Siemens NAN 67 Series Manual de servicio descargar pdf (Pagina 88)

6-14

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

C79000-G7076-C504-02

Bit 2 of the status word is called the status bit (STA bit, see Figure 6-6). The

status bit stores the value of a bit that is referenced. The status of a bit

instruction that has read access to the memory (Normally Open Contact,

Normally Closed Contact) is always the same as the value of the bit that this

instruction checks (the bit on which it performs its logic operation). The

status of a bit instruction that has write access to the memory (Set Coil, Reset

Coil, or Output Coil) is the same as the value of the bit to which the

instruction writes or, if no writing takes place, the same as the value of the bit

that the instruction references. The status bit has no significance for bit

instructions that do not access the memory. Such instructions set the status bit

to 1 (STA=1). The status bit is not checked by an instruction. It is interpreted

during program test (program status) only.

Bit 3 of the status word is called the OR bit (see Figure 6-6). The OR bit is

needed if you use Contact instructions to perform logical Or operations with

an And function. Logical Or operations correspond to arranging contacts in

parallel. The And function corresponds to arranging contacts in series (see

Section 6.2). An And function may contain the following instructions:

Normally Open Contact and Normally Closed Contact. The OR bit shows

these instructions that a previously executed And function has supplied the

value 1 and thus forestalls the result of the logical Or operation. Any other

bit-processing command resets the OR bit.

Bit 5 of the status word is called the overflow bit (OV bit, see Figure 6-6).

The OV bit indicates a fault. It is set by a math instruction or a floating-point

compare instruction after a fault occurs (overflow, illegal operation, illegal

number). The bit is set or reset in accordance with the result of the math or

comparison operation (fault).

Bit 4 of the status word is called the stored overflow bit (OS bit, see

Figure 6-6). The OS bit is set together with the OV bit if an error occurs.

Because the OS bit remains set after the error has been eliminated (unlike the

OV bit), it indicates whether or not a error occurred in one of the previously

executed instructions. The following commands reset the OS bit: JOS (jump

after stored overflow, STL programming), the block call commands, and the

block end commands.

Bits 7 and 6 of the status word are called condition code 1 and condition

code 0 (CC 1 and CC 0, see Figure 6-6). CC 1 and CC 0 provide information

on the following results or bits:

S Result of a math operation

S Result of a comparison

S Result of a digital operation

S Bits that have been shifted out by a shift or rotate command

Status Bit

OR Bit

Overflow Bit

Stored Overflow

Bit

Condition Code 1

and Condition

Code 0

Configuration and Elements of Ladder Logic

1 2 ... 83 84 85 86 87 88 89 90 91 92 93 ... 317 318

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