Campbell CR6 Series, Руководство оператора

Страница: 225 / 690

Section 7. Installation
225
argument TRUE is predefined in the CR6 operating system to only equal -1 , so
only the argument -1 is
always
translated as TRUE . Consider the expression
If Condition(1) = TRUE Then ...
This condition is true only when Condition(1) = -1 . If Condition(1) is any other
non-zero, the condition will not be found true because the constant TRUE is
predefined as -1 in the CR6 system memory. By entering = TRUE , a literal
comparison is done. So, to be absolutely certain a function is true, it must be set
to TRUE or -1 .
Note TRUE is -1 so that every bit is set high (-1 is &B11111111 for all
four bytes). This allows the
AND
operation to work correctly. The
AND
operation does an AND boolean function on every bit, so TRUE AND X
will be non-zero if at least one of the bits in X is non-zero (if X is not zero).
When a variable of data type BOOLEAN is assigned any non-zero
number, the CR6 internally converts it to -1 .
The CR6 is able to translate the conditions listed in table Binary Conditions of
TRUE and FALSE
(p. 225)
to binary form (-1 or 0), using the listed instructions and
saving the binary form in the memory location indicated. Table
Logical
Expression Examples
(p. 226)
explains some logical expressions.
Non-Zero = True (Sometimes)
Any argument other than 0 or -1 will be translated as TRUE in some cases and
FALSE
in other cases. While using only
-1
as the numerical representation of
TRUE is safe, it may not always be the best programming technique. Consider
the expression
If Condition(1) then ...
Since = True is omitted from the expression, Condition(1) is considered true if it
equals any non-zero value.
Binary Conditions of TRUE and FALSE
Condition
CRBasic
Instruction(s) Used
Memory Location of
Binary Result
Time TimeIntoInterval() Variable, System
IfTime() Variable, System
TimeIsBetween()
Variable, System
Control Port Trigger WaitDigTrig() System
Communications VoiceBeg() System
ComPortIsActive()
Variable
PPPClose() Variable
Measurement Event DataEvent() System