Rosemount 3491, Reference Manual, Page: 45 / 152

Reference Manual
00809-0100-4841, Rev. AA
May 2007
4-13
Rosemount 3490 Series
Setting up Input Channel 1 for a 4-20mA input
This section is applicable if a transmitter with a 4-20mA output is connected.
Models 3491 and 3493 support the connection of a
single 4-20mA transmitter
Do not connect a second 4-20mA transmitter.
It is important at this stage to ensure that the units of the 3490’s PV are set
correctly. Navigate now to parameter
P200 and select the required units.
NOTE:
The "Duty" Wizard is recommended for configuring an application after P111
is configured.
Menu: SETUP / [CONTROL UNIT /] DUTY(Mode) / Duty Wizard
To configure parameter P111 for analogue measurements, do the following:
1. Navigate to the Ch1 I/P Source menu for parameter P111.
2. Select "mA in 1" from the option list.
Figure 4-15 on page 4-15 illustrates an overview of the data flow through
Input Channel 1.
Analog 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 a 3490 Series Control Unit 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%).
P112 is a +ve or -ve % input offset for optimal adjustment of the normalised
percentage i.e. D802 = D842 + P112. 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.
P114 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 P114 will change the signal from a % value to a
new scaled value, typically in level units where:
D801 = (D802/100) * P114
The value of D801 is the Secondary Value (SV) of the 3490.
The level measurement can be converted into a content (or flow)
measurement using a selected profile calculation (P113). The programming
for various supported profiles can be found in Section “Profile Calculations for
Contents and Flow Applications“ on page 4-24.
P117 is a low cut-off parameter that allows the user to force channel 1 output
(D851) to be zero when the calculated value falls below a user defined value: