviceNotificationReq: " << nErr << '\n';

    // release handle
    nErr = AdsSyncWriteReq(pAddr, ADSIGRP_SYM_RELEASEHND, 0, sizeof(hUser), &hUser); 
    if (nErr) cerr << "Error: AdsSyncWriteReq: " << nErr << '\n';

    // Close the communication port
    nErr = AdsPortClose();
    if (nErr) cerr << "Error: AdsPortClose: " << nErr << '\n';
    }

    // Callback-function
    void __stdcall Callback(AmsAddr* pAddr, AdsNotificationHeader* pNotification, ULONG hUser)
    {
    int         nIndex; 
    static ULONG     nCount = 0; 
    SYSTEMTIME     SystemTime, LocalTime; 
    FILETIME        FileTime; 
    LARGE_INTEGER     LargeInteger; 
    TIME_ZONE_INFORMATION TimeZoneInformation; 

    cout << ++nCount << ". Call:\n"; 

    // print (to screen)) the value of the variable 
    cout << "Value: " << *(ULONG *)pNotification->data << '\n'; 
    cout << "Notification: " << pNotification->hNotification << '\n';

    // Convert the timestamp into SYSTEMTIME
    LargeInteger.QuadPart = pNotification->nTimeStamp;
    FileTime.dwLowDateTime = (DWORD)LargeInteger.LowPart;
    FileTime.dwHighDateTime = (DWORD)LargeInteger.HighPart;
    FileTimeToSystemTime(&FileTime, &SystemTime);

    // Convert the time value Zeit to local time
    GetTimeZoneInformation(&TimeZoneInformation);
    SystemTimeToTzSpecificLocalTime(&TimeZoneInformation, &SystemTime, &LocalTime);

    // print out the timestamp
    cout << LocalTime.wHour << ":" << LocalTime.wMinute << ":" << LocalTime.wSecond << '.' << LocalTime.wMilliseconds << 
    " den: " << LocalTime.wDay << '.' << LocalTime.wMonth << '.' << LocalTime.wYear << '\n';

    // Größe des Buffers in Byte
    cout << "SampleSize: " << pNotification->cbSampleSize << '\n';

    // 32-Bit Variable (auch Zeiger), die beim AddNotification gesetzt wurde // (siehe main)
    cout << "hUser: " << hUser << '\n';

    // Print out the ADS-address of the sender
    cout << "ServerNetId: ";
    for (nIndex = 0; nIndex < 6; nIndex++)
    cout << (int)pAddr->netId.b[nIndex] << ".";
    cout << "\nPort: " << pAddr->port << "\n\n";
    cout.flush();
    }