Xylem System 5000 User Manual - Manual (Page 76)

OUTPUTS
74
This is the minimum value to expect from the Task’s Input. For example, if a temperature probe
was being measured and had a range of -50 to +50 degrees, the Max level would be set to 50
and the Min level set to -50. With this setup, a temperature of -50 would cause a milliamp
output of 4.00 milliamps and +50 degrees would produce a 20.0 milliamp output.
Min
This is the maximum value to expect from the Task’s Input.
Note that as the range is enlarged the resolution of the output current is decreased, and the
opposite if the range is reduced. The resolution is calculated as:
(mA Output Range) / (Input Range) = mA per Input Unit Change.
In the above example, presuming the temperature probe produces only integer values:
(20 - 4) / (50 - (-50)) = 16/100 = 0.16mA per degree change.
The resolution of the output signal can also be calculated based on the resolution of the DAC
(digital to analog converter). As the DAC has a resolution of 16 bits, it is 1 part in 65536 (2^16),
allowing a very accurate current to be produced based on the measured Input.
Input Range / 65536 = Resolution
In this example: 100 / 65536 = 0.001526 degrees.
Max
The auto-update mode updates the current in the 4-20mA loop every 1.0 second. If Auto-Update
is turned Off, the current will only be updated when the 4-20mA Output is run (each scan).
Generally, this will only be an issue when external power is not maintained on the current loop.
Auto-Update is enabled by default.
Auto-Update
Defines what the output current should be if it were measured with a current meter. This is based
on the value of the data source, the Max value and the Min value.
If a current meter is used to verify the output and it does not match this value, check the wiring
and voltage levels to verify they are ok. If the current meter reads zero milliamps, the wires
could be connected backwards or there is no power source for the loop. Remember that this
output controls the current in the loop but does not provide the power for the loop. An external
power source is required.
If the current displayed on a meter is in the range of 4 to 20 milliamps but is less than what is
displayed by this option, it may be the external power source is not big enough. In most cases a
12 volt battery will not be sufficient to power the loop; a 24 volt battery is required. This reading
and the actual current value may track at lower values but as the current approaches the 20.0
milliamp level the actual current as displayed on an external meter will start to drop off,
indicating the battery voltage is too low.
4-20mA Output