Siemens NAN 67 Series Wartungshandbuch PDF herunterladen (Seite 87)

6-13

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

C79000-G7076-C504-02

Bit 0 of the status word is called the first-check bit (FC bit, see Figure 6-6).

At the start of a ladder logic network, the signal state of the FC

bit is always

0, unless the previous network ended with –––(SAVE). (The bar over the FC

indicates that it is negated, that is, always 0 at the beginning of a ladder logic

network.)

Each logic instruction checks the signal state of the FC

bit as well as the

signal state of the contact that the instruction addresses. The signal state of

the FC

bit determines the sequence of a logic string. If the FC bit is 0 (at the

start of a ladder logic network), the instruction stores the result in the result

of logic operation bit of the status word and sets the FC

bit to 1. The

checking process is called a first check. The 1 or 0 that is stored in the RLO

bit after the first check is then referred to as the result of first check.

If the signal state of the FC bit is 1, an operation then links the result of its

signal state check with the RLO formed at the addressed contact since the

first check, and stores the result in the RLO bit.

A rung of ladder logic instructions (logic string) always ends with an output

instruction (Set Coil, Reset Coil, or Output Coil) or a jump instruction related

to the result of logic operation. Such an output instruction resets the FC

bit

to 0.

Bit 1 of the status word is called the result of logic operation bit (RLO bit,

see Figure 6-6). This bit stores the result of a string of bit logic instructions or

math comparisons. The signal state changes of the RLO bit can provide

information related to power flow.

For example, the first instruction in a network of ladder logic checks the

signal state of a contact and produces a result of 1 or 0. The instruction stores

the result of this signal state check in the RLO bit. The second instruction in

a rung of bit logic instructions also checks the signal state of a contact and

produces a result. Then the instruction combines this result with the value

stored in the RLO bit of the status word according to the principles of

Boolean logic (see First Check above and Chapter 8). The result of this logic

operation is stored in the RLO bit of the status word, replacing the former

value in the RLO bit. Each subsequent instruction in the rung performs a

logic operation on two values: the result produced when the instruction

checks the contact, and the current RLO.

You can, for example, use a Boolean bit logic instruction on a first check to

assign the state of the contents of a Boolean bit memory location to the RLO

or trigger a jump.

First Check

Result of Logic

Operation

Configuration and Elements of Ladder Logic

1 2 ... 82 83 84 85 86 87 88 89 90 91 92 ... 317 318

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