The great advantage of the ISO transport protocol is the package-oriented transfer of data. However, with the increase in networking, the missing routing functionality became an ever growing disadvantage.
Since the routing-compatible TCP/IP protocol has become predominant due to the Internet, an attempt was made to combine the advantages of both protocols. In the extension RFC1006 (RFC = Request for Comments) "ISO on top of TCP", also called "ISO on TCP", the properties of the ISO transport on the TCP/IP protocol are defined. The ISO-on-TCP protocol is also on Level 4 of the ISO-OSI reference model and defines Port 102 as default port for data transfer.
This protocol can be used in the current modules of SIMATIC S7, SIMATIC PC and, by implementing the CP 1430 TCP, also in SIMATIC S5.
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Fig. 01 - Position in the ISO-OSI reference model
SR is a service for actively transferring data between two stations. Data is exchanged here between the stations using send and receive blocks
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.
|Data area per job
||1 - 8192 bytes|
||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 ISO-on-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)
- package-oriented data transfer
- dynamic data lengths are possible
Disadvantages/special features of the ISO-on-TCP protocol:
- can be used mainly only in homogeneous SIMATIC structures
- increased programming workload for data management
- separate router configuration/settings required (router must be entered in the HW configuration of the CP)
- Entry ID 20987359 provides a complete example of communication via the ISO-on-TCP protocol.
- General information on communication via SIMATIC S7 is available in Entry ID 20982954.