There is nothing different in the schematic diagram, except that all slave stations are connected in parallel on the bus. However, in the actual wiring, daisy chain adopts the mode of one in and one out at the equipment end, which requires a short bifurcation.
If it is on a straight line, if the straight line is very long, the bifurcation position of the bifurcation solution reaches the device. It is another topology, which will affect communication.
RS485 only supports daisy-chain topology, and does not support any branching topology.
Further reading: Why RS485 does not support arbitrary topology?
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RS485 does not support arbitrary topology, mainly because arbitrary topology will produce a large number of standing waves and reflections at the branches. It is mainly caused by impedance discontinuity.
1. Discontinuous impedance: When the signal suddenly meets the cable impedance at the end of the transmission line, the signal will be reflected at this place. The principle of this signal reflection is similar to the reflection caused by light entering from one medium to another. In order to eliminate this reflection, it is necessary to bridge a terminal resistor at the end of the cable, which is as large as the characteristic impedance of the cable, so that the impedance of the cable is continuous. Because the signal transmission on the cable is bidirectional, a terminal resistor of the same size can be bridged at the other end of the communication cable.
2. Impedance mismatch: Another reason for signal reflection is impedance mismatch between data transceiver and transmission cable. In high frequency circuits, when the frequency of the signal is high, the wavelength of the signal is very short. When the wave length is short enough to match the length of the transmission line, the shape of the original signal will change when the reflected signal is superimposed on the original signal. If the characteristic impedance of the transmission line does not match the load impedance, the load end will reflect. The reflection caused by this reason is mainly manifested in the confusion of data in the whole network when the communication line is in idle mode.
When the signal is transmitted to the end of the bus, the instantaneous impedance will suddenly change (taking RSM485ECHT as an example, the impedance will change from120 Ω to 96kΩ), which will lead to the reflection of the signal and affect the quality of the signal.
In the case of long power consumption and communication distance, it is still recommended to choose buses that support any topology, such as PowerBus and MBUS.