Entry type: FAQ, Entry ID: 28866566, Entry date: 07/24/2008
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What properties, advantages and special features does the global data communication offer?

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Description:
Global data communication permits cyclic data communication between SIMATIC S7 CPUs via the MPI interface. Data communication is cyclic during updating of the process output and input images.

In addition to cyclic data transfer, it is also possible to have event-controlled data transfer in the S7-400 CPU via prepared function blocks. For this, function blocks are called in the S7 program for sending and receiving the data.

The data to be transferred is defined statically in the program and can be transferred consistently in different global data rings, i.e. defined groups of nodes that exchange global data with each other. The data can only be transferred to modules that have been parameterized in the same STEP 7 project and that use either the same communications bus (K bus) on the backplane bus or the MPI bus.

Global data communication and MPI are located as follows in the ISO-OSI reference model.

Fig. 01: ISO-OSI reference model

Global data communication services:
 

Services Description
Cyclic data transfer Cyclic data transfer covers all the configured global data rings. The data is transferred when the process image is updated.
GD_SND / GD_RCV The S7-400 sends and receives global data packages through the event-controlled data transfer via the function blocks GD_SND and GD_RCV respectively. The global data ring number and global data package number are specified on the function block.

Properties of the services:
The volume of the data to be transferred is low with a maximum of 22 bytes (S7-300 CPU) and 54 bytes (S7-400 CPU).
 

Service / Properties S7-300 S7-400
Max. data length 22 bytes 54 bytes
Max. number of GD packages for sending 4 / 8
depending on the S7-300 CPU
8 / 16
depending on the S7-400 CPU
Max. number of GD packages for receiving 4 / 8
depending on the S7-300 CPU
16 /32
depending on the S7-400 CPU
Max. number of GD rings 4 / 8
depending on the S7-300 CPU
8 / 16
depending on the S7-400 CPU
Blocks - SFC 60 "GD_SEND" / SFC 61 "GD_REC"

Advantages of global data communication:

  • Simple configuration of communication.
  • Consistent data transfer.

Disadvantages of global data communication:

  • Global data communication can be used only in homogeneous SIMATIC structures.
  • Data transfer is static.
  • Only small volumes of data can be transferred.
  • Global data communication is not acknowledged.

Notes:

  • Global data communication is not available in the S7-400H.
  • You can find general information about communication via SIMATIC S7 in Entry ID: 20982954.
  • Information on the SFCs for S7 global data communication is available in the manual "System Software for S7-300/400 System and Standard Functions" in Entry ID: 1214574.