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Industrial Ethernet S7-300/400 CPs -- Product information -- System description (protocols)
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 dynamic DNS and can use it to communicate with each other?
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?
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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?
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Using C-PLUG with network components and SIMATIC CPs
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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?
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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
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?
Part number:

Description
The information below applies to the following Industrial Ethernet CPs:

  • 6GK7443-1EX40-0XE0
  • 6GK7443-1EX41-0XE0
  • 6GK7443-1GX20-0XE0
  • 6GK7443-1EX20-0XE0
  • 6GK7443-1GX30-0XE0
  • 6GK7443-1EX30-0XE0

These modules support open TCP/IP communication via Industrial Ethernet using the following communication blocks (T blocks):
 
FB Block name Function
FB63 TSEND For transmitting data packets via an established communication connection
FB64 TRCV For receiving data packets via an established communication connection
FB65 TCON For establishing a communication connection
FB66 TDISCON For ending an existing communication connection

Here, the communication processors only support the ISO-on-TCP connection type. Consequently, the RFC 1006 protocol extension of the TCP protocol is used. Thus a maximum of 1452 bytes of user data are transferred.

With Industrial Ethernet the maximum frame size is 1460 bytes (maxSegmentSize). Consequently, the 1452 bytes of data fit in one Ethernet message; this means that the above-mentioned communication processors can send the data in one message.

The CPUs with integrated PROFINET interface also support open TCP/IP communication via Industrial Ethernet using T blocks.

The Industrial Ethernet CP sends the 1452 bytes of data in one message. The S7 CPU sends the same volume of data in two messages.

Explanation
To understand this difference, you must consider two LAN (Local Area Network) recordings.

When a connection is established, the two communication partners negotiate the connection parameters.
They include the maxSegmentSize parameter, among others. This is proposed by the active partner and acknowledge or negotiated by the communication partner. This returned value then counts for the communication connection.
Another parameter is the MaxTPDUSize parameter. This is the maximum size of each protocol element. If the value of this parameter is greater than that of the maxSegmentSize parameter, a corresponding protocol element has to be transferred segmented.

Below, we first consider a connection of the TCP/IP communication between two IE CPs using T blocks.
Then we consider a connection of the TCP/IP communication between CPU and IE CP using T blocks. This makes clear why the data has to be sent in two messages here.

  1. Communication connection between two IE CPs

    Fig. 01 shows that the IE CP with the IP address 140.90.36.1 actively establishes the connection and proposes a MAXTPDUSize parameter of 2048 bytes. In the total recording you see that the communication partner (IE CP) acknowledges the MAXTPDUSize of 2048 bytes. This then counts for the communication connection.


    Fig. 01

    This means that only one message has to be sent. The maximum user data length of 1452 bytes is not limited by the maxSegmentSize and the maxTPDUSize. Both values are greater than 1452 bytes.
    The IE CP only has to send one message!
     
  2. Communication connection between a CPU and an IE CP

    Fig. 02 shows that the CPU with the IP address 140.90.37.10 actively establishes the connection and proposes a MAXTPDUSize parameter of 1024 bytes. In the total recording you see that the communication partner (IE CP) acknowledges the MAXTPDUSize of 1024 bytes although it supports a greater maxTPDUSize (see Fig. 01). The maxTPDUSize for the communication connection between CPU and IE CP is 1024 bytes.


    Fig. 02

    This means that the maximum user data length of 1452 bytes is sent in two messages. The maximum user data length is limited by the maxSegmentSize. This value is less than 1452 bytes.
    The CPU has to send two messages.

Effects for the user
Initially, the user does not see any of the behavior at the application level. The mechanisms are coordinated independently by the protocol levels of the two communication partners. However, if you compare the number of messages on the LAN, you see that more messages are visible in the second constellation using a CPU.

 Entry ID:28375758   Date:2014-03-19 
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