Xylem H-3521 - Manual (Page 18)

2-8 Installation
H-3521
Example of a H-3521 Fluid Extended "Set Current Stage" command:
Command
Response Time Values Description
"aXSCS2.3!" "a0061" 6sec 1 Set the Stage to 2.3
Subsequent Command Response Description
"aD0" a+12.80 The new Offset
2.10.2 4-20mA Output
The H-3521 has a 12-bit digital-to-analog converter (DAC), precision voltage reference and a 4-
20mA current transmitter. The SDI-12 and 4-20mA sections are isolated from each other with a
high voltage digital opto-coupler. The Stage is scaled into a 12-bit value and loaded into the
digital-to-analog converter to control the current transmitter. The 4-20mA output is updated
whenever a measurement is made. If no measurements are made, the 4-20mA output becomes
“stale”. For industrial applications where the H-3521 is connected to a SCADA or PLC system
instead of a SDI-12 data logger, the H-3521 can be programmed to automatically make
continuous measurements. To program the H-3521 to make continuous measurements, issue an
extended SDI-12 command and set Power_Mode=3. See Chapter 4 for details. This setting can
be made at the factory if requested.
Note: When the H-3521 is first powered up, the output current is set to 4.0mA. It remains at
4.0mA until the first measurement sequence. The digital-to-analog converter is powered from
the loop side of the opto-isolator. If the loop power is disconnected or is applied after the SDI-12
side is powered up, the data in the digital-to-analog converter will be lost. When the loop power
is restored, the 4-20mA output will be at an unknown value. Once a fresh SDI-12 measurement
is made, the digital-to-analog converter is loaded with new valid data.
2.10.3 Programming the 4-20mA Output Range
The H-3521 scales the current Stage data to drive the 4-20mA output. The 4-20mA_Hi and 4-
20mA_Lo
settings control how the Stage data is processed. The 4-20mA_Lo should be set to the
desired Stage corresponding a 4.00mA output. The 4-20mA_Hi should be set to the desired Stage
corresponding to a 20.00mA output. For testing purposes, the H-3521 comes from the factory
with 4-20mA_Hi = 20.0 and 4-20mA_Lo = 4.00. See Chapter 4 for details on programming these
settings. The extended “aXS!” command allows convenient testing of the 4-20mA output. This
command allows the user to temporarily force the Stage to a test value. For example, the user
can force the Stage to several different values while calibrating or monitoring the attached 4-
20mA instrumentation. Once a fresh measurement is made via a SDI-12 measurement , the
temporary Stage data is overridden.