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Siemens releases Open Modbus/TCP block

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Existing automation systems require modernisation or expansion to accommodate the increasing use of Ethernet-based data communication in both industrial and office environments. The result is often a heterogeneous system landscape. Linking it up to a powerful higher-level system necessitates the use of open and preferably standardised communications protocols. The Open Modbus/TCP block developed by Siemens is based on Modbus, a protocol that is used all over the world and is already supported by many manufacturers. The communications protocol is an open Internet draft standard that has already been incorporated in the IETF (Internet Engineering Task Force). As a result of this disclosure, any manufacturer or user can implement this protocol.

Open Modbus/TCP now also supports central processing units with Profinet capability (PN-CPUs), which therefore includes the entire range of Siemens Simatic automation devices. There are three Modbus variants available for integrating different Simatic systems: the Modbus block for communications processors is suitable for Simatic S7-300 and S7-400 systems. For Modbus communication, a CP343-1 or a CP443 1 communications processor is needed as well. The Modbus block for central processing units with Profinet capability is used to link up Simatic S7-PN CPUs and ET200S-PN CPUs. For Modbus communication in this case the integrated Profibus interface is used. For Simatic S7-400H systems, the Modbus block with redundancy functionality is used and, for communication purposes, a CP443-1 communications processor is needed for each subsystem as well.

All Modbus variants perform a range of functions supporting server mode, client mode and conformance class 0 with the function codes 3 and 16. The block with redundancy functionality also makes it possible to establish a communications link that is redundant and therefore fail-safe at the same time. For communication between Simatic stations and other Modbus nodes, native TCP connections that are based on the standard functions of the Simatic S7 library are set up. All Modbus blocks have multi-instance capability.

Configuration is carried out with Simatic STEP 7. A software wizard is also available for configuring the Modbus blocks for Profinet CPUs.

For more information, visit www.siemens.com/s7modbus.

03 December 2008

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