Emerson Bristol 2808 Series, Instruction Manual

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Appendix B Page 1 Loop Powered Indicator
Section 1
INTRODUCTION
1.1 INTRODUCTION
The loop powered indicator (LPI) option is used to provide local indication in engineering
units of the measurand represented by a 4-20 mA current loop. The LPI may be installed in
a Model 2808 Transmitter with the 2808 Display Cover Assembly or in a stand-alone
housing. The LPI is powered by the 4-20 mA current loop using less than 500 uA @ 3 V for
the electronic circuitry.
The LPI option is a circuit board assembly with a micro-controller, a liquid-crystal display
(LCD) and active electronic circuitry contained on a single board, i.e., the “Meter/Display
Board.” The Meter/Display Board plugs into the “Meter Motherboard” that provides the
electrical connections from the transmitter interface and allows the display to be rotated in
90-degree increments.
1.1.1 Features
•
Powered by a 4-20 mA current loop using less than 500 uA @ 3V
• Dual Board Set - Meter Motherboard allows the Meter/Display Board to be rotated in
90-degree increments.
• 41⁄2 Digit Display allows display of numeric values as large as 19999.
• Eight selectable unit labels: mA, %, psi, IN H2O, bar, kg/cm2, °C, and °F.
•
One selectable “no-label” position.
•
Reverse polarity protection.
•
Over current protection
1.1.2 Hardware Circuit Overview
The LPI option uses a micro-controller with integral LCD display drivers. The current
flowing through the LPI is sampled and converted to a corresponding digital word. Based
on user-configuration the digital value is displayed in engineering units along with a unit
label.
The 41⁄2 digit display can show numeric values as large as 19999. The display contains eight
integral unit labels. These are: mA, %, psi, IN H2O, bar, kg/cm2, °C, and °F.
Calibration is done at the board level by injecting known current levels into the assembly,
reading the current and computing correction coefficients that are then stored in the LPI.
The coefficients are then used in a correction algorithm to linearize the current signal and
achieve a minimum accuracy of 0.1%FS at room temperature. The circuit uses precision
resistors to sample the current. The Meter/Display Board is burned-in for long-term
stability and reliability. Operating temperature is limited by the LCD display to -30 to +70
°C. Calibration is done once at the factory, but unit display selection may be done as often
as required by the user (see Section 2.3.1).
Reverse polarity protection is achieved by shunting the entire circuit with a diode. A 250
mA fuse provides overcurrent protection. A shunt capacitor is also included to minimize
EMI effects and provide secondary transient protection.