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6-7

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

C79000-G7076-C504-02

Figure 6-2 shows two representations of a relay logic circuit with one contact

between a power rail and a coil. The normal state of this contact is closed. If

the contact is not activated, it remains closed. The signal state of the closed

contact is 0 (not activated). If the contact remains closed, power from the

power rail can cross the contact to energize the coil at the end of the circuit.

Activating the contact (signal state of the contact is 1) opens the contact,

interrupting the flow of power to the coil.

The circuit on the left in Figure 6-2 shows a normally closed control relay

contact as it is sometimes represented in relay logic diagrams. For the

purpose of this example, the circuit on the right indicates that the contact has

been activated and is therefore open.

Power Rail

Normally

Closed

Contact

Coil

Standard Representation Representation Indicating

Activated Contact

Figure 6-2 Relay Logic Circuit with Normally Closed Control Relay Contact

You can use a Normally Closed Contact instruction (see Section 8.3) to check

the signal state of a normally closed control relay contact. By checking the

signal state, the instruction determines whether power can flow across the

contact or not. If power can flow, the instruction produces a result of 1; if

power cannot flow, the instruction produces a result of 0 (see Table 6-7). The

instruction can either store this result or use it to perform a Boolean logic

operation.

Table 6-7 Result of Signal State Check by Normally Open and Normally Closed Contact

Instruction

Result if Signal State of Contact is 1

(Contact Is Activated)

Result if Signal State of Contact Is 0

(Contact Is Not Activated)

1 (Available power can flow because the

normally open contact is closed.)

0 (Available power cannot flow because the

normally open contact is open.)

0 (Available power cannot flow because the

normally closed contact is opened.)

1 (Available power can flow because the

normally closed contact is closed.)

Normally Closed

Contact

Configuration and Elements of Ladder Logic

1 2 ... 76 77 78 79 80 81 82 83 84 85 86 ... 317 318

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