RS485 communication cable shielded communication twisted pair characteristic impedance 120 ohm foamed polyethylene insulation

RS-485 又名 TIA-485-A,ANSI/TIAVEIA-485 或 TIA/EIA-485.

Product specification: characteristic impedance 120 ohms, conductor is 2-wire to 4-core (2-x2x24AWG) multi-strand tinned copper wire, PE insulated sheath; In order to be suitable for complex industrial noise environment, aluminum foil/polyester composite tape shielding rate + tinned copper woven net 90% shielding rate is specially used for double shielding, with independent TC grounding conductor and industrial gray PVC outer sheath, which conforms to ULAWM2919 document specifications.

When high-speed, long-distance communication, or in a harsh industrial environment, the choice of transmission medium is very heavy. It is a common cable model for building automation, industrial control network, power automation and other communication networks.

The use of impedance matching RS485/422 special cable is conducive to reducing the echo reflection of the signal: its twisted pair symmetry, multiple isolation shielding, armor and other unique structures, through the correct grounding, improve the anti-interference performance of the channel: its high frequency and low attenuation characteristics, can realize the signal long-term transmission distance is larger!

Its flexible combination form, a variety of sheaths more applicable to different occasions.

A complete solution, in addition to listening to the user's voice, but also for the sake of users.

Twisted pair shielded cables with a sexual impedance of 120Q are widely used in RS485/422, CANBUS and other buses. This series of cables have many specifications. Please provide parameters such as cable laying environment, communication rate, zui large transmission distance without relay, etc. We will recommend zu appropriate products according to specific conditions. The general recommend are as follows:

Ordinary twisted pair shielded cable STP-120Q(forRS485 & CAN)one pair 20AWG, cable outer diameter about 7.7mm, blue sheath. Suitable for indoor, pipeline and general industrial environment. When in use, one end of the shielding layer is grounded!

The CC-Link bus cable is a 3-core stranded shielded cable with a characteristic impedance of 110 10Q. Domestic model and specification: STP-110Q(forCANopen & CC-Link). When in use, the shielding layer should only be grounded!

Product specification: characteristic impedance 120 ohms, conductor is 1 wire to 2 cores, 2 wires to 4 cores (2 x2x24AWG) multi-strand stranded tinned copper wire PE insulation sheath; In order to be suitable for complex industrial noise environment, aluminum foil/polyester composite tape shielding rate + tinned copper woven net 90% shielding rate are specially used for double shielding, with independent TC grounding conductor and industrial gray PVC outer sheath, which conforms to the ULAWM2919 document specification. Type 9842 communication cable is a standard product designed and produced strictly according to the RS-485 communication protocol specification. It is not only a designated model of Rockwell (AB) automation DH-485 control network, but also a standard 2-wire CAN bus cable.

RS485 is a standard that defines the electrical characteristics of drivers and receivers in balanced digital multipoint systems. The standard is defined by the Telecommunications Industry Association and the Electronics Industry Alliance. Digital communication networks using this standard can effectively transmit signals in long-distance conditions and in environments with high electronic noise. RS-485 enables the configuration of connecting local networks and multi-drop communication links.

RS485 has two-wire system and four-wire system two kinds of wiring, four-wire system can only achieve point-to-point communication, is rarely used, mostly using the two-wire system wiring, this wiring method for the bus topology, in the same bus can be connected to 32 nodes.

In the RS485 communication network is generally used in the master-slave communication mode, that is, a host with multiple slave. In many cases, when connecting RS-485 communication links, a pair of twisted pairs is simply used to connect the "A" and "B" ends of each interface, while ignoring the connection of signal ground. This connection method can work normally in many occasions, but it has buried a great hidden danger. Reason 1 is common mode interference: RS-485 interface uses differential mode to transmit signals, there is no need to detect signals relative to a certain reference point. The system only needs to detect the potential difference between the two wires. However, it is easy to ignore that the transceiver has a certain common mode voltage range. RS-485 the transceiver has a common mode voltage range of -7 to 12V, the whole network can work normally only if the above conditions are met. When the common mode voltage in the network line exceeds this range, the stability and reliability of communication will be affected, even the second reason for the damage to the interface is the problem of EM1: the common mode part of the output signal of the sending driver needs a return path. If there is no low-resistance return channel (signal ground), it will return to the source end in the form of radiation, and the whole bus will radiate electromagnetic waves outward like a huge antenna.

Ordinary twisted pair can be used in low-speed, short-distance and non-interference situations. On the contrary, in high-speed and long-line transmission, impedance-matched (generally 120Q) RS485 special cables (STP-120Q (for RS485 & CAN) a pair of 18AWG) must be used, while armored twisted pair shielded cables (ASTP-120Q (for RS485 & CAN) a pair of 18AWG) should also be used in harsh interference environments.

When using the RS485 interface, for a specific transmission line, the large cable length allowed for data signal transmission from the RS485 interface to the load is inversely proportional to the baud rate of the signal transmission. This length data is mainly affected by factors such as signal distortion and noise. In theory, when the communication rate is 100Kbps and below, the transmission distance of RS485 can reach 1200 meters, but the transmission distance in practical application is also different due to the transmission characteristics of chips and cables. In the transmission process, the signal can be amplified by increasing the relay, and eight relays can be added, which means that the transmission distance of RS485 can reach 10.8 kilometers in theory. If long-distance transmission is really needed, optical fiber can be used as the propagation medium, and a photoelectric converter can be added at each end of the transceiver. The transmission distance of multi-mode optical fiber is 5 to 10 kilometers, while the transmission distance of single-mode optical fiber can reach 50 kilometers.

We put the industrial network into three categories: RS485 network, HART network and fieldbus network.

HART network: HART is a transitional bus standard proposed by Emerson. It mainly superimposes digital signals on 4~20 mA current signals. The physical layer adopts BELL202 frequency shift keying technology to realize the functions of some intelligent instruments. However, this protocol is not a real open standard.

FieldBus fieldbus network: Fieldbus is one of the hot technologies in the field of automation today, and is known as the computer local area network in the field of automation. Its appearance marks the beginning of a new era of automatic control technology. The field bus is a digital, serial, multi-station communication network that connects the instruments that control the field and the control devices in the control room. Its key symbol is to support two-way, multi-node, bus-type all-digital communication. Fieldbus technology has become a hot spot in the development of automation and instrumentation in the world. Its appearance has revolutionized the structure of the traditional control system, making the automatic control system move forward in the direction of "intelligence, digitization, informatization, networking and decentralization", and forming a new type of fully distributed control system for network communication-fieldbus control system FCS(FieldbusControlSystem). However, the fieldbus has not yet formed a truly unified standard, ProfiBus, CANbus, CC-Link and other standards exist in parallel, and have their own living space. when reunification will take place is a long way off. The types of instruments supporting the field bus are still relatively small, the room for choice is small, the price is high, and the amount is small.

RS485 network: RS485/MODBUS is a popular way of network, the implementation is simple and convenient, and there are many instruments supporting RS485.

The network topology generally adopts a bus-type structure with terminal matching. When building a network, you should pay attention to the following points:

(1) A twisted pair cable is used as the bus to connect each node in series. The length of the outgoing line from the bus to each node should be as short as possible so that the reflected signal in the outgoing line has a low impact on the bus signal. Although some network connections are incorrect, they may still work normally at short distances and low speeds, but with the extension of the communication distance or the improvement of the communication rate, the adverse effects will become more and more serious. The main reason is that the signal is reflected at the end of each branch and superimposed with the original signal, which will cause the signal quality to decline.

(2) Pay attention to the continuity of the bus characteristic impedance, and the reflection of the signal will occur at the impedance discontinuity. This discontinuity is easy to occur in the following situations: different cables are used in different sections of the bus, or too many transceivers are installed close together on a certain section of the bus, and moreover, too long branch lines are led out to the bus. In short, a single, continuous signal path should be provided as a bus.

(3) Pay attention to the problem of terminal load resistance. The whole network can work well without terminal load resistance under the condition of short distance of equipment, but the performance will decrease with the increase of distance. In theory, when sampling at the midpoint of each received data signal, matching can be disregarded as long as the reflected signal is attenuated sufficiently low at the beginning of sampling. However, this is difficult to grasp in practice. An article by MAXIM Company in the United States mentioned an empirical principle that can be used to determine the data rate and cable length that need to be matched: when the conversion time (rise or fall time) of the signal exceeds 3 times the time required for one-way transmission of the electrical signal along the bus, it can not be matched.

The general terminal matching adopts the terminal resistance method, and the RS-485 should be connected in parallel with the terminal resistance at the beginning and end of the bus cable. The termination resistance takes 120Ω in the RS-485 network. It is equivalent to the resistance of the cable characteristic impedance, because the characteristic impedance of most twisted pair cables is about 100 ~ 120Q. This matching method is simple and effective, but there is a disadvantage that the matching resistor consumes a large amount of power, which is not suitable for systems with strict power consumption restrictions. Another relatively power-saving matching manner is RC matching. Using a capacitor C to cut off the DC component can save most of the power. However, the value of the capacitor C is a difficult point, which requires a compromise between power consumption and matching quality. There is also a matching method using diodes. Although this scheme does not achieve real "matching", it uses the clamping effect of diodes to quickly weaken the reflected signal and achieve the purpose of improving signal quality.