RS-422 can support 10 nodes, and RS-485 supports 32 nodes, so multiple nodes form a network. The network topology generally adopts a terminal-matched bus type structure and does not support a ring or star network. When building a network, you should pay attention to the following points:
1. A twisted pair cable is used as the bus, and the nodes are connected in series. The length of the lead line from the bus to each node should be as short as possible so that the reflected signal in the lead line has the least influence on the bus signal. Figure 8 shows some of the common connection methods (a, c, e) and the correct connection methods (b, d, f) that are common in practical applications. Although the three network connections a, c, and e may not work properly, they may work normally at short distances and low speeds. However, as the communication distance increases or the communication rate increases, the adverse effects will become more and more serious. The signal is superimposed on the end of each branch and superimposed on the original signal, which will cause the signal quality to drop.
2. Attention should be paid to the continuity of the characteristic impedance of the bus, and the reflection of the signal occurs at the point where the impedance is discontinuous. This kind of discontinuity is easy to occur in the following cases: different sections of the bus use different cables, or too many transceivers on a certain bus are installed close together, and too long branch lines are led out to the bus.
In summary, a single, continuous signal path should be provided as the bus.
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