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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?
Which IO controllers and IO devices support the following functions: IRT, prioritized startup, MRP, PROFIenergy, Shared device, I device and clock-synchronized mode?
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 are the different data block lengths for the various protocols of Send/Receive communication?
Part number:

Configuration Notes:
This entry is designed to explain a comment in the CP443-1EX40 device manual. Section 5.3 "Characteristic data for SEND/RECEIVE interface" provides information about data block lengths for the various protocols of Send/Receive communication.
 
LAN interface - data block length generated by the CP per protocol unit
For sending ISO Transport, ISO-on-TCP,
TCP: 400 bytes / TPDU
For receiving ISO Transport: 512 bytes /
ISO-on-TCP: 1024 bytes /
TCP: 1460 bytes / TPDU

The CP443-1EX40 device manual is available in Entry ID: 19308871

The values specified there are the number of user data bytes that are sent or received simultaneously by the module. It might well happen that the user sends/receives jobs to/from the communication blocks with longer user data lengths. These are divided into segments of appropriate length by the transport layer of the CP. These values are valid for the LAN interface of the module (local area network).

Values for sending user data blocks
The block size is limited to 400 bytes. But this doesn't depend on the protocol used. The buffer sizes are organized in the firmware so that always data blocks of a maximum length of 400 bytes are sent.
This means that a user data package of 401 bytes is sent in 2 separate messages. The first message has a length of 400 bytes and the second a length of one byte. The behavior described here applies for all 3 protocols of the Send/Receive interface used:

  • ISO transport protocol
  • ISO-on-TCP protocol
  • TCP protocol

Values for receiving user data blocks
The maximum data block length here cannot be considered as a generic absolute value. A distinction between cases of the 3 protocols is necessary here.
An explanation of the different cases is given below and illustrated by LAN recordings in the sniffer format.

  • ISO transport protocol
    The maximum number of bytes to be received is determined by the "Maximum TPDU Size" parameter. This is negotiated in a handshake procedure when the connection is set up. The active partner proposes a value and the passive partner accepts it or reduces it to a lower value.
    In the example below a "Maximum TPDU Size" of 512 bytes has been negotiated.

    ( 38 KB )
    Fig. 01: Maximum TPDU Size for ISO Transport

    LAN recording in the sniffer format:

    ISO_Connection.zip ( 388 bytes )  

     
  • ISO-on-TCP protocol
    Here, there is not just one parameter that could represent a limit, but there are two relevant parameters:

    -
    MaximumSegmentSize = 1460
    -
    Maximum TPDU Size = 1024

    The "MaximumSegmentSize" is negotiated as 1460 bytes when the connection is set up. You can see this clearly in the following figure.

    ( 37 KB )
    Fig. 02: MaximumSegmentSize ISO-on-TCP

    Since with the ISO-on-TCP protocol it is a matter of porting the mechanisms of the ISO transport protocol to TCP, there is also a "Maximum TPDU Size" parameter for the ISO-on-TCP protocol. This is set to 1024 bytes in the example below.

    ( 38 KB )
    Fig. 03: Maximum TPDU Size ISO-on-TCP

    Note:
    The smaller of the two values must be considered as the relevant limit. Consequently the maximum size of a data package that can be received with the ISO-on-TCP protocol is 1024 bytes. Here too please note that the user can also receive larger data blocks. The CPs do the segmenting automatically themselves.

    LAN recording in the sniffer format:

    ISO-on-TCP_Connection.zip ( 584 bytes )

     
  • TCP Protocol
    The TCP protocol doesn't have the "Maximum TPDU Size" parameter. The maximum number of bytes to be received is determined by the "MaximumSegmentSize" parameter. It is set to 1460 bytes.

    ( 38 KB )
    Fig. 04: MaximumSegmentSize TCP Protocol

    TCP_Connection.zip ( 443 bytes )

 Entry ID:24271223   Date:2006-12-15 
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