Xylem H-424MS V1.1 - Manual (Page 5)

Chapter 1
Operation
1.1 Introduction
The W ATER LOG ® H-424MS is a spread spectrum SDI-12 radio bridge. The radio link is easy to
use and works with any SDI-12 data recorder. The radio link provides transparent, SDI-12
compliant communication between your data recorder and one or more remote sites. All SDI-12
commands are supported, including manufacturer specific extended commands.
Two or more H-424MS modules are required; one connected to the data recorder (master) and
one located at each remote site (slaves). Each remote site can have multiple SDI-12 sensors.
The master and slave H-424MS modules are identical, a user accessible circuit board jumper
determines whether the H-424MS operates as a master or slave.
The radio link uses modern frequency hopping spread spectrum telemetry radios. Spread
spectrum technology is highly secure and has good interference immunity. The radio operates in
the license-free, 900 Mhz ISM Band eliminating the need for any FCC licensing.
The radio system has an optional power-save feature which allows operation with less than 2mA
of average standby battery current.
1.2 Operation
Each H-424MS contains a small microprocessor and a spread spectrum telemetry radio. The
microprocessor buffers commands and data between the SDI-12 port and the radio. All SDI-12
compliant commands will work, including manufacturer specific extended commands. Note
however, your data logger must support SDI-12 compliant “retries” to work with the H-424MS.
Most data loggers support retries while logging but certain models do not provide retries for
transparent commands. Transparent SDI-12 commands will not work with these data loggers.
A circuit board jumper selects either full power or power-save operation. When jumpered for
full power operation, each H-424MS draws approximately 100mA continuous. The full power
mode has the advantage of providing completely transparent SDI-12 communication with no
waiting for the remote sites to power-up.
Unfortunately, for power save operation the SDI-12 protocol has no handshake mechanism for
delaying the timing of the sensor response. This makes it difficult to implement a radio link
which has remote stations operating in a low power sleep mode. When operated in the power
save mode, the H-424MS radio link uses the wakeup scheme described in the next section.
H-424MS
Operation 1-1