Watlow LogicPro - Manual (Page 154)

Chapter 10: Function Blocks LogicPro User’s Guide
138 Watlow Anafaze Doc.# 28002-00 Rev 3.00
integers in the stack is equal to the size of the stack. A False
to True transition of the unload bit (UNLD) moves the last
integer entered in the stack to the output Variable (OUTP) and
causes the output bit (Q) to be True. Every succeeding False
to True transition of the unload bit continues to move the
newest integer from the stack to the output Variable, and
causes the output bit to remain True until the entire stack is
unloaded.
At this point, the count Variable equals zero, the output bit is
False, and the empty bit (EMP) is True. This bit only remains
True as long as the count of integers in the stack is equal to
zero. A True condition of the clear bit (CLR) clears the stack
of any integer values and resets the count Variable to zero,
thus causing the empty bit to be True. The output Variable
retains its last value when the stack is cleared.
Figure 10.19 Last In - First Out Function Block
Inputs
LD is the LIFO load bit. As long as the clear bit is False, each
False to True transition of this bit loads the input Variable
value into the stack until the count value equals the size value.
In a program using Ladder logic, the value of LD is determined
by the Boolean evaluation of elements placed between the left
power rail and LD in the function block. In FBD programs, the
value of LD is determined by the Variable connected to LD. If
no contacts are placed on the rung before LD, the value of LD
is always True, but a transition never occurs. This bit has the
lowest evaluation priority of the input bits in this function
block.
UNLD is the LIFO unload bit. As long as the clear bit is False
and the count value is not zero, each False to True transition
of this bit unloads the newest integer in the stack to the output
Variable. In a Ladder Diagram program, the value of UNLD is
determined by the evaluation of Boolean elements placed
between the left power rail and UNLD in the function block. In