RS485: more on Transmission Line Termination & Material + Reality Check > 자유게시판

Note that conceptually the termination should happen at the end of the TML (there are two of them) and is in no way correlated to the actual location of the fieldbus protocol master (or any other bus node for that matter). It supports maximum data rates at a length of 1200 meters, making the RS485 protocol an excellent choice for communication between physically distant pieces of equipment. With RS485 communication distances of 1200 m are possible.Differential signal lines also allow higher bit rates than possible with non-differential connections. This is, because on the other interfaces the communication channel is shared by multiple receivers and-in the case of RS485-by multiple senders.RS232 has a separate communication line for transmitting and receiving which-with a well written protocol-allows higher effective data rates at the same bit rate than the other interfaces. RS485 will support 32 drivers and 32 receivers (we are talking about bi-directional - half duplex - multi-drop communications over a single or dual twisted pair cable !!).

Because of this, the resulting noise current is many factors lower than with an ordinary straight cable. Be the first to get exclusive content straight to your email. 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. Shielding-which is a common method to prevent noise in RS232 lines-tries to keep hostile magnetic fields away from the signal lines. 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. Therefore RS485 will be in use for many years in the future. 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. Again the amount of loss depends of the values of the series and parallel resistance and the loss will increase with frequency.

But, it should be noted that the network running in the reference was running at 300 baud and had other tweaks such as isolated transceivers and high resistance bias resistors on every node. This problem can also be fixed by adding bias resistors to force the line to idle condition when a driver is not connected, but this has to be done on a network basis, not a device basis. Therefore RS485 can overcome the practical communication speed limit of RS232. RS232 signals are represented by voltage levels with respect to system common (power ground). This has been done to avoid reflections of signals. To avoid reflections on longer cables it is necessary to use appropriate termination resitors. 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. There are many cables available meeting the recommendations of RS-422 and RS-485, made specifically for that application.

RS-422 specifies a number of other things, but they are all electrical values. This means that the drivers are able to tolerate short-circuit conditions and continue functioning properly. This means that since only one transmission direction is available, only one device can send data. Our company, Chongqing Blue Jay Electronic digital meters can show 50 digitals in one second with high precision and strong stability. Default, all the senders on the RS485 bus are in tri-state with high impedance. In this situation, bandwidth can be used for almost 100%.There are other implementations of RS485 networks where every node can start a data session on its own. This is comparable with the way ethernet networks function. The designer has to define the logic function of the driver and receiver. For an off, mark or logic 1 state, the driver's A terminal is negative relative to the B terminal. The RS-232 standard title is "Interface Between Data Terminal Equipment and Data Circuit Terminating Equipment Employing Serial Binary Data Interchange". RS232, RS423, RS422, and RS485 are few popular serial communication protocols widely used across the globe.

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