Little Known Facts About What Is Rs485 Cable - And Why They Matter > 자유게시판

I think you may be able to add a few more devices and still have a fully operational network depending on the load presented by the terminating resistors. The combination of the cable impedances and/or termination resistors will load the network and can make communications unreliable. We will pull the enable pin high to put the MAX485 module in receiver mode. 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. Let’s talk about what we now know about RS485.

Network topology is probably the reason why RS485 is now the favorite of the four mentioned interfaces in data acquisition and control applications. You may be wondering, what can I do with serial communications, especially RS485. It is a half duplex protocol, meaning that only one party at a time may transmit data. The advantage of such a multi-drop RS-232 network is that the communications are full duplex, with each communicating party capable of simultaneous transmission and reception of data. The RS485 standard addresses the problem of data transmission, where a balanced (differential) transmission line is used in a multi-drop (party line) configuration (or point-to-point if only two devices are on the network). RS485 can handle up to 32 connected devices. Modbus is restricted to addressing 247 devices on one data link, which limits the number of field devices that may be connected to a parent station (again, Ethernet TCP/IP is an exception). Another advantage over RS232 that I mentioned earlier is the number of connectable devices. What made it such an advancement over RS232 was the fact that it could transmit not only a single device to device transmission, but also a communications bus to connect multiple devices at once.

Differential signals and twisting allows RS485 to communicate over much longer communication distances than achievable with RS232. This is possible since the HMI is connected to your PLC via RS485. A simple network of a PLC, VFD, and an HMI allows remote control of motors in an industrial setting. This makes utilizing RS485 simple. With your RS485 network, you can remotely monitor and control that pump from a control center. Let’s say you were monitoring a water pump filling a vessel. The PLC is programmed to monitor the level of the water in the vessel. You decide that the water is not filling fast enough to keep up with demand. 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. We recommend that you keep the faster Serial1 port as the default serial link as you work through the exercises in this book. While the default baud rate of the primary serial port is 9600 baud, you can speed your communications and download times appreciably by switching to a faster baud rate.

The maximum slew rate also limits the maximum communication speed on the line. The SPI can transfer data much more rapidly than an asynchronous serial link - its maximum rate is 2 Megabits/second. For both other interfaces-RS422 and RS485-the slew rate is indefinite. We also see that there is a maximum slew rate defined for both RS232 and RS423. The maximum RS485 cable length is typically listed as 1200 meters or about 4000 feet. Just like it’s older brother RS232, RS485 is a form of serial communication. Much like RS232, RS485 doesn’t have an actual standard connector. Twisted pairs in RS485 communication however adds immunity which is a much better way to fight noise. The picture shows the magnetic field lines and the noise current in the RS485 data lines that is the result of that magnetic field. Think of RS485 as the younger, faster brother of RS232. RS232 allows both communicating parties to transmit and receive data at the same time; this is referred to as full duplex communications. Although the devices would share the same network, communications would only be understandable by members of the same group. Some vendors may make chips which can drive more but they can't push the standard too far or it may present an electrical risk to the other devices.

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