Purpose
Indicates a
logical start and creates an ON/OFF execution condition based on the ON/OFF
status of the specified operand bit.
Ladder Symbol
Variations
Applicable Program Areas
Operand Specifications
Description
LD is used for the first normally open bit
from the bus bar or for the first normally open bit of a logic block. If there
is no immediate refreshing specification, the specified bit in I/O memory is
read. If there is an immediate refreshing specification, the status of the Basic
Input Unit's input terminal is read and used.
LD is used in the following circumstances as
an instruction for indicating a logical start.
•
When directly connecting to the bus
bar.
•
When logic blocks are connected by AND LD or OR
LD, i.e., at the beginning of a logic block.
The AND LOAD and OR LOAD instructions are
used to connect in series or in parallel logic blocks beginning with LD or LD
NOT.
At least one LOAD or LOAD NOT instruction is
required for the execution condition when output-related instructions cannot be
connected directly to the bus bar. If there is no LOAD or LOAD NOT instruction,
a programming error will occur with the program check by the
CX-Programmer.
When logic blocks are connected by AND LOAD
or OR LOAD instructions, the total number of AND LOAD/OR LOAD instructions must
match the total number of LOAD/LOAD NOT instructions minus 1. If they do not
match, a programming error will occur. For details, refer to 3-2-7AND LOAD: AND
LD and 3-2-8 OR LOAD OR LD.
FLAGS
There are no flags affected by this
instruction.
Precautions
Differentiate up (@) or differentiate down
(%) can be specified for LD. If differentiate up (@) is specified, the execution
condition is turned ON for one cycle only after the status of the operand bit
goes from OFF to ON. If differentiate down (%) is specified, the execution
condition is turned ON for one cycle only after the status of the operand bit
goes from ON to OFF.
Immediate refreshing (!) can be specified for
LD. An immediate refresh instruction updates the status of the input bit for CPU
Unit built-in inputs just before the instruction is executed.
For LD, it is possible to combine immediate
refreshing and up or down differentiation (!@ or !%). If either of these is
specified, the built-in input is refreshed from the CPU Unit just before the
instruction is executed and the execution condition is turned ON for one cycle
only after the status goes from OFF to ON, or from ON to OFF.
Example
