RS 485 is a symmetrical (balanced) two-wire transmission system: information is transferred by the voltage difference between the two conductors and not by the voltage of one conductor against frame-ground or earth. Symmetrical two-wire lines are much less sensitive to interference than single-wire or asymmetric lines. RS 485 is a multidrop connection, i.e. numerous stations can send and receive on the same cable (but only one station can send at any one time!).
RS 485 transmission technology
All stations on our PROFIBUS are connected to the bus with high impedance in their idle state. Receiving stations put little load on the bus and transmitting stations, when they are not active, go into a high impedance state (tri-state).
The bus cable can therefore accept any choice of voltage. This can lead to undefined states or even to the destruction of individual driver modules. To prevent this from happening, active bus terminations are used. The bus termination takes the form of a voltage divider and ensures an idle level of approx. 1 V as the bias voltage on the cable. For reliable PROFIBUS operation, at least one termination must have a supply. For greater ease in project planning, both terminations are implemented as active terminations.
Schematics of RS485
Connector assignment allows for the removal of these supply signals in the connector, so that the active termination can be included directly in the cable plug. When the plug is withdrawn from the device, the termination stays on the cable and the bus therefore remains terminated, but loses one of its two supplies.
If it is not possible to guarantee that at least one supply is active during runtime, a separate supply for the bus termination should be provided. Suitable, active bus terminations are available on the market as separate devices.