Siemens NAN 67 Series Instrukcja Obsługi Pobierz pdf (Strona 80)

6-6

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

C79000-G7076-C504-02

6.2 Boolean Logic and Truth Tables

A ladder logic program tracks power flow between power rails as it passes

through various inputs, outputs, and other elements and boxes. Many Ladder

instructions work according to the principles of Boolean logic.

Each of the Boolean logic instructions checks the signal state of an electrical

contact for 0 (not activated, off) or 1 (activated, on) and produces a result

based on the findings. The instruction then either stores this result or uses it

to perform a Boolean logic operation. The result of the logic operation is

called the RLO. The principles of Boolean logic are demonstrated here on

the basis of normally open and normally closed contacts.

Figure 6-1 shows two conditions of a relay logic circuit with one contact

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

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

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

power rail cannot energize the coil at the end of the circuit. If the contact is

activated (signal state of the contact is 1), power will flow to the coil.

The circuit on the left in Figure 6-1 shows a normally open 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 closed.

Power Rail

Normally Open

Contact

Coil

Standard Representation Representation Indicating

Activated Contact

Figure 6-1 Relay Logic Circuit with Normally Open Control Relay Contact

You can use a Normally Open Contact instruction (see Section 8.2) to check

the signal state of a normally open 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.

Power Flow

Normally Open

Contact

Configuration and Elements of Ladder Logic

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