Specifications

12/16/97 4 - 1
WEATHERLINK
®
RADIO COMMUNICATIONS Application Note 4
INTRODUCTION
The purpose of this note is to provide an overview of the alternatives available for communication of
WeatherLink data and control commands between a field weather station and the base computer. The
emphasis is on radio communications.
COMMUNICATION OPTIONS
Because the WeatherLink bus uses standard RS-232 conventions operating at 1200 or 2400 Baud, half-
duplex, (see below for details) many communication modes may be used. The following four are presently
supported by Davis Instruments’ products:
Short-range Modem. A pair of Short-range Modems can transmit over a two-twisted-pairs cable for
distances of four miles or more, depending on wire gauge. The Davis model 7875 is an example.
Telephone (POTS). The conventional “plain old” telephone system may be used to “dial up” a weather
station from the base computer to transmit commands and receive data. Davis supplies a model 7870
Adapter to enable the connecting of the WeatherLink’s modular cable to the DB-25 connector of a standard
external telephone modem. The WeatherLink software supports entry of telephone numbers, automatic
scheduled dialing, and maintenance of databases for multiple stations.
Cellular Phone. A cellular phone modem-transceiver may be installed at the weather station, enabling it to
be called from any telephone for transferring of data. The Davis model 7652-003 includes an antenna and all
necessary components for connection of a Motorola CTM2400 3-Watt Cell-phone Transceiver to the
WeatherLink module. If the station is solar-powered it will be necessary to use a Timer, model 7690, or an
Alarm Output Module, model 7736, to control power to the transceiver. The Davis Instruments 7708/7711
solar-panel/battery combination provides sufficient power to support a small number of relatively short calls
per day under typical solar conditions.
Radio. Just about any radio modem/transceiver pair that can accept 1200- or 2400-Baud RS-232 data in a
data-only mode is suitable for use with WeatherLink data. The alternatives supported by Davis products and
the factors involved in selecting an approach are discussed below.
If there is a question regarding the selection or installation of communications equipment, we recommend
that the services of a communications consultant or technician be employed.
LINK CHANNEL DESCRIPTION
The WeatherLink data channel may be described as follows:
• Point-to-point, master-slave. The base station (computer with WeatherLink software) initiates all
communications. The GroWeatherLink, working with YDI or RF Neulink radios, can address a network
of field stations.
• RS-232, 1200 or 2400 Baud, switch-selectable.
• Half-duplex. The GroWeather and EnviroMonitor stations are true half-duplex. The Perception,
Wizard, and Monitor are designed to operate full-duplex, but they will operate on a half-duplex channel
if some limitations are accepted, the principal one being that if the user requests a long bulletin display it
cannot be interrupted. It must be allowed to complete.
• Data-only. No RTS/CTS handshake. 20 msec allowed for turn-time.
• CRC error-check. Data are error-checked in all systems. Control commands are error-checked by
GroWeather and EnviroMonitor stations.
• Power-conserving mode available.