Xylem H-4401 - Manual (Page 4)

1-2 SDI-12 Bridge
H-4401
the wake-up beacon it remains awake to receive the subsequent data packet.
1.3 Operation
With the H-4401 non-transparent architecture the data logger is configured to initiate and collect
SDI-12 commands/responses from a RS-232 port instead of its normal SDI-12 port. SDI-12
commands from the data logger are forwarded via the radios to the remote H-4191. When the
remote data radio receives an inbound data packet, it pulses the CTS input which awakens the H-
4191 in preparation to receive the inbound message. When the H-4191 detects the “!” character,
it transmits the contents of its buffer to the remote SDI-12 bus in a contiguous frame with proper
parity and bus timing. The H-4191 waits for and collects any sensor response and forwards it to
the radio. The sensor response is sent over the radios to the data logger. If one or more of the
radio transmissions is lost or corrupted, the data logger can retry the entire sequence as needed.
The H-4191 has an internal inactivity timer which keeps the module awake to process the service
request and receive subsequent radio packets. If both the RS-232 and SDI-12 ports become
inactive (and both CTS and RTS are not asserted) for longer than 10-seconds, the timer expires
and the H-4191 enters its low power sleep mode. The H-4191 examines the
“aTTTN” sensor response initiated by an “aM!” measure command and initializes a
second “keep-awake” timer to TTT + 3-seconds. This ensures the H-4191 will remain awake to
process the service request. When the service request actually arrives, the keep-awake timer is
zeroed. While the keep-awake timer is running the H-4191 sends a null (00h) to the radio
once/second to keep the radio link awake while waiting for the service request.
1.4 Installation
The H-4401 may be used with a 50-ohm, 900 Mhz antenna providing the effective radiated power
is 6dB or less. The antenna connector is a reverse type-TNC connector which is required to
meet FCC part 15.203 regulations. A simple vertical whip antenna will work up to 2 or 3 miles.
Longer distances require a directional antenna with gain. A 6dB or 9dB YAGI will work fine, do
not use a 12dB model. At 900 MHz, RG58 type coaxial antenna feed lines have excessive loss
(1.8dB/10 feet). Consider using newer low loss RF cable such as Times Microwave LMR-400 or
similar (0.8dB/10 feet). The LMR-400 requires special RG8 connectors.
The H-4401 is packaged in a weather tight enclosure and is best mounted as close to the antenna
as possible. The H-4401 has a ground lug which should be connected to a good earth ground
with a heavy copper wire. The ground helps provides lightning protection for the radio, your
sensors and the data logger.
The H-4401 radios all operate at the same frequency. If multiple data logger sites are deployed,
make certain the radios of each separate logging site cannot “hear” each other. The radios can be
ordered with alternate identification codes, please contact the factory for further information.