(Home — January 30, 2018) Hinwil, Zurich —
Once an RS485 network exceeds about 32 nodes on a network, serious consideration should be given to using galvanic isolation. Even though some IC manufacturers offer light loading devices, that can accommodate 256 or even 400 nodes on one RS485 network, you may NOT want to build such a network for a few reasons. One reason is that, large networks accumulate distributed electrical noise which can make communications unreliable. In general it is very important not to run communications wires in the same trough or conduit or in parallel with AC power cables. Maintain as much distance as possible and cross any power cable at a right angle. While shielding is not specified for RS485 systems, it can help in many instances. By “isolating” sections of a large network, the accumulated noise on one isolated leg is not so likely to cause a data error that will propagate to another leg of the network. Galvanic isolation will break a large problem into several small, but manageable ones. Galvanic isolation can also help eliminate “ground loops.”
Another potential problem with large networks without isolation is that severe damage can occur to your entire system, if a high voltage source is connected (accidentally or otherwise) to your communications lines. Your entire network could be damaged. With galvanic isolation the damage is generally limited to only one leg of the network, except in extreme cases of very high voltage (induced by lightening for example). While it goes against conventional wisdom, and can potentially cause a problem with circulating currents by grounding a shielded cable at both ends, this method is very effective at keeping induced lightening noise away from the communications lines. In the alternative, ground one end of the shield and connect the other end to ground through a bi-directional transient protector (from a few volts to a few hundred volts depending on the situation). Telbit AG provides an extensive line of telby TM Broadband Isolation Transformer, rated at 10kV, 15kV and 20kV which provide very high isolation. These products are extremely effective in applications involving industrial control, large RS485 networks, outdoor data links between buildings, etc. The advantage of Wire-Line Isolation Transformers is the fail safe operation; this is a passive component and does not require a power supply. Applications include locations where Critical (Must not Fail) Communications are the Priority.
In general RS485 is designed for multi-drop, “daisy-chain” operation over a single twisted pair cable with a nominal characteristic impedance of 120 Ohms. This cable is usually 24AWG.
Category-5 cable will generally work well in most instances even though its characteristic impedance is 100 Ohms. “Tap points” or “T” connections should be short to eliminate reflections. It is possible to connect several RS485 circuits in parallel if the distances are below about 200 feet per leg @ 9600bps. At greater distances and higher data rates, the cable impedances add up and load the network. In addition there is no good way to add terminations resistors at the ends of a “star” network. The combination of the cable impedances and/or termination resistors will load the network and can make communications unreliable.
About Telbit AG :: Telecommunication – Electronic
Telbit AG engineers and manufactures telecommunications equipment for high reliability and critical service applications, including communication protection for high voltage environments (GPR). For over 40 years, Telbit AG has been providing high voltage isolation products to protect telecommunications circuits entering substations, power generating plants and wireless communications facilities located within Zone of Influence (ZOI).