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RS485 – the basics |
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Two wires are used (known as A & B), and arranged as a “twisted
pair”. This arrangement greatly reduces both radiated EMI and susceptibility to
received EMI. The twisted pair will have an overall screen that is earthed. RS485 is a “balanced” system, the electrical signal on each cable is ideally the exact opposite of the other – when one is high, the other is low. The twisted pair cable will have what is known as a “characteristic impedance” the standard for RS485 state that ideally this should be 120 ohms. In addition, the standards suggest that a terminating resistor should be added at the ends of each network. The terminating resistor should ideally match the characteristic impedance of the cable. In practice a 124 ohm resistor is normally employed. This resistor is wired across A & B and effectively provides a current path to remove unwanted reflections. If these signal reflections are not trapped they will distort the communication signals and could ultimately cause the network to fail. All RFM1 and RFM 4 have provision for a termination resistor to be fitted. Where we are configuring our cabling as a loop then a single termination resistor would be fitted to the RFM at the notional mid-point of the loop. The limit of 247 units on a single network is due to 2 factors 1) The electrical loading of each node – initially the standard defined that 32 nodes should be allowed on a network. With component development the RS485 driver chips became ½ node, ¼ node, and then 1/8 node load. 32 x 8 = 256 nodes 2) Modus standard defining that node addresses having to be in the range 1 to 247. We reserve any ident numbers above 247 for calibration and factory / Commissioning tests. These are not accessible to the user or on a network in normal operation. |
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