Ground Wire for RS485 or Not? > 자유게시판

• 목록
• 답변
• 글쓰기
• 게시판 리스트 옵션
  • 수정
  • 삭제

In order to avoid errors when many devices are connected, cables of the same colour should be used for all the connections to the terminals A and cables of the same colour should be used for all the connections to the terminals B of the various devices (e.g. white for A and blue for B). For higher speeds and longer lines, the termination resistances are necessary on both ends of the line to eliminate reflections. To avoid reflections on longer cables it is necessary to use appropriate termination resitors. This has been done to avoid reflections of signals. Differential signals and twisting allows RS485 to communicate over much longer communication distances than achievable with RS232. If high noise immunity is needed, often a combination of twisting and shielding is used as for example in STP, shielded twisted pair and FTP, foiled twisted pair networking cables. We also see that the maximum allowed voltage levels for all interfaces are in the same range, but that the signal level is lower for the faster interfaces.Because of this RS485 and the others can be used in situations with a severe ground level shift of several volts, where at the same time high bit rates are possible because the transition between logical 0 and logical 1 is only a few hundred millivolts.Interesting is, that RS232 is the only interface capable of full duplex communication.

Well, either you know that your grounds are at the same voltage due to other connections, or you provide a ground connection for a reference along with the data wires. Grounds between buildings may vary by a small voltage, but with very low impedance and hence the possibility of catastrophic currents - enough to melt signal cables, PCB traces, and transceiver devices. However, if the differences in signal grounds is too great, further attention is necessary. With such an implementation of a RS485 network it is necessary that there is error detection implemented in the higher level protocol to detect the data corruption and resend the information at a later time.There is no need for the senders to explicity turn the RS485 driver on or off. DH 485 is a proprietary communications protocol used by Allen-Bradley in their line of industrial control units. That is why RS485 is currently a widely used communication interface in data acquisition and control applications where multiple nodes communicate with each other. Currently available high-resistance RS485 inputs allow this number to be expanded to 256.RS485 repeaters are also available which make it possible to increase the number of nodes to several thousands, spanning multiple kilometers.And that with an interface which does not require intelligent network hardware: the implementation on the software side is not much more difficult than with RS232.

There are other implementations of RS485 networks where every node can start a data session on its own. Because there is a chance of data collosion with this implementation, theory tells us that in this case only 37% of the bandwidth will be effectively used. We will concentrate on the RS485 interface here. RS232 is an interface to connect one DTE, data terminal equipment to one DCE, data communication equipment at a maximum speed of 20 kbps with a maximum cable length of 50 feet. The maximum slew rate also limits the maximum communication speed on the line.For both other interfaces-RS422 and RS485-the slew rate is indefinite. First of all we see that the speed of the differential interfaces RS422 and RS485 is far superior to the single ended versions RS232 and RS423.We also see that there is a maximum slew rate defined for both RS232 and RS423. Noise is easily picked up and limits both the maximum distance and communication speed.With RS485 on the contrary there is no such thing as a common zero as a signal reference. Because of this, the resulting noise current is many factors lower than with an ordinary straight cable.Shielding-which is a common method to prevent noise in RS232 lines-tries to keep hostile magnetic fields away from the signal lines.

The transmitter and receiver compare the voltages of the data- and handshake lines with one common zero line.Shifts in the ground level can have disastrous effects. Ideally, the two ends of the cable will have a termination resistor connected across the two wires. If it were a local one then there would be two different ground symbols. If distance between router and master inverter 80m, then use extender or repeater. In most higher level protocols, one of the nodes is defined as a master which sends queries or commands over the RS485 bus. Depending of the information in the sent data, zero or more nodes on the line respond to the master. N nodes are connected in a multipoint RS485 network. If a star configuration is unavoidable, special RS-485 repeaters are available which bidirectionally listen for data on each span and then retransmit the data onto all other spans. However then I run the risk of a ground loop which I'm not sure how to remove. SC, G or reference, the common signal reference ground used by the receiver to measure the A and B voltages. In the straight cable, all noise current is flowing in the same direction, practically generating a looping current just like in an ordinary transformer.When the cable is twisted, we see that in some parts of the signal lines the direction of the noise current is the oposite from the current in other parts of the cable.

If you loved this report and you would like to get additional facts regarding what is rs485 cable kindly pay a visit to the web-page.