Chapter 4 _______________________________________________________________ Installation
VAISALA _______________________________________________________________________ 85
Multipoint Modem Connection
If a multipoint connection (several pieces of equipment on the same line)
is used, the only available connection standard is 300 bps V.21. In this
configuration, the data collection computer or the weather station acts as
a master that polls the LT31 Transmissometers and the other possible
sensors.
The master modem must be in the Originate mode. All other modems on
the line must be in the Answer mode and the carrier signal must be
disconnected by default. In this mode the modem turns the carrier signal
on only when it gets the appropriate poll signal and turns it off when the
message has been sent. If there are several transmissometers on the line,
they must have different identifiers. For details, see Chapter 5, Operation,
on page 145.
When using the multipoint modem connection mode, have the limited
transfer capability of the 300 bps V.21 mode in mind. For an example,
see the following:
- LT31 message 2 (option; includes present weather data) is configured.
- Polling interval is 15 seconds.
A maximum number of three transmissometers can be used on one line.
Installation Procedure
The installation of LT31 has several separate procedures which are
described in the following sections. The procedures should be followed
in the given order to make installation as easy as possible.
NOTE
Direct sunlight into the LT31 transmissometer light transmitter included
in LTM112 and light receiver included in LTM212 should be avoided.
In case this recommendation cannot be followed due to the runway setup
that dictates an LT31 orientation where sunlight can unavoidably enter
one of the LT31 measurement units for a few days of the year during
sunrise or sunset, it is recommended that the LT31 receiver unit LTR111
is installed pointing towards the low sun.
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