Siemens NAN 67 Series Manual de servicio descargar pdf (Pagina 239)

20-5

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

C79000-G7076-C504-02

The enable output (ENO) of a Ladder box corresponds to the BR bit of the

status word (see Section 6.3). 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 Ladder, you are responsible for managing the BR bit. You should use

the SAVE instruction (in STL) or the –––(SAVE) coil (in Ladder) 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.

Warning

Possible unintentional resetting of the BR bit to 0.

When writing FBs and FCs in LAD, if you fail to manage the BR bit as

described above, one FB or FC may overwrite the BR bit of another FB

or FC.

To avoid this problem, store the RLO at the end of each FB or FC as

described above.

Figure 20-2 shows the effects of a conditional and an unconditional call of a

block on the bits of the status word (see Section 6.3).

Conditional:

Unconditional:

BR CC 1 CC 0 OV OS OR STA RLO FC

Write x – – – 0 0 1 1 x

Write – – – – 0 0 1 – x

Figure 20-2 Effect of a Block Call on the Bits of the Status Word

Enable Output

Effect of the Call

on the Bits of the

Status Word

Program Control Instructions

1 2 ... 234 235 236 237 238 239 240 241 242 243 244 ... 317 318

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Siemens NAN 67 Series Manual de servicio Pagina 239