Cabo modbus rtu
Wrong in the sense to determine the value of terminating resistors now requires measurements and calculations. If they quote an impedance per meter/foot you have chosen the wrong kind. If they quote a number such a 100Ohms you have a good cable. A clue to knowing if you selected one is to look at the cables Nominal Impedance. Some cables are designed so that the impedance is relatively independent of distance. Cable selection does make a difference.Īll cables offer impedance (resistance). Another big culprit are Variable Frequency Drives.ĥ. People are often surprised to find that the worst source of induced noise are switching DC loads. It seems obvious not to wind your cable around other cables or sources of electricity/magnetism. The twisted pair used for Tx and Rx is more effective at noise cancellation than the shield.Ĥ. Always connect the ground reference conductor first if you are connecting a device that is powered up or you are connecting your laptop to an operating network.Īlways choose devices that have optical isolation this almost always will protect the RS485 transmitter/receivers.ģ. You can (if you must) use a shield drain wire as the 3rd conductor (ground reference conductor)Ģ. The voltage level on the Tx/Rx conductors are measured relative to this voltage level. Remember this statement: The so-called Ground Terminal on an RS485 interface is not a connection to ground. The more power sources used to power devices, the greater the physical separation of devices, the less well-grounded devices, and power sources are the greater the risk.
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For a more detailed discussion read this article. You risk blowing 485 ports, you risk unstable operation (works sometimes and doesn't work other times) and finally, you risk re-installation. You take a huge risk by not installing the 3rd conductor. What he means is that RS485 is often non-trivial and getting a network working can rely more on experience and experimentation.ġ. It is his reference to Art that makes RS485 bad. Search the Internet on RS485 you will find Bob Perrins's article called THE ART AND SCIENCE OF RS-485.
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