Entry type: FAQ, Entry ID: 28866132, Entry date: 04/01/2008
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What properties, advantages and special features does the S7 Basis Communication offer?

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Description:
S7 Basis Communication involves unconfigured communication connections via which data is transferred on the MPI bus or on the PROFIBUS. The communication connections used are dynamic, i.e. they are set up and released again by the application.

S7 Basis Communication is on Layer 7 ("Application Layer") of the ISO-OSI reference model:

Fig. 01: ISO-OSI reference model

S7 Basis Communication services:
 

Services Description
I_PUT / I_GET This unidirectional service permits you to fetch or write the data of an I slave connected to your own PROFIBUS DP master system.
X_PUT / X_GET This unidirectional service permits you to fetch or write the data of a module connected to the same MPI bus.
X_SEND / X_RCV This bidirectional service permits you to fetch or write the data of a module connected to the same MPI bus.

Properties of the services:
The data volume of the data to be transferred is a maximum of 76 bytes.

Service / Properties I_PUT / I_GET X_PUT / X_GET X_SEND / X_RCV
Max. user data length 84 bytes / 94 bytes 76 bytes 76 bytes
Communication concept Client / Server Client / Server Client / Client
Connection resources1) 0-12 (S7-300)
16-64 (S7-400)
- see CPU specification
0-12 (S7-300)
16-64 (S7-400)
- see CPU specification
0-12 (S7-300)
16-64 (S7-400)
- see CPU specification
Possible address areas E, A, M, D E, A, M, D E, A, M, D
Blocks SFC 72 "I_GET" / SFC 73 "I_GET" SFC 67 "X_GET" / SFC 68 "X_PUT" SFC 65 "X_SEND" / SFC 66 "X_RCV"

1) The connections resources to be reserved for the S7 Basis Communication are set in the STEP 7 Hardware Configuration in the Properties dialog of the configured CPU -> "Communication" tab.

Advantages of S7 Basis Communication with the service

  • There is no need to configure communication connections.
  • Data can be transferred dynamically and variably.
  • Data is sent and received consistently.
  • The connection resources can be controlled by the S7 program in the CPU.
  • The Client / Server and Client / Client communication concepts are possible.

Disadvantages of S7 Basis Communication with the service

  • S7 Basis Communication can be used only in homogeneous SIMATIC structures.
  • Only small volumes of data can be transferred.

Notes:

  • You can find general information about communication via SIMATIC S7 in Entry ID 20982954.
  • There is information available on the SFCs for S7 Basis Communication is available in the manual "System Software for S7-300/400 System and Standard Functions" in Entry ID: 1214574.

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