Westermo TR-23 - Handbuch (Seite 8)

8 6600-2002
Modem description
The TD-23 modem is designed for communication on 2- or 4-wire copper cable (twisted pair)
using the V.23 standard. Multidrop and point-to-point configurations are possible. The TD-23
will support half duplex on a 2-wire system and full duplex using 4-wire at baud rates up to 1200
bit/s
The TD-23 is designed with a wide range of applications in mind and is consequently very
versatile with DIP-switches allowing easy configuration under different operating conditions.
The TD-23 has a RS-232 interface (DCE) and a RS-422/485 interface. The RS-232 connection
can be made through either the DB-9 connector or screw terminal on the face of the unit.
When using the RS-232 interface the control of the line is normally done using the RTS
handshake signal. If the DTE equipment does not generate RTS or when using
RS-485 (RS-485 uses no control signals) the TD-23 can be set to detect incoming data and
adjust the direction of communication accordingly. In this mode the data is buffered in the
modem for the fraction of a second it takes to change the transmission direction.
When connecting in multidrop configurations the maximum number of units that can be
connected on the bus and the maximum bus length depends on the line quality.
A typical value is 16 units 25 km apart.
The TD-23 can communicate with other equipment conforming to the V.23 mode 2 standard,
but to obtain optimal performance we recommend that all the devices on the network are TD-
23’s.
Both the transmission level and the carrier detect level can be set on DIP-switches allowing
the modems to be optimized for different operating conditions.
When there is no data to transmit the modems transmitter is disconnected from the line. This
means that loading on the line is kept to a minimum enabling large numbers of modems to be
connected together. TD-23 is also equipped with an activity timer that will disconnect the
transmitter after a predetermined time (enable with DIP-switch).
This feature eliminates the risk of one DTE transmitter blocking the line in the event
of an RTU problem.
The typical application for the TD-23 is a multidrop network where long distance
communication is required over a 2-wire circuit. Another use is to extend the
RS-422/485 bus over the 1200 m stipulated in the RS-422/485 standard.
The TD-23 can be used in traffic control installations, multidrop remote telemetry systems
and to drive radio systems.
The TD-23 is tested and approved to European standards CTR-15 (2-wire leased line) and
CTR-17 (4-wire leased line). On leased lines (supplied by telecoms providers) the above
standards limit the maximum output level to –9dBm but for private lines the output can be set
up to 3dBm which allows long distance communication despite poor
line quality.
The modem has a potential free contact integrated. When the line is activated by the modem,
i.e. when data is transmitted on the line side, this contact is closed.
The contact is typically when TD-23 is used together with radio modem. When the power
supply is connected via this potential free contact the radio modem receives power only when
data is transmitted.
DTE=Data Terminal Equipment (PC, workstation etc.)
DCE=Data Communication Equipment (modem)