Specifications

4 - 2 12/16/97
COMMUNICATION PROTOCOL
A data transfer comprises the following sequence:
1. The master (base) station sends a control command or request for data (6 to 12 bytes, including CRC).
2. The field station responds with a single character: CRC ERROR or ACKNOWLEDGE valid request.
3. If data were requested, the field station then sends the data.
Any one of six types of data transfer may be requested. Example transfers and their typical lengths:
a. Read Archive Memory 32 data bytes + 2 CRC
b. Memory Dump 256 bytes + 2 CRC
c. Archive Dump 32k bytes in XMODEM blocks (128 bytes data + header & CRC).
If data are transferred at least once per Archive Interval, a single good Read Archive transfer each time is
sufficient to convey all necessary data.
If power conservation is not an issue, the station may be requested to send data continuously for real-time
display of the “bulletin.”
SELECTING A RADIO
In selecting a radio communication channel, the factors to be considered include cost, whether the
transmitters will be licensed or unlicensed, the range required, the nature of the transmission path, and the
frequency of data downloads versus the power available.
UNLICENSED, USA
Spread-spectrum. The frequency band of 902 to 928 MHz is available for unlicensed transmission in the
U.S. and Canada. Power output must be less than 1 Watt; the maximum allowed antenna gain for a 1-Watt
transmitter is 6 dBm.
At this frequency the transmission path must be line-of-sight: a completely unobstructed path between the
antennas of the communicating radios. Even vegetation can affect the transmission. Hills or buildings will
block transmission.
The spread-spectrum radio specifically supported by Davis Instruments is the YDI model RM910-DAVIS.
The radio is available from YDI; Davis Instruments provides two kits, each of which includes an antenna
and all necessary cables, connectors, and mounting hardware, plus power supply and programming disk.
One kit includes a 3 dB omni-directional antenna, the other an 8.5 dB directional antenna. A 12 dB antenna
is available from YDI (Model 918-10); if this is used the 7632-912 Installation Kit should be purchased from
Davis. The higher-gain antennas are permitted because the output power of the transmitter is 20 mW.
Information regarding antenna choice, model numbers, and procurement of radios is given below.
The range of the YDI radio is one-half to four miles, depending on the antennas used, as discussed below.
Unlicensed spread-spectrum transmission is also permitted in the U.S. and Europe at 2.4 GHz.
Transmissions at this frequency are even more demanding in their requirement for a line-of-sight path.
Low Power. Other frequency bands are available for low-power (10 to 20 mW) unlicensed transmission.
Davis Instruments does not directly support any of these at this time.
LICENSED, USA
To prevent interference between radios, the Federal Communications Commission (FCC) requires that a
specific frequency be assigned and a license issued before a narrow-band higher-power transmitter is placed
in operation. The license process takes two to six weeks. The FCC fee is $60; a fee of $160 is required by
the Personal Communications Industry Association, which assigns a recommended frequency and submits