LD

LD

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