Omron NT21 Reference Manual page 223

Inputting Numeric Values
Reference: When the Storage Type is BCD, the numeral memory table entry's signed binary
data is converted to data for storage in the host word (BCD data) and written to
the host. If the value is negative the most significant digit in the word is set as F.
This means that the maximum number of digits for a negative number (integer
part + decimal fraction part) is seven.
When the Storage Type is Binary, the contents of the numeral memory table
entry are written without alteration to the host word.
However, when a negative number is written to a numeral memory table entry
whose number of allocated words at the host is one, it is stored immediately after
input as a 16-bit negative number.
The data is stored in the numeral memory table entry as a 32-bit negative num-
ber in this case as well.
Upper/Lower (Max./Min.) Limit Check
The PT has a function to check the correctness of an input numeric value to pre-
vent a numeric value outside the allowable range from being input so that the
input of an illegal value will not adversely affect the system.
The upper/lower (max./min.) limit check is executed in the timing indicated be-
low according to the type of a numeral setting input field.
For a thumbwheel type numeral setting input field, the check is executed each
time the increment/decrement key of each digit position is pressed.
With a thumbwheel type numeral setting input field, if n...n < m...m [upper limit:
1n...n, lower limit: 0m...m (n and m are arbitrary values at each digit)], carry over
and underflow are not possible at the highest digit position.
Example:
If the upper limit value and the lower limit value are 1200 and 201, respectively,
the numeric value can be changed only in the following range: from 201 to 999 if
an initial value is a 3-digit value and from 1000 to 1200 if an initial value is a 4-digit
value.
The reason for this is that the upper/lower (max./min.) limit check function does
not allow the numeric value at the highest digit position to change from 0 to 1 or
from 1 to 0.
To avoid this, set the upper and lower limit values so that n...n y m...m, or pro-
vide a touch switch separately to write a numeric value for carry over or under-
flow.
Note To ensure greater safety for numeric inputs, use the upper/lower limit setting
function. Otherwise the system may operate unpredictably.
Section 2-12
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