The DeltaV Control Network examples in this chapter are designed according to the IEEE 802.3 Local and Metropolitan Area Networks standards. The topologies shown in the examples are a small subset of the control network topologies that conform to the Ethernet 802.3 standard. Network topologies vary depending upon the plant layout. One plant layout may require a large number of hubs in one wiring closet rather than single hubs in several wiring closets or rack rooms located throughout the plant. Another plant layout may require a centralized hub or switch in a star topology that branches to several remote locations. These and other network configurations are possible and fully supported by Emerson. Consult your sales representative for assistance with network configuration.
The examples begin with simple 10 megabit per second (abbreviated here as 10Mbit) twisted pair network implementations, and end with more complex 10/100Mbit twisted pair networks with a combination of fiber-optic and twisted pair segments. In all cases, the examples show the maximum cable distances and the maximum number of intermediate Ethernet hubs. These maximum limits assure you that any network design that exactly matches the example, or is a subset of the example, is guaranteed to operate according to Ethernet rules and will provide the necessary performance for your DeltaV system.
Care must be taken when designing your network to not have large amounts of network traffic running through the area control network across switches which are also used for control (for example, controllers and CIOCs). Failing to do so can result in the control integrity being compromised causing ACN switchovers and device dropouts.
Advanced Ethernet models used for the calculation of maximum packet transmission distances over copper cabling can be found in Ethernet 802.3 standards. For simplicity, this section uses general Ethernet rules. For example, the number of repeater hops is limited to four (four repeater hubs physically connected together in series) for 10Mbit networks, and to two (two repeater hubs physically connected together in series) for 100Mbit networks. This limitation on the number of repeater hops allows for a minimum amount of space (Inter Frame Gap) between each packet that is transmitted over the network so that the physical medium’s receivers and transmitters located on each repeater port have time to distinguish between the end of one packet and the beginning of the next. As a packet travels between end nodes on a network, each repeater through which the packet travels reduces the Inter Frame Gap by a small amount. If too many repeater hubs are in the path between two end nodes, the overall net Inter Frame Gap shrinkage will result in an Inter Frame Gap that is too small and packet information will be corrupted by the time it reaches the receiving device. When all repeater hubs used in a network, such as the repeater hubs that are qualified for use with the DeltaV system, are designed in compliance with Ethernet standards, the generalized two hop/ four hop rules can be used. Otherwise, a custom calculation would have to be performed on a case by case basis for every network connection.