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6-16

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

C79000-G7076-C504-02

Table 6-13 CC 1 and CC 0 after Comparison Instructions

CC 1

CC 0 Explanation

0 0 IN2 = IN1

0 1 IN2 < IN1

1 0 IN2 > IN1

1 1 IN1 or IN2 is an illegal floating-point number

Table 6-14 CC 1 and CC 0 after Shift and Rotate Instructions

CC 1

CC 0 Explanation

0 0 Bit shifted out last = 0

1 0 Bit shifted out last = 1

Table 6-15 CC 1 and CC 0 after Word Logic Instructions

CC 1 CC 0 Explanation

0 0 Result = 0

1 0 Result <> 0

Bit 8 of the status word is called the binary result bit (BR bit, see Figure 6-6).

The BR bit forms a link between the processing of bits and words. This bit

enables your program to interpret the result of a word operation as a binary

result and to integrate this result in a binary logic chain. Viewed from this

angle, the BR represents a machine-internal memory marker into which the

RLO is saved prior to an RLO-changing word operation, so that it is still

available for the continuation of the interrupted bit chain after the operation

has been carried out.

For example, the BR bit makes it possible for you to write a function block

(FB) or a function (FC) in statement list (STL) and then call the FB or FC

from ladder logic (LAD).

When writing a function block or function that you want to call from Ladder,

no matter whether you write the FB or FC in STL or LAD, you are

responsible for managing the BR bit. The BR bit corresponds to the enable

output (ENO) of a Ladder box. You should use the SAVE instruction (in

STL) or the or the –––(SAVE) coil (in LAD) to store an RLO in the BR bit

according to the following criteria:

S Store an RLO of 1 in the BR bit for a case where the FB or FC is

executed without error.

S Store an RLO of 0 in the BR bit for a case where the FB or FC is

executed with error

You should program these instructions at the end of the FB or FC so that

these are the last instructions that are executed in the block.

Binary Result Bit

Configuration and Elements of Ladder Logic

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