Could you please provide more details [:blush:]
[quote user="Ates"]
1. for example we have a program which gets the data from plc every1 minutes, but how can i get data upon change ?
i mean i want to get a data when it changes, must i look to it every 100ms and must i catch it when it changes ?

What kinda program do you have? Is an Application around S7API,orOPC or isWinCC;
What PLC are you communicating with (S7300 or 400), how (MPI, Profibus, Ethernet etc)?

If you use WinCC then it depends on the specific of the application used in PLC & SCADA: You want to catch data change in PLC (an analog tag with dead band, a digital tag / in OB 1 or OB 3x) and then notify WinCC? Or let WinCC decide upon an S7 cyclic service (see below).

[quote user="Ates"]
2 . so how is wincc's scripts are working if you trigger them to an external tag with upon change ? or if you create a global script with asynchron tag variable,how is it working actually?

In WinCC you can configure tagtriggers only for actions (either VBS or C Script);
If you configure tag trigger upon change then depends on various aspects (CPU type 300-400 communication resources & connections) S7 Channel chouse the type of S7 communication service will use. In general it will subscribe to an S7 cyclicvariable service @ 1s_cycle so the PLC will send every 1s update values for particular tag(s). On some AS400 there is a true upon changeservice but is used rare (STEP7 and WinCC uses based on SZL 0x131 index 3 SmartAnyRead capability)
If tag changes WinCC Data Managerwill notify Global Script RT (Action RT) to execute the action.

[quote user="Ates"]
3. has plc has got its buffer for opc variables ? for example when i create a group and i create active items is it automaticly doing something in plc side or is it sending only when my program demands it ?

Are you talking about a Clientapplication that uses SIMATIC S7 OPC Server?
When you create a group (in the client app)you define an update rate. All Items within that group will be updated at that interval in OPC Server side. The client can specify to be notified only upon change but this is a deal between OPC Server and the client application; has nothing to do with the communication between OPC Server and the PLC. That communication is based again on S7 cyclic service @ group_update_rate (See above WinCC S7 Channel)
If the maximum number of cyclic read exceeds the CPU capability (SZL 0x131 index 3MaxCyclicRead) then OPC Serverstart reading the variables in pooling mode (as J_Bell explained).