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P-NET on IP for Real Time Ethernet

One of the main jobs of the International P-NET User Organization (IPUO) is to support, and be active within, various international standardisation committees and Work Groups (WGs) associated with Industrial Communications in general, and Fieldbus technology in particular. In this respect, a process of overhauling fieldbus specifications has been underway for a while now, and the current situation can now be reported upon.

Engineers working with process and machine automation will undoubtedly be aware of the increasing popularity and use within the industrial sector of Ethernet (ISO/IEC 8802-3) as a physical medium, together with associated media access and transport protocols, to enable both local and worldwide communication between machines, plants, control stations etc. Such is the growing industrial usage of this Internet technology, that the International Electrotechnical Commission (IEC), proposed that all the current international standardised fieldbus types, as specified within IEC 61158 (P-NET being type 4), plus some additional serial protocols already utilising Ethernet, should be included in extensions to specification IEC 61784 - Real Time Ethernet (RTE). The required international voting procedures for agreeing the communication protocols to be included, and to have this activity approved as a new work item, has now been completed, with overwhelming international support. The use of the Internet as a medium for the inter-communication of P-NET data is not new. Indeed, many versions of the well known VIGO Fieldbus Management System have provided the means to interconnect VIGO-enabled PCs together using IP for some time. More recently, programmable P-NET modules that include an Ethernet port have been made available, where the in-built operating system provides transparent P-NET communication between two points connected locally or via the Internet. Standardised Ethernet wiring and connectors also provide the means of interconnection of switches, to avoid media access collisions, and wireless access points and routers (WLAN), as a highly convenient media alternative. When connecting two or more industrial networks together using this method, it will, more often than not, contain real-time measurement or process data. This technology is therefore now referred to as Real Time Ethernet (RTE). The work of submitting the draft RTE extensions to the P-NET specification in the format required, is now complete, although it may be some while before a completed IEC 61784-2 specification will be generally available. In essence, the P-NET protocol and message structure has not been changed. What happens, is that the familiar message structure used, say via RS 485, is 'wrapped' in a 'user datagram' using the transport protocol within IP called UDP. Using this technology, it is necessary to define specific usage ports as a means of defining the underlying message protocol (i.e. P-NET). The IPUO have already officially reserved ( via IANA - Internet Assigned Numbers Authority), UDP port 34378 for 'normal' P-NET communication, and port 34379 for 'secure' P-NET communication, where a message using the latter port requires the inclusion of a password. The P-NET on IP extensions to the current P-NET standard includes a description of the structure of UDP/IP packages for P-NET messages. These enhance current definitions of P-NET message addressing modes (e.g. simple, extended, complex) with IP, including associated routing definitions. Implementation of P-NET on IP for Real Time Ethernet, means that P-NET packages can be routed through IP networks in exactly the same way as they can be routed through non-IP networks. Routing can be through any type of P-NET network and in any order. Nodes on an IP network are addressed with two P-NET Route elements, but this is entirely handled by the IP nodes. This means that any P-NET client (master) can access servers on an IP network, without knowing anything about IP addresses! To further discuss the implications of P-NET on IP, or to ascertain availability of operational equipment, please contact the International P-NET User Organization. Phone numbers and email adresses are available on www.p-net.org .

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