Satel SATEL-EASy+ YM6010, Руководство пользователя, Страница: 21 / 85

SATEL-EASy+
User Manual
Version 1.19
20
5.3 RS-485 interface
RS-485 is an extension of the RS-422 standard and enables the connection of more than two
devices on to the same bus. Communication is half-duplex so there is only one cable pair
compared to two when using the RS-422. One cable pair carry both, transmitter and receiver
signals, but not at the same time. The RS-485 standard defines the electrical characteristics of
the connections to prevent possible data contention states as well as cable shorts etc. from
harming the devices themselves. RS485 can be set on by altering the user settings.
5.4 Termination of RS-422/485 lines
Each differential pair of wires is a transmission line. A transmission line must be terminated
properly to prevent, or at least minimise, harmful reflections formed between the transmitting
and receiving end of the transmission line. A common method of terminating RS-485 type of
transmission line is to connect so-called termination resistor between the wires at both ends of
the transmission line. Even when there are more than two devices on the same transmission line
the termination resistors are needed only at the ends of the transmission line. The termination
resistor must be selected so that its resistance matches the characteristic impedance of the
transmission line as close as possible (typical values range from 100 to 120 Ohm). When using an
RS-422 type of connection the termination resistor is connected only at each of the receiving
ends. Termination resistors are particularly important when using long transmission lines
and/or high data transfer speeds.