RS-485 Line Design Recommendations

  1. RS-485 interface is a widely used high-speed and noise-proof industrial serial interface for data transfer. Almost all the modern computers in industrial version, most digital sensors and actuators, programmable logic controllers together with the traditional RS-232 interface utilize a certain implementation of RS-485 interface.

    A twisted-pair with ≈120 Ohm impedance is used as communication line.

  2. On long-distance lines (>100 m), maximum length hardly depends on the number of blocks and even increases with the increase of the sensor number. For anti-interference protection, the shielded (casing) twisted pair should be grounded at any point, but only once: this excludes high current flow in the shield because of ground potential inequality. Choosing the cable grounding point is not regulated by any standard, but typically the communication line shield should be grounded at one of its ends. Recommended cable parameters
    Length (m) Rate (kbps)
    200 230.4
    400 115.2
    800 57.6
    1,600 38.4 and less
  3. Stable operation of RS-485 network requires proper coordination (against impedance), correct screening and correct line topology. Physically, the RS-485 network topology is of bus-type (see the figure below):
    Физически топология сети RS-485 — шинная

    Bus topology must be as compact as possible for bus structure so that to minimize the reflections.

  4. The long branches (stubs) from the backbone to peripheral devices are not allowed, length must not exceed 10 cm. The connection as shown on the figure is recommended:
    Длинные ответвления (шлейфы) от магистрали до периферийных устройств не допускаются