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6-9

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

C79000-G7076-C504-02

Figure 6-3 shows a ladder logic diagram that you can use to program two

normally open contacts connected in series to a coil. The first Normally Open

Contact instruction in the logic string checks the signal state of the first

contact in the series (input I 1.0) and produces a result based on the findings

(see Table 6-7). This result can be 1 or 0. A result of 1 means that the contact

is closed and any available power could flow across the contact; a result of 0

means that the contact is open, interrupting the flow of any power available

at the contact. The first Normally Open Contact instruction copies this 1 or 0

to a memory bit in the status word of the programmable logic controller. This

bit is called the “result of logic operation” (RLO) bit.

The second Normally Open Contact instruction in the logic string checks the

signal state of the second contact in the series (I 1.1) and produces a result

based on the findings (see Table 6-7). This result can be 1 or 0, depending on

whether the contact is closed or open. At this point, the second Normally

Open Contact instruction performs a Boolean logic combination. The

instruction takes the result it produced upon checking the signal state of the

second contact and combines this result with the value stored in the RLO bit.

The result of this combination (either 1 or 0) is stored in the RLO bit of the

status word, replacing the old value stored there. The Output Coil instruction

(see Section 8.4) assigns this new value to the coil (output Q 4.0).

The possible results of such a logic combination can be shown in a “truth

table.” In such a logic combination, 1 represents “true” and 0 represents

“false.” The possible Boolean logic combinations and their results are

summed up in Table 6-8, where “contact is closed” and “power can flow”

correspond to “true” and “contact is open” and “power cannot flow”

correspond to “false” (see Figure 6-3 for the contacts).

Table 6-8 Truth Table: And

If the result produced by

checking the signal state

of contact I 1.0 is

and the result produced

by checking the signal

state of contact I 1.1 is

the result of the logic

operation shown in

Figure 6-3 is

1 (contact is closed) 1 (contact is closed) 1 (power can flow)

0 (contact is open) 1 (contact is closed) 0 (power cannot flow)

1 (contact is closed) 0 (contact is open) 0 (power cannot flow)

0 (contact is open) 0 (contact is open) 0 (power cannot flow)

Using Normally

Open Contact in

Series

Configuration and Elements of Ladder Logic

1 2 ... 78 79 80 81 82 83 84 85 86 87 88 ... 317 318

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