Technical data

www.westermo.com Theoretical and general applications 31
Installation of RS-422 and RS-485
General recommendations for installation
… Twisted pair wire should be used.
… Star networks are not permitted and distance from the bus to the device
must be a maximum of 30 cm (1ft).
… Receivers at the end of the bus are to be terminated with a 120 ohm
resistor.
… The RS-232/V.24 connection should not be longer than 15 metres
(50 ft).
… RS-422/485-supports transmission ranges up to 1200 m (4000 ft) at 100
kbit/s. However, great ranges can be achieved at lower transmission rates.
Range and short-haul modems
As mentioned earlier, the RS-232/V.24 standard does not recommend cabling longer
than approximately 15 metres (50 ft). Short-haul modems are used to allow longer
links to be made. These convert the RS-232/V.24 into defined electrical or optical sig-
nals, which are transmitted on e.g. a permanent 4-wire connection or fibre up to a dis-
tance of several kilometres. The short-haul modem at the receiver then converts the
signal back to RS-232/V.24. The modem must use a common standard and an identical
interface for communications over the cable.
20 mA current loop (TTY)
Current Loop is the oldest technique. RS-232/V.24 signals are coded onto a
20 mA current loop as the absence or presence of a current on a wire pair.
The transmitter is either connected active and the receiver passive, or vice
versa, to feed each wire pair with current. Current Loop provides reliable com-
munications, but is relatively sensitive to interference as the current loop is not bal-
anced (see page 40). In addition, problems can be experienced with the equipment as
there is no recognized standard for Current Loop.
+5 V
0 V
B
A
Tristate
T+
T -
R+
R -
R+
R -
T+
T -
20 mA
20 mA
Start bit Data bits Parity bit Stop bit
B A C K