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7-2

Ladder Logic (LAD) for S7-300 and S7-400

C79000-G7076-C504-02

7.1 Overview

Many ladder logic instructions work together with one or more addresses

(operands). This address indicates a constant or a place where the instruction

finds a variable on which to perform a logic operation. This place can be a

bit, a byte, a word or a double word of the address.

Possible addresses are, e.g.:

S A constant, the value of a timer or counter, or an ASCII character string

S A bit in the status word of the programmable logic controller

S A data block and a location within the data block area

The following types of addressing are available:

S Immediate addressing (enter a constant as the address)

S Direct addressing (enter a variable as the address)

Figure 7-1 shows an example of immediate and direct addressing. The

function of the box is to compare two input parameters (in this case, two

16-bit integers) to see if the first input is less than or equal to the second. The

constant 50 is entered as input parameter IN1 Memory word MW200, a

location in memory, is entered as input parameter IN2.

Because the constant 50 in the example is the actual value with which IN1 of

the box is to work, 50 is considered an immediate address of the instruction

box. Because MW200 points to a location in memory where there is another

value with which IN2 of the box is to work, MW200 is considered a direct

address. MW200 is a location, not the actual value itself.

CMP_I

IN1

MW200

IN2

Figure 7-1 Immediate and Direct Addressing

What Is

Addressing?

Immediate and

Direct Addressing

Addressing

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