Xylem H-4271 - Manual (Page 8)

H-4191 RS-232 to SDI-12 Interface
1.9 Repeater (Mesh)
The 9XTend module supports communication topologies which employ repeaters. When
configured as a repeater, a node re-sends RF data unless the transmission is addressed to it or if
the transmission has already been detected. A ‘repeater end node” handles repeated messages but
will not repeat the message over the air. Repeater topologies require careful design and setup of
the system. Repeaters will NOT work with Sleep & Sniff in that all nodes must be continually
powered . The recommenced setting is ATMD=5 (repeater). This setup allows all of the nodes
in the system be configured identically.
1.10 SDI-12 Gateway
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 H4271 data radio, a H-4191 module 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 such as the H-4271 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 of the H-
4191 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 back 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.
1-4 Introduction
H-4271