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Entry type: FAQ Entry ID: 26483647, Entry date: 09/24/2007

What properties, advantages and special features does the S7 protocol offer?

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All SIMATIC S7 and C7 controllers have integrated S7 communication services with which the user program can read and write data. The S7-400 controllers use SFBs, the S7-300 and C7 controllers use FBs. These functions are available to you regardless of the bus system used, so that you can use S7 communication via Industrial Ethernet, PROFIBUS or MPI.

The S7 Protocol can be inserted as follows in the ISO-OSI reference model:

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Fig. 01 - Position in the ISO-OSI reference model

S7 protocol services:

Services Description
PUT / GET A unidirectional read/write service for transferring small volumes of data to and from a station.
USEND / URCV A bidirectional uncoordinated service for transferring medium-sized volumes of data between two stations.
BSEND / BRCV A bidirectional block-oriented service for transferring large volumes of data between two stations.

Service properties:
The S7 protocol permits transfer of data from 1 byte to 64 Kbytes. The volume spectrum depends on the service and hardware used.

Service / Properties BSEND / BRCV USEND / URCV PUT / GET
Max. data length
S7-300 / S7-400
32 KB / 64 KB 1) 160  bytes / 440 bytes 2) 160  bytes / 400 bytes 2)
Possible address ranges
S7-300 / S7-400
M, D/M, T, C, I, Q, D M, D/M, T, C, I, Q, D M, D/M, T, C, I, Q, D
Data consistency
S7-300 / S7-400
Total length per job  4) Total length per job  4) 8-32 bytes / 32 bytes to total length 3,4)
Communication concept Client / Client Client / Client Client / Server
Max. number of connections  See CPU specification See CPU specification See CPU specification
S7-300 / S7-400
1) Corresponds to the max. length of a data block of the system concerned in each case.
2) Corresponds to the total volume of user data for the SFB / FB in the case of Industrial Ethernet.
3) Depending on the CPU used.
4) In the user program you must make sure that the data block is not modified during data transfer.

Advantages of the S7 protocol:

  • Independent of the bus medium (PROFIBUS, Industrial Ethernet (ISO or TOP), MPI).
  • Can be used on all S7 data areas.
  • Transfer of up to 64Kbytes in one job.
  • Level 7 protocol ensures automatic acknowledgment of the data records.
  • Low processor and bus load during transfer of large volumes of data, because it is optimized for SIMATIC communication.

The S7 protocol is supported by all available S7 controllers and communication processors. Furthermore, PC systems with appropriate hardware and software support communication via the S7 protocol.


  • Entry ID 20987358 provides a complete example of S7 communication with BSEND/BRCV via PROFIBUS.
  • General information on communication via SIMATIC S7 is available in Entry ID 20982954.

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