Xylem H-4191 - Manual (Page 8)

1-6 Operation
H-4191
When monitoring traffic between a data recorder and its SDI-12 sensors, the display may
sometimes have spurious characters which prefix the command. For example:
p0M!00153
p0D0!0+4.56+0.0000+50.0
The lower case “p” in this example was caused by the break pulse generated by the data recorder.
A “break” is a 12mS or longer pulse on the data line which wakes the sensors from their low
power sleep mode. The serial asynchronous receiver transmitter (UART) in the H-4191
misinterprets the break as a random character.
1.5 Low Power Operation
The H-4191 normally reverts to a low power or “sleep” mode. If either the DSR or CTS RS-232
inputs are active (asserted) the H-4191 awakens, powers up its RS-232 transceiver and is
available for full operation. The red LED on the side of the enclosure is illuminated when the H-
4191 is awake. Most PC terminal programs automatically assert both DSR and CTS by default.
When the H-4191 is connected to a PC the H-4191 will normally awaken and remain awake as
long as it is connected.
1.6 Operation with a Radio
Applications often arise where a SDI-12 sensor must be physically located hundreds or thousands
of feet from the data logger. Transparent wireless SDI-12 bridges are available for these
applications. Unfortunately, transparent bridges can miss or drop measurements because of
corrupted radio packet transmissions. Wireless bridges are problematic because the SDI-12
protocol provides insufficient time to make retries or other recovery.
When coupled with a data radio, the H-4191 can be used to construct a “non transparent”
wireless SDI-12 link. With this architecture the H-4191 is permanently connected to one or more
sensors at the remote site. A pair of RS-232 data radios are installed between the data logger and
the remote H-4191. The data logger must be setup 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