Xylem H-3521 - Manual (Page 15)

H-3521
Installation 2-5
2.6.1 SDI-12 Interface
The user must connect pins 5,6, and 7 to the data recorder or the user can power the H-3521
separate of the data recorder and connect just the SDI-12 data line(pin 7).
2.6.2 RS-232 Output
When the user wakes up the H-3521 with this port the H-3521 makes a new measurement and
updates the outputs (if enabled) and then prints out the measured stage and temperature value. If
the user sends a carriage return(CR) following the wake up then the H-3521 simple menu
interface is initiated and the H-3521 waits for a command. After 3 minutes of inactivity the
menu mode times out and the H-3521 returns to sleep.
2.6.3 4-20mA Output
Current loop sensors output a current rather than a voltage. The 4-20mA output will drive
standard industrial telemetry and process control instrumentation. Since the signal to noise
margin of 4-20mA is not large, take care to protect the wiring from noise and interference. The
loop power supply must be sufficient to maintain 8.5 to 35V across the H-3521's output
terminals, in addition to whatever voltage is needed to maintain 20mA across the loop receiver
and interconnect wiring. The +12.0V SDI-12 power source will work only if the resistance of
your loop receiver and wiring is less than 150 ohms.
8.5V + (150ohms * 20mA) = 11.5V
The 4-20mA output is reverse diode protected. The H-3521 is not loop powered, continuous
+12V instrument power must be supplied.
! Make certain there is 8.5 to 35V across the 4-20mA output terminals.
! Make certain the H-3521 is receiving +12V power.
! Use shielded 4-20mA cables in noisy environments.
2.6.4 Modbus Interface
Modbus is an industry standard serial digital interface for interconnecting Programmable Logic
Controllers (PLCs), intelligent sensors and other devices. The H-3521 is a Modbus slave and has
a serial RS-485 port for connecting to a Modbus compatible host device. See Appendix B for the
Modbus register definitions. Continuous +12Volt power must be supplied to the H-3521.
To activate the Modbus interface, issue an extended SDI-12 command or use the RS-232 menu
interface and set Power_Mode=4. See section 4/5 for details. This setting causes the H-3521 to
remain awake and monitor the RS-485 port for Modbus messages. This setting can be made at
the factory if requested.