Maple Systems OIT Family - Manual (Page 57)

The linear scaling conversion is accomplished using the simple equation Y=MX+B,
where “X” is the raw data read from the PLC, “Y” is the data presented to the OIT
operator after the conversion, “M” is used to convert the scale of one unit of measure to
another unit of measure, and “B” is the offset variable used for alignment.
For example, if an 8-bit A/D converter (255 steps) is used to measure voltage ranging
from 0 to 15 volts, the raw data needs to be reduced so that a value of 255 A/D units
equals 15 volts. The OIT needs to multiple the raw data by 1/17 (255/15=17) to properly
convert the data into volts. Since the A/D value of 0 correctly represents 0 volts, the
variable B is not used. The equation for this example would be Y = (1/17)X + 0.
Let’s look at a similar example. Suppose an 8-bit A/D converter (255 steps) is used to
measure voltage ranging from 5 to 20 volts. Now we need to do two things, adjust the
A/D range of 1-255 down to a range of 15 volts (20 volts-5 volts) and offset the data so
that an A/D value of 0 represents 5 volts. Let’s start with the range conversion. As in the
previous example, M is calculated by dividing the A/D range by the voltage range
(255/15=17). Since we are reducing the raw data, M must be 1/17. Now to offset the raw
data so that an A/D value of 0 represents 5 volts, set B to 5. The equation for this
example would be Y = (1/17)X + 5.
The multiplier M can be entered in as a fraction which allows both range expansion and
range reduction. To expand the range of the raw data, make sure the numerator (top
number) is greater than the denominator (bottom number). To reduce the range of the raw
data, make sure the numerator is less than the denominator. In the previous examples, M
would be specified as 1/17.
16-bit PLC registers using the Signed or Decimal formats and 32-bit PLC registers using
the Long format can be converted using linear scaling. Therefore, the input variable X,
which represents the raw PLC register value, can be a signed integer, unsigned integer, or
signed long integer value. The output variable Y is determined by the linear equation.
Therefore, it may be a positive or negative number with a maximum range of
-2,147,483,648 to +2,147,483,647.
When using low and high limits, the values should be entered in the unit of measure
being used by the OIT operator not the unit of measure being used in the PLC register.
The OIT always performs integer arithmetic when calculating the linear scaled values to
be displayed. Therefore, you should be aware of what kind of accuracy you wish to have
when using linear conversion. For example, if the OIT converts degrees Celsius to
degrees Fahrenheit using the formula Y = (9/5)X + 32, 1°C would be converted to 34°F.
Technically, this is inaccurate since 1°C=33.8°F. Fortunately, this can be corrected by
simply using a different formula. If the conversion accuracy must be to one decimal
point, then multiply the given equation on the left side by 10 (for example, Y = (90/5)X +
320) and select a decimal point of 1 for the register monitor display. Finally, the above
formulas only work if the Celsius temperature values stored in the PLC register are
always integer whole number values. If the Celsius temperature values in the PLC
register are to one decimal point and the Fahrenheit temperature values are to be
displayed to one decimal point, the following formula should be used Y = (9/5)X + 320.
PLC Register Monitors 57
1010-0099, Rev. 07