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Selection and wiring method of RS485 / RS232 cable

written words: [big] [middle] [small] QR code of mobile phone page 2019/9/18     Number of visits:     

In low-speed, short distance, non-interference places, general twisted pair can be selected. On the contrary, when transmitting on high-speed, medium and long-distance lines, it is necessary to select RS485 special cable (stp-120 Ω (for RS485 & can) one pair 18 AWG) with impedance matching (usually 120 Ω). In the natural environment with extreme interference, the armored cable type twisted pair shielded cable (astp-120 Ω (for RS485 & CAN) one pair 18 AWG)。 In the application of RS485 socket, according to the special transmission route, the large cable length allowed for data signal transmission from RS485 socket to load is inversely proportional to the baud rate of data signal transmission. The key to the long and short statistical data is the harm of data signal frame loss and noise. In theory, when the communication speed is 100kpbs and below, the maximum transmission distance of RS485 can reach 1200 meters. However, in practical application, the transmission distance is also different due to the transmission characteristics of integrated IC and cable. In the whole process of transmission, the way of lifting relay can be selected to expand the data signal, and the maximum number of relays can be 8. In other words, in theory, the larger transmission distance of RS485 can generally exceed 9.6 axiom. If long-distance transmission is really necessary, optical fiber line can be selected as the spreading material, and a photoelectric converter is added on both sides of the receiver. The transmission distance of multimode optical fiber is 5 ~ 10 km, while the single-mode optical fiber reaches 50 km. The network topology usually adopts the system bus structure of terminal equipment pairing, which is not compatible with ring or Star Internet.

When building a network, we should pay attention to the following aspects:


(1) This twisted pair cable is selected as the system bus, and each connection point is connected in pairs. The length of the outgoing line from the system bus to each connection point should be as short as possible, so as to minimize the harm of the reflective surface data signal in the outgoing line to the system bus data signal. Although some data connections are wrong, they will still work normally in short distance and low speed. However, with the increase of communication distance or communication speed, the negative impact will be more and more serious. The key reason is that the data signal accumulates with the original data signal behind the reflection surface at the end of each branch, which will lead to the degradation of data signal quality.
(2) Attention should be paid to the continuity of the system bus impedance, where the characteristic impedance does not persist, the reflecting surface of the data signal will be generated. The following types of conditions are easy to cause such discontinuities: different cables are selected for different sections of the system bus, or too many transceivers are installed close to each other on a certain section of the system bus, followed by too long branch lines to find out the system bus.

In other words, it is necessary to show this single, continuous data signal security channel as the system bus.


In the whole process of RS485 networking scheme, another problem that must be considered is the load resistance problem of terminal equipment. In the case of few machines and short spacing, all the Internet can work stably without load resistance of terminal equipment, but with the increase of distance, the characteristic will be reduced. If the signal attenuation of each center is not taken into account, it is necessary to start sampling at the beginning. However, this is not sure in fact. An article by Maxim in the UK mentioned that this empirical standard can be used to distinguish which statistical data speed and cable length must be paired: when the transformation time (rise or fall time) of data signal exceeds about 3 times of the time required for electronic signal to be transmitted along the system bus, pairing is not required.


Generally, terminal device resistor is selected for terminal equipment pairing. RS-485 should be connected with terminal device resistor at the beginning and end of system bus cable. end

120 Ω in RS-485 Internet. A resistor that is equal to the cable impedance, since most of the twisted pair cable impedance is about 100-120 Ω. This kind of matching method is simple and reasonable, but one defect is that the matching resistor consumes a lot of output power, so the system software with strict power limit is not suitable. Another energy-saving pairing method is RC pairing. Most of the output power can be saved by using a capacitor C to decorate and isolate the DC component. But the value of capacitor C is a difficult problem, which must be carried out well between power and matching quality. There is also such a matching method of selecting the secondary tube. Although this kind of scheme does not maintain the real "matching", it can quickly weaken the data signal of the reflecting surface by using the clamping effect of the secondary tube, exceeding the goal of improving the quality of the data signal, and the environmental protection and energy saving effect is remarkable.

In recent two years, according to the implementation of some enterprises' technological innovation, some enterprises have already paved the intranet which has been extended to each company's office and main control room in the production workshop, and issued serial server to replace multiple serial port cards. The key is to use the company's existing intranet resources to reduce route project investment and save costs, which is equivalent to placing multiple serial port cards according to TCP / IP On the spot.

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