Rosemount 3491, Reference Manual, Page: 53 / 152

Reference Manual
00809-0100-4841, Rev. AA
May 2007
4-21
Rosemount 3490 Series
Setting up Channel 2 for a HART input (Model 3492 only)
This section is applicable only if a second HART transmitter is connected to
model 3492.
NOTE:
The "Duty" Wizard is recommended for configuring an application.
Menu: SETUP / [CONTROL UNIT /] DUTY(Mode) / Duty Wizard
The HART transmitter digitally communicates pre-calculated values of the
four variables (PV, SV, TV and FC) to the 3490 Series Control Unit. The
values are received continuously by the 3490 Series Control Unit and stored
in parameters D900 to D903. Parameter P121 nominates one of these four
variables to be the source for the PV value of Input Channel 2.
Figure 4-18 on page 4-23 illustrates an overview of the data flow through
Input Channel 2.
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 of the 3490 Series Control Unit.
Where pre-calculated content or flow values are coming from the transmitter,
P123 need only be set to "scaled". Parameter P124 can be used only to
adjust the value into alternative units that match the display units you may
have changed (P200).
Where level values are coming in from the HART transmitter, P123 can be set
to "scaled" if wanting just level measurement, otherwise P123 can be set to a
profile for generating content or flow values. For the programming details
involving various supported profiles, refer to Section “Profile Calculations for
Contents and Flow Applications“ on page 4-24.
Parameter P150 determines how corresponding values from Channels 1 and
2 are processed - sum, difference or product calculation - before being output
to parameters D800 - D803 ('Answers'). P150 can also allow values from
Channel 1 to go straight to D800 - D803; the factory default is for Input
Channel 1 ("Ch1") to do this.
P127 is a low cut-off parameter that allows the user to force the channel 2
output (D852) to be zero when the calculated value falls below a user defined
value:
Table 4-6. P127 Options
This feature is particularly useful in an Open Channel Flow application (OCF)
where a small standing or remaining liquid level in the channel would cause
continued totalising of flow when no actual flow exists. For example, set P127
to a +ve (positive) value of flow units, usually around 2% of maximum flow, to
overcome this problem.
P210 allows the user to apply damping to the calculated value of D852.
P127 Channel 2 Output Action
+ve value Drop immediately to zero.
None Continue measuring.
0.0 Hold output at zero.
-ve value Rise immediately to zero.