Xylem H-4401 - Manual (Page 3)

Chapter 1
Introduction
H-4401
SDI-12 Bridge 1-1
1.0 Introduction
The W ATER LOG ® H-4401 is a wireless “DAA” SDI-12 bridge. The wireless bridge is ideal where
one or more SDI-12 sensors must be physically located hundreds or thousands of feet from the
data logger. The radio link is easy to use and provides SDI-12 communication between a
W
ATER
LOG ® XL data recorder and one or more remote sites. All SDI-12 commands are
supported, including manufacturer specific extended commands. The radio system includes a
“sleep and sniff” feature which provides low power operation for use with battery operated
gauging stations. No device addresses or setup is required, the master broadcasts to all slaves at
the same time, addressing is done by the individual SDI-12 sensors connected to the system.
The radio link uses modern direct sequence spread spectrum telemetry radios which operate in the
license-free 900 MHz ISM band. Spread spectrum technology is highly secure and has good
interference immunity.
1.1 Architecture
Wireless bridges are problematic in that the SDI-12 protocol was originally designed for direct
wired connections and has no provisions for additional latency inherent to radio communications.
Design Analysis Associates manufactures two types of wireless SDI-12 bridges. The H-424 is a
transparent bridge which works with any SDI-12 data logger. It works by forcing the data logger
to issue retries while simultaneously communicating with the remote sensors. This scheme
normally provides sufficient time to send one data packet and collect the sensor response.
Unfortunately, the transparent bridge can drop measurements because of corrupted radio packet
transmissions. If a packet is missed, the retry scheme provides insufficient time for the radio to
resend the packet.
The H-4401 is a non-transparent bridge which works only with W
ATER
LOG ® XL data loggers.
Two H-4401 components are required; a H-4401M (master) is connected to a RS-232 serial port
on the data logger (instead of the SDI-12 port). One or more H-4401S (slave) units are located
at the remote sensor sites. The-H-4401 bridge works by sending and receiving SDI-12
communications as ASCII messages, without the timing restrictions of the SDI-12 protocol. The
H-4401M has a 9600 baud packet radio. The H-4401S has both a packet radio and a H-4191
RS-232 to SDI-12 interface. The H-4191 provides the RS-232 to SDI-12 electrical interface,
generates the critical bus timing, parity and command retries needed for the SDI-12 bus protocol..
1.2 Low-power Operation
The radios used in the H-4401 are programmed for “sleep & sniff” low power operation. The
master radio is configured for “pin wake-up” and awakes from its low power sleep state when
transmit data is available. When the master radio is awakened it transmits a 4.5 second beacon
before sending the data packet. The slave radios are configured for “cyclic sleep”. The slave
radios awake every 4-seconds and “listen” for several milliseconds. When a slave radio detects