What properties, advantages and special features does the S7 protocol offer?
The following functions are available to you for the S7 CPUs and C7 CPUs regardless of the bus system used, so that you can use S7 communication via Industrial Ethernet, PROFIBUS or MPI:
- System function blocks (SFBs): in STEP 7 V5.x for S7-400 CPUs
- Function blocks (FBs): in STEP 7 V5.x for S7-300 CPUs and C7-CPUs
- Instructions: in TIA Portal for S7-300 CPUs, S7-400 CPUs, S7-1200 CPUs and S7-1500 CPUs
Position of the S7 protocol in the ISO-OSI reference model.
Services of the S7 protocol
Overview of the S7 protocol services.
|PUT / GET||This service is a unidirectional read/write service for transferring small volumes of data to and from a station.|
|BSEND / BRCV||This service is a bidirectional and block-oriented service for transferring large volumes of data between two stations.|
|USEND / URCV||This service is a bidirectional and uncoordinated service for transferring small volumes of data between two stations.|
User data size
The S7 protocol permits transfer of data from 1 byte to 64 Kbytes. The maximum data size depends on the service used and the S7 CPU used.
|Service||S7-300 CPU||S7-400 CPU||S7-1200 CPU||S7-1500 CPU|
|PUT / GET||160 bytes||400 bytes||160 bytes||880 bytes|
|BSEND / BRCV||32768 bytes / 65534 bytes||65534 bytes||-|
|USEND / URCV||160 bytes||440 bytes||-|
Properties of the S7 protocol
The following table shows the properties of the S7 protocol.
|Properties||PUT / GET||BSEND / BRCV||USEND / URCV|
|Memory areas||M, D, E, A, T, Z||M, D, E, A, T, Z||M, D, E, A, T, Z|
|Data consistency||Total length per job2)||Total length per job2)|
|Communication principle||Client / Server||Client / Client||Client / Client|
|Maximum number of connections||See CPU specification||See CPU specification||See CPU specification|
1) Depending on the CPU used.
2) 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, MPI).
- Can be used on all S7 data areas.
- Transfer of up to 64 Kbytes in one job.
- The S7 protocol ensures automatic acknowledgment of the data records.
- Low processor and bus load during transfer of large volumes of data.
The S7 protocol is supported by all available S7 CPUs 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 about communication via SIMATIC S7 is available in Entry ID: 20982954.
- Examples of S7 communication with PUT / GET are available in the following Entry IDs:
92269951: Data transfer between an S7-300 CPU and an S7-1200 CPU
82212115: Data transfer between an S7-1200 CPU and an S7-1500 CPU
18610307: PUT / GET in the user program of the S7-300 CPU (STEP 7 V5.x)
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