Ошибка на частотнике

Deltarqtity | 07.04.2018

the RS-485 line isn’t isolated from the nodes it connects to. The power supplies can be batteries or floating AC supplies. This arrangement is useful if the nodes’ circuits are relatively quiet but you want to isolate the nodes and line from variations in earth ground. A system where each node is battery powered has this type of isolation. In Figure 7-15C, the data lines are isolated, but the grounds aren’t isolated. This partial isolation offers some protection to the nodes if a voltage surge hits the line. Because the line shares its ground with the nodes, the grounds must be within the common-mode limits of the components. If for some reason the line can have only two wires, the line can use a common earth ground instead of a ground wire as the return path. Figure 7-15D shows another partially isolated line. The line shares its ground with just one node, while all of the other nodes are isolated from the line. Because the line has a single ground connection, the common-mode voltage is small. 139 Chapter 7 A FULL ISOLATION nk rik B FULL EARTH-GROUND ISOLATION, FLOATING SUPPLIES C DATA ISOLATION, COMMON GROUND OPTOI SOLATED ^ LINK 3 E TRANSFORMER J £ ISOLATION nk EARTH GROUND ^ SIGNAL v GROUNDS Figure 7-15: RS-485 lines can use a variety of methods to isolate data and ground. 140 Designing RS-485 Links and Networks Using Multiple Buses A typical half-duplex RS-485 network has one pair of signal wires that connect to every node. Some networks can increase their capabilities or efficiency by using multiple RS-485 buses. Examples includes networks that have large numbers of nodes, span very long distances, or have a physical layout that benefits from a star topology. Adding a Repeater TIA-485-A says that a bus can have up to 32 unit loads. As Chapter 6 explained, if you need more than 32 transceivers, you can use transceivers that are less than a unit load each. Every node adds capacitance to the bus, however, and interface chips that are a fraction of a unit lo prom-electric.ru

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