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Entry type: FAQ Entry ID: 24658964, Entry date: 03/11/2009

How can you synchronize CPU clocks with each other?

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If multiple CPUs are linked with each other in a subnetwork, you parameterize the clock of one CPU as the master clock. When parameterizing the CPU you also specify the synchronization interval after which all the clocks in the subnetwork are automatically synchronized with the master clock. You call system function SFC48 "SNC_RTCB" in the CPU with the master CPU. The call synchronizes all the clocks in the subnetwork regardless of the automatic synchronization. If you set a master clock with SFC0 "SET_CLK", all the other clocks in the subnetwork are automatically synchronized with that value.

No. Description
1 Open the Hardware Configuration in the CPU that you wish to parameterize as master CPU and open the Properties of that CPU.

In the Properties of the CPU you select the "Diagnostics/Clock" tab and set the type of Synchronization Mode and Time Interval as shown in Fig. 01. Then save with OK.

Fig. 01

3 Then save and compile the hardware configuration with "File > Save and Compile".

Repeat Steps 1 to 3 for all other CPU modules to be synchronized. However, for these groups you must set the type of synchronization to "As Slave".

4 Then open the dialog "Open Project" with "File > Open ..." and select the "Libraries" tab. As shown in Fig. 02 mark "Standard Library" with the cursor and then click the OK button.

Fig. 02

5 The Standard Library is now loaded into the SIMATIC Manager. Under "System Function Blocks" (Fig. 03) you will find the system functions. Copy system functions SFC0 and SFC48 by drag-and-drop to the block folder of your master CPU.

Fig. 03

6 How to set the date and time using system function SFC0 is described in the FAQ with Entry-ID 21222026.
More information on parameterizing SFC0 and SFC48 is also available in the Online Help of STEP 7 (F1 key). After loading both system functions into the CPU (master clock) all the other clocks in this subnetwork are automatically synchronized.

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