Xylem H-4271 - Manual (Page 7)

1.7 Point to Point
By default the modems operate in a transparent mode. The modems act as a serial line
replacement. If multiple modems are installed, data transmitted by one modem is received by all
of the other modems within range. In this mode the standby (receive) power is 65mA typ. This
mode is often used with SCADA applications where AC power is available and power is not of
concern.
1.8 Sleep and Sniff
Sleep & Sniff is a power management scheme normally used for solar panel powered systems.
With Sleep & Sniff one modem is configured as a Master and is connected to the data logger.
One or more modems are configured as Slaves and are connected to the remote sensing sites.
With Sleep & Sniff both the master and slave station(s) normally operate in a low power state.
The slave stations periodically awaken and listen (sniff) for a beacon signal. In this mode the
slaves only draw 65mA for a few milliseconds every 4-seconds or so. The Sleep & Sniff interval
is programmable depending on the desired power profile and system response time. The Master
is configured to awaken via “serial port sleep” when it detects an inbound message from the data
logger. The master prefixes the transmission with a beacon signal which is detectable by the
slave modems. After sending the beacon for a (programmable) period, the master transmits the
data packet. The recommended setup configures the slaves with cyclic sleep = 4.0 seconds and
the Master wake-up initializer = 10 seconds. With this setup the slaves will have sufficient time
for two attempts to detect the beacon.
The protocol for Sleep & Sniff is implemented by the 9XTend module and requires no
intervention by the data logger other than the data logger must be patient. The first response
from the remote site will require 10 seconds or more. An inactivity timer setting can be used to
keep both the master and slaves awake for subsequent transmissions without the overhead of
sending a beacon for every transmission. Sleep & Sniff is workable with multiple slave stations.
However, the communication protocol must include an in-band addressing scheme (such as SDI-
12) where only one slave will attempt to respond to a specific message.
With Sleep&Sniff operation a fault condition can occur is where the slave fails to detect the
beacon. If subsequent retry messages are sent they will also fail because the slave is asleep. The
data logger should issue retries by either waiting for the “time before sleep” inactivity timer to
expire before sending a retry or by forcing the retried message to be sent with a beacon. This can
be done by using the ATFH (force wake-up initializer) command. See Chapter 2 for a driver
which implements retries.
H-4271
Introduction 1-3