User`s manual

Input and Output Variables Input and Output Variables • 2-1
2 Building Equations
Equations are made up of different combinations of
four major elements: inputs, defined in the Flexible Com-
biner application, constants such as numerical values used
in mathematical computations, operators such as plus
signs (+) or greater than signs (>) used to perform basic
math functions, and functions such as if-then statements
or boolean logic (AND/OR).
2.1 Input and Output
Variab les
The most commonly used variables in an equation will
be the Flexible Combiner’s input values. These are repre-
sented in equations by their variable names, AI1-AI8 and
DI1-DI8.
To use input values in an equation, just enter their
names (AI for analog input or DI for digital input, and a
number from 1 through 8 corresponding to the input num-
ber). The E2 will look for input variable names and create
a link to the input’s value when parsing the equation.
Analog Input Values and SI Units
The value of an AI input will always be equal to the
current input value in the E2’s internal standard (SI)
units. This is important to remember because any mathe-
matical operations that involve this value will use its SI
units, which may cause the equation to be wrong if you are
assuming the wrong units.
Example: AI1 is reading a value of 212°F. If you
wanted to write an equation that added 10°F to the value
of AI1, the equation “AI1+10” would be incorrect because
this would add 10 to the value of AI in Celsius, since E2’s
SI unit for temperature is Celsius.
The way to correct this would be convert the number
10 to an equivalent number of degrees C (“AI1+5.6”), or
else use a type suffix so the E2 knows you want to add 10
degrees (“AI1+10DDF”). Type suffixes are explained in
further detail in Section 2.2.2, Specifying a Constant’s
Engineering Unit with Type Suffixes.
Table 2-2 lists the SI units used by E2:
Variable Name Description
AIx (x=input
number)
The analog value of input AIx, in SI
units
DIx (x=input
number)
The digital state of input DIx, converted
to analog:
1.0 if input is ON
0.0 if input is OFF
AOx (x=output
number)
The analog value of output AOx, in SI
units
DOx (x=output
number)
The digital state of output DOx, con-
verted to analog:
1.0 if input is ON
0.0 if input is OFF
Table 2-1 - Input and Output Variable Names
Input Type Choices SI Unit
Temperature degrees Fahrenheit (DF)
degrees Celsius (DC)
degrees C (DC)
Change in
Temperature
delta degrees Fahrenheit
(DDF)
delta degrees Celsius
(DDC)
delta degrees C
(DDC)
Pressure inches of water (INW)
centimeters of water
(CMW)
pounds per sq. inch (PSI)
bars (BAR)
kilopascals (KPA)
pascals (PA)
pascals (PA)
Change in
Pressure
delta inches of water
(DINW)
delta centimeters of water
(DCMW)
delta pounds per sq. inch
(DPSI)
delta bars (DBAR)
delta kilopascals (DKPA)
delta pascals (DPA)
delta pascals
(DPA)
Air Velocity feet per minute (FPM)
meters per minute (MPM)
meters per second (MPS)
meters per sec-
onds (MPS)
Liquid
Ve lo ci t y
gallons per minute
(GPM)
liters per second (LPS)
liters per sec-
ond (LPS)
Vo l u m e
Flow
cubic feet per minute
(CFM)
cubic meters per second
(CMS)
cubic meters
per second
Table 2-2 - SI Units for Analog Inputs