Reference Manual
00809-0100-4841, Rev. AA
May 2007
Rosemount 3490 Series
4-16
Setting up Input Channel 2 for a 4-20mA input (model 3492 only)
Model 3492 supports the connection of a single 4-20mA transmitter. Do not
connect a second 4-20mA transmitter.
This section is applicable if a transmitter with a 4-20mA output is connected
and supplying the same analog measurements to both input channels; this
may be appropriate in some applications, perhaps where an offset is used.
It is important at this stage to ensure that the units of the 3490’s PV are set
correctly. Navigate now to P200 and select the required units.
NOTE:
The "Duty" Wizard is recommended for configuring an application after P121
is configured.
Menu: SETUP / [CONTROL UNIT /] DUTY(Mode) / Duty Wizard
To configure parameter P121 for analogue measurements, do the following:
1. Navigate to the Ch2 I/P Source menu for parameter P121.
2. Select "mA in 1" from the option list.
Figure 4-16 on page 4-18 illustrates an overview of the data flow through
Input Channel 2.
Analogue measurements are presented to the 3490 Series Control Unit as a
4-20mA signal (mA input), and pass through a conditioning (to remove spikes)
and damping stage. The mA input damping can be set using P321.
The signal is then checked to ensure it is within the expected range (3.7mA to
20.75mA). Signals outside this range cause an alarm condition, which may be
externally signalled using the 3490’s relay - see P545 in Section “Alarm
indication selection“ on page 4-60.
By default, the mA signal (readable on D840) is normalised into a percentage
in the range 0 - 100% (readable on D842), where 4mA is 0% and 20mA is
100% (although in practise the 3490 Series Control Unit can process mA
input values in the range 3.8mA to 20.5mA (-1.25% to +103.125%).
P122 is a +ve or -ve % input offset for optimal adjustment of the normalised
percentage i.e. D802 = D842 + P122. As an example, this feature may be
used to accommodate a standing value of level in a tank. The output is
readable on D802 and is the Tertiary Value (TV) of the 3490 Series Control
Unit.
P124 is not applied when the PV units (see P200) are configured to be "%"
i.e. D801 = D802.
If the 3490’s PV units have been changed from a percentage (see P200), the
4-20mA signal is still normalised into a percentage as shown on D802.
However, a value entered into P124 will change the signal from a % value to a
new scaled value, typically in level units where:
D801 = (D802/100) * P124
D801 is the Secondary Value (SV) of the 3490 Series Control Unit.
The level measurement can be converted into a content (or flow)
measurement by using a selected profile calculation (P123). The
programming for various supported profiles can be found in Section “Profile
Calculations for Contents and Flow Applications“ on page 4-24.
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