User’s Guide ______________________________________________________________________
68 __________________________________________________________________ M211840EN-C
If external line termination is used, resistor range 100 ... 180
Ω
is
suitable for twisted pair lines. Resistors are connected across RX- to
RX+ and across TX- to TX+ (with RS-485 only one resistor needed).
Figure 35
Termination Jumper Positions
The termination resistors increase power consumption significantly
during data transmission. If low power consumption is a must, connect
a 0.1 uF capacitor in series with each external termination resistor or use
internal RC termination.
Note that the RS-485 interface can be used with four wires (as RS-422).
The main difference between the RS-485 and RS-422 interfaces is their
protocol:
-
In the RS-422 mode the transmitter is constantly enabled
-
In the RS-485 mode the transmitter is enabled only during
transmission (for allowing the host’s transmission in the two-wire
case).
The RS-232 output swings only between 0 ... +4.5 V. This is enough for
modern PC inputs. The recommended maximum for the RS-232 line
length is 100 m (300 ft) with 1200 Bd data rate. Higher rates require
shorter distance, for instance, 30 m (100 ft) with 9600 Bd.
1
=
NC, no termination
2
=
R, 121 ohm termination
3
=
RC, 121 ohm series with 4.7 nF capacitor termination
