Xylem H-4191 - Manual (Page 9)

H-4191
Operation 1-7
process the service request. When the service request actually arrives, the keep-awake timer is
zeroed. While the keep-awake timer is running the H-4191 sends a null (00h) to the radio
once/second to keep the radio link awake while waiting for the service request.
The radios used in this architecture are normally programmed for “sleep & sniff” low power
operation. When properly configured, the data logger, radios and H-4191 module can provide a
low-power wireless SDI-12 bridge capable of performing retries.
1.7 Sensor “Z”
When implementing a wireless SDI-12 bridge as described in the previous section it is useful for
the data logger to be capable of monitoring the battery voltage in the remote station. In addition
to functioning as a RS-232 to SDI-12 gateway, the H-4191 has a built-in “sensor” which responds
to SDI-12 address “Z”. The “Z” sensor can measure the H-4191's +12V input voltage, is used to
facilitate turning flow control On or Off and is useful for testing and maintenance. The “Z” sensor
responds to all SDI-12 compliant commands, however, it is only accessible via the RS-232 port.
The built-in sensor will not respond to a data logger connected to the SDI-12 port. The sensor
address can be changed from “Z” to another value if needed with the extended
Change_Address_Command. See Section-2 for further details.
1.8 RS-232 Hardware Flow Control
The RS-232 port of the H-4191 supports optional RTS/CTS hardware flow control for slow
PLC’s or RTU’s which cannot input or receive the entire SDI–12 response message. The H-4191
monitors its CTS input and transmits RS-232 characters only if CTS is asserted. The H-4191 can
buffer up to 60 SDI-12 response characters, any additional input characters will be lost.
While the H-4191 is busy sending a SDI-12 command or doing retries, it can buffer up to 32
command characters from the RS-232 port. The H-4191 de-asserts the RTS output when more
than 28 characters are queued in its host input buffer.
The H-4191 comes from the factory with the RTS/CTS hardware flow control set to Off. An
extended SDI-12 command allows the hardware flow control feature to be turned On or Off.
The On/Off setting is stored in EEPROM internal to the H-4191 and remains in effect even if the
power is lost. See Chapter 2 for details.