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Industrial Ethernet S7-300/400 CPs -- Product information -- System description (protocols)
How many devices can you connect to an S7-300/S7-400 using the Modbus/TCP protocol?
When compiling a CFC chart, why is the error message "Time stamp conflict" displayed in connection with the Modbus/TCP FBs for S7-400 CPUs?
How many licenses are required for the Modbus/TCP block in the S7-300/S7-400?
Which IO controllers and IO devices support the following functions in STEP 7 (TIA Portal): IRT, prioritized startup, MRP, MRPD, PROFIenergy, Shared device, MSI/MSO, I device, Isochronous mode, System redundancy and Option handling?
How are the communication resources assigned in the S7-300?
How do you load the 3D, CAx and ePLAN data for the SIMATIC products?
What are the differences between OPEN MODBUS / TCP Redundant V1 and OPEN MODBUS / TCP Redundant V2?
What should you watch out for when you enable the "SNMP" function in the configuration of the security module?
Which security modules support the dynamic DNS function (DDNS)?
What should you watch out for when enabling and using the security functions of CP343-1 Advanced and CP443-1 Advanced?
What should you watch out for when upgrading from previous modules to the successor module CP443-1 or CP443-1 Advanced?
How do you verify the authenticity of the Siemens modules CP343-1 and CP443-1?
What should you watch out for when operating a port of CP343-1, 6GK7343-1CX10-0XE0 and 6GK7343-1EX30-0XE0 with firmware V2.1 or higher, with 10 Mbit/s half duplex?
Which IO controllers and IO devices support the following functions in STEP 7 V5.5: IRT, prioritized startup, MRP, MRPD, PROFIenergy, Shared device, I device, sochronous mode, System redundancy and Option handling?
Where do you procure the firmware extension for the CP 343-1 ERPC for database connection?
Which SIMATIC S7-300/S7-400 modules support the NTP time-of-day message and how do you activate this kind of time synchronization?
Which modules can you use when implementing software redundancy?
Which types of connection/protocols do the S7-300/400 CPUs and the CPs support by default?
Which PROFINET nodes support automatic commissioning (device replacement without interchangeable medium) in the case of topological configuration and device replacement without topological configuration?
Which PROFINET nodes support the extended PN diagnostics and what do you have to configure?
Using C-PLUG with network components and SIMATIC CPs
What properties have the communication services and protocols that can be used in the Industrial Ethernet network?
Behavior of the communication processors during data transfer via TCP connections without RFC 1006
What should you watch out for with the ICMP Redirect (Internet control message protocol) function regarding the IP address of the router used for Industrial Ethernet CPs?
What should you pay attention to with the autonegotiation function for negotiating the LAN mode in Industrial Ethernet CPs?
What is the connection between subnet masks and IP addresses with regard to subnetting and supernetting (Classless Inter Domain Routing CIDR)?
Which modules support the S7 routing function?
In the case of open TCP/IP communication via Industrial Ethernet using T blocks with IE CPs and CPUs, why are different amounts of messages sent when the maximum length of 1452 bytes of user data is transferred?
What properties, advantages and special features does the ISO protocol offer?
What properties, advantages and special features does the UDP protocol offer?
What properties, advantages and special features does the ISO-on-TCP protocol offer?
What properties, advantages and special features does the S7 protocol offer?
What properties, advantages and special features does the TCP protocol offer?
What are the requirements for disabling the PROFINET CBA interface for CP443-1EX40?
"Time Wait" with TCP connection setup
What are the different data block lengths for the various protocols of Send/Receive communication?
What is RFC1006 and what do I need this service for?
Meaning and function of the "Keep Alive" in TCP connections
What properties, advantages and special features does the TCP protocol offer?
Part number:

If data is transferred with the TCP protocol, the transfer takes the form of a data stream. There is no information transferred either about the length or about the start and end of a message. The recipient cannot tell where one message ends in the data stream and the next one begins. Therefore, the sender has to specify a message structure which can be interpreted by the recipient. The message structure can, for example, comprise the data followed by a control character, such as "carriage return", signaling the end of a message.

In most cases TCP uses the IP (Internet protocol) which is why people also frequently (and often not altogether correctly) talk about the "TCP/IP protocol". It is in layer 4 of the ISO-OSI reference model.

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

Socket and ports:
Several processes can run simultaneously on multitasking operating systems, whereby each process can provide multiple services. They should be able to be addressed separately.
For this reason, special interfaces, or ports, are defined for data communication both in the TCP/IP and the UDP protocol. This expansion of the host-to-host transfer to a process-to-process transfer is termed application multiplexing and demultiplexing.

Together with the IP address for a station, the port number forms a socket which is defined as the unique address of the user program in the entire network. Therefore, any of a process's services can be addressed to a station within a network using a socket.

In the SIMATIC environment, multiple "simultaneous" processes or user programs cannot be created on one CPU. However, multiple communication jobs may be initiated simultaneously. In order to facilitate unique addressing here as well, assignment must take place via the connection configuration.

Protocol services:

  • Send/Receive
    The SEND/RECEIVE interface is used for actively transferring data between two client stations. Data is exchanged here between the stations using send and receive blocks
  • Fetch/Write
    Fetch/Write is a service which is used to read data from a server station or to write data to it. Access takes place via the operating system for the passive server station without any further function calls within the server station. In SIMATIC S7, the fetch/write service is only implemented in server functionality. Therefore, the SIMATIC S7 is unable to actively read or write data. Clients are able to actively read/write data to or from the S7.
Quantity framework:


Value range
Data area per job 1 - 8192 bytes
Interface Level 4 of the ISO-OSI reference model
Number of possible connections  Up to 16 per S7-300 CP
Up to 64 per S7-400 CP

Advantages of the TCP protocol:

  • high-speed communication
  • suitable for transferring medium-to-large data volumes (<=8192 bytes)
  • routing capability (i.e. can be used in the WAN)
  • can be used flexibly in systems of other makes
  • acknowledges

Disadvantages of the TCP protocol:

  • can only transmit static data lengths
  • increased programming workload for data management
  • Data is transferred as a data stream.


  • Entry ID 19033929 contains a complete example of the transmission of data with variable message lengths via the TCP protocol.
  • You can find general information about communication via SIMATIC S7 in Entry ID 20982954.

 Entry ID:26171811   Date:2007-11-29 
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