RS485 communication to the terminal?
In the RS485 (RS485 converter) during the process of constructing network another idea problem is the terminal load resistance, less equipment in shor...
In the RS485 (RS485 converter) during the process of constructing network another idea problem is the terminal load resistance, less equipment in short distance under the condition of not adding terminal load resistance of the whole network can be a very good work (i.e., generally below 300 meters without terminating resistance), but with the distance and load quantity increased performance will decrease.
General terminal matching method using terminal resistance, RS-485 bus lines should be in the beginning and the end and the terminal resistance. Terminating resistance in the RS-485 network 120. Resistance equal to the characteristic impedance of the cable, because most twisted pair cable characteristic impedance in about 100 ~ 120. This matching method is simple and effective, but there is a drawback, matching resistance to high power consumption, more stringent restrictions on the power system is not suitable for. In addition to a more efficient way of matching the RC matching. Use of a capacitor C can partition the DC component to save most of the power. But the value of capacitance C is a difficult, power consumption and the need to match the quality of a compromise between. There is a diode using the matching method, this scheme is not real "match", but its use diode clamp can quickly weaken the reflected signal, to achieve the purpose of improving the signal quality, remarkable energy saving effect.
Terminal resistance is in order to eliminate the communication cable in the reflection of the signal in the communication process, there are two kinds of signals caused by the reflection of the signal: impedance discontinuities and impedance mismatch.
Impedance discontinuities, signal in the transmission line impedance of the cable end suddenly encountered little or no, signal caused by reflection in this place will. The principle of signal reflection, and light into another from one medium to cause reflection is similar. Method for eliminating this reflex, must cross terminal resistor is connected with a characteristic impedance and cable of the same size in the end of the cable, the cable impedance continuity. Since the transmission of signals on the cable is two-way, therefore, at the other end of communication cable can be connected across the same size of a terminal resistor.
The other reason is the impedance between the signal reflection data transceiver with transmission cables do not match. Reflecting this causes, mainly in the idle mode in communication lines, the entire network data confusion.
To weaken the influence of the reflected signal lines of communication, the methods of noise suppression and bias resistor is usually adopted. In practical application, the reflected signal is relatively small, simple and convenient method, often used with bias resistors.
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