Weak current College】 【RS485 data bus lightning introduction】.

<br> 1 RS485 bus application areas, industrial control, data acquisition system DCS highway toll system for process control and manufacturing power system acquisition and control system for remote terminal interconnection <br> 2. lightning over-voltage protection of necessity due to the introduction .of long distance transport RS485 bus (1200 meters above the sea level), and its transmission lines are often exposed to outdoor and therefore extremely vulnerable because of the introduction of lightning and overvoltage. The RS485 transceiver operating voltage is low (5V or so), .its own pressure is very low (-7V to + 12V), once the pressure the introduction, the breakdown is damaged. The strong surge energy appears, you can even see the transceiver burst, circuit board jiaohu phenomenon. <br> 3. protection .methods and principles <br> The above is the RS485 bus circuit diagram of a two-level protection. When lightning occurs, the induced voltage by T and R-introduction of a common defence, G1.G2, G3 for differential mode protection, overvoltage .is significantly reduced to approximately 500V, after the current limiting resistor R1R2, TVS1/2/3 second time limit, to the transceiver's voltage is clamped around in 6.8V, enabling the transceiver. Model (G1/G2/G3) Vs DC open voltage .(100V / S) insulation resistance Ir (DC100V) electrostatic capacitance (1KHZ) flow energy (10/700us) GS41-181N 120V ~ 240V ≥ 100 M Ω ≤ 2PF 4KV/500A4. programme selection and contrast G1/G2/G3 R1 / .R2 TVS1/2/3 comparison programme a GS41-181N 10 Ω / 1 W P6KE6.8CA small / middle / low cost protection scheme 2 3R090 10 Ω / 1 W P6KE6.8CA volume / high / high cost protection programme three P0640 10 Ω ./ 1 W P6KE6.8CA small / protection in low / costs <br> 5. knowledge quiz <br> Question: selection of the various devices is based on the? a: G1G2G3 choice first consider its ability to withstand resistance to flow. If .GS41-181N can withstand 10/700us, 4KV lightning test; 90V Ceramic tube (3R090) can accommodate 10/700us, 8KV lightning test; 64V solid pipe (P0640) can only take 10/700us, 3KV lightning test. R1R2 can choose current .limiting best PTC resistance of high-cost or low-cost wire wound resistors. After the actual test, the scenario of wire-wound resistors select 10 Ω / 1 W, low prices, the effect is good; you may use the PTC 10 euro .cent, 200 ~ 300mA, voltage withstand 600V ceramic or polymer thermistor. TVS1/2/3 to select according to chip voltage and pressure resistance, generally slightly higher than the maximum working voltage chip. <br> Q: overvoltage protection standards are based on .the? a: IEC61000-4-5, ITU-T K20/K21 and GB are about GB9043 lightning surge immunity test standards. Its communication lines of the highest test standard is 10/700us, 4KV. 10/700us for communication lines of mine .in the induction voltage waveform, represents a zero value rose to a peak time of 10us dropped from a peak of $ 700us half. <br> Q: lightning over-voltage protection earthing requirements? a: lightning surge protection in addition to the need to .select high-quality protective devices for good circuit board design, the ground is the most important requirements. General lightning and are necessary for reliable connection to the Earth, and Earth resistance not more than 10 euro. Reliable grounding can greatly enhance the protective effect, .but bad ground will greatly weakening the protection effect. <br> Q: in order to reduce the cost and size, can only use one level protection? a:. Can withstand great energy lighting devices (such as RS485) is not possible once the .lightning strike voltage clamp to the chip can bear. TVS while you can clamp voltage a lightning strike to the chip can bear, but can't bear large lightning energy, therefore must be two-level protection. <br> Q: RS232, RS422 .and RS485 of protection are the differences? a: protection methods are identical. Only in accordance with its operating voltage, fine voltage protection devices TVS should choose different parameters. Such as RS232 Max voltage 15v, TVS selection is P6KE18CA (18V), RS422 maximum .working voltage of 12 v, TVS selection is P6KE15CA (15V). Q: RS232, RS422, RS485 difference <br> 1, RS-232-C <br> RS-232-C is United States electronic industries Association of EIA (Electronic .Industry Association) developed a serial physical interface standard. RS is in English "recommendation", 232-identification number, c for change frequently. RS-232-C bus standard equipped with 25 of signal lines, including a main channel and a secondary channel .. <br> In most cases, mainly using the main channel, duplex communication in General, you only need a few signal lines can be achieved, such as a send line, one receive line and a ground wire. <br> RS-232 .-C standards for data transmission rate is per second, 50, 75, 100, 150, 300, 600, 1200, 2400, 4800, 9600, 19200 Porter. <br> RS-232-C standard requirements drive allows 2500pF of capacitive load ., communication distance will be affected by this capacitance limits, for example, the use of communication 150pF / m cable, maximum communication distance is 15m; if every metre cable capacity decreases, the communication distance can be increased. Transmission distance short another reason is that .in the case of single-ended signal transmission RS-232, there is a total and noise and common mode interference cannot be suppressed, so typically used within 20m communications. <br> 2, RS-485 <br> RS-485 bus, .in theRequires communication distance of dozens of meters to km, widely adopted RS-485 serial bus <br> RS-485 adopts balanced send and receive, and therefore have differential inhibition of common mode interference. Coupled with the bus transceiver with high sensitivity, detection .of low-voltage 200mV, transmitting signals can be restored at km away. <br> RS-485 using half-duplex working mode, any time you can only have one point in the sending State, therefore, send circuit shall be determined by the .signal control. <br> RS-485 for multipoint interconnect handy, you can save a lot of signal lines. Application constitutes interconnection RS-485 can distributed system, which allows up to 32 drives in parallel, and 32 single sink. <br> .3, RS-422 <br> RS422, RS485, and RS422 bus circuit is essentially the same, to differential mode to send and receive, without the need for digital earth. <br> Differential work is the same rate conditions transmission distance of root .cause, this is both the fundamental difference with RS232, because it is single-ended input output RS232, duplex jobs at least digital earth. Send and accept line three lines (asynchronous transfer), can also be combined with other control features such as line .-complete synchronization. <br> RS422 by two pairs twisted pair transceiver can be full-duplex jobs do not affect each other, but only half duplex RS485, hair collection cannot simultaneously, but it only needs a pair. <br> RS422 and RS485 .transmission can next in 19kpbs 1200 m. Use new transceiver line can be connected to the device..