Technical data

+5 V 0 V
R+
R-
30 Theoretical and general applications www.westermo.com
Termination and Fail-Safe
The line should be terminated using a resistor that has the same value as the charac-
teristic impedance for the line. This resistance should be approximately 120 ohm.
Termination should be applied as shown in the diagrams on page 29. Termination
should be made at each end of the bus. Termination prevents reflections in the cable.
“Fail-safe” is a resistance from each wire to the + supply on the one hand, and to the
0V on the other. This means the line is drawn to a predetermined passive level, other-
wise the line will fluctuate with the risk of disturbances being detected as data.
Polarity
The interconnection of the transmitter and receiver must be done with the right
polarity, in relation to each other. By connecting equipment from different suppliers we
know from experience that standards can be interpreted differently. A polarity error in
relation to other equipment means that this equipment will interpret the data incor-
rectly. According to the standard, the transmitter is designated by
A and B, these are connected to A´ and
B´. We have chosen to clarify these desig-
nation with T+, T–, R+ and R-
(transmit/receive + and –).
RS-232/V.24 to RS-422/485 converter – RTS support
Systems with RS-422/485 converters in a multidrop network, only permit one trans-
mitter at a time to be active on the bus. Other devices’ transmitters must be in “tri-
state” mode, i.e. passive. In order to achieve this, it must be possible to control con-
nected equipment by means of a hardware signal. RTS or DTR signals are usually used
for this. When a device wishes to transmit on the bus it must first set its RTS or DTR
signal high, so that the converter switches its transmitter, it can then send data. When
no hardware signal is available, it is possible to use a special converter, which switches
on its transmitter as soon as data is sent via RS-232 and switches off as soon as the
data stops.
A (T+) A' (R+)
B (T–) B' (R–)
B A C K