Siemens NAN 67 Series Manual de servicio descargar pdf (Pagina 39)

3-15

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

C79000-G7076-C504-02

3.8 Basic Guidelines for Entering Ladder Logic Instructions

A Ladder network can contain several elements in different branches. All

elements and branches must be connected together; however, the power rail

on the left does not count as a connection (IEC 1131-3).

When you program in Ladder, you must observe certain guidelines.

Any errors are reported with an error message in the Program Editor.

Every Ladder network must end with a coil or a box. You cannot use the

following elements to close a network:

S Comparison boxes

S Midline outputs (#)

S Positive (P) or Negative (N) RLO edge detection

Branches that cause reverse power flow (from right to left) cannot be edited.

Figure 3-8 shows an example. With signal state “0” at I 1.4, a power flow

from right to left would be possible at I 6.8. This is not allowed.

I 1.0

Q 2.6

I 1.2 I 1.4 I 4.2

I 6.8

Q 4.4 I 2.8

Q 6.0

✗

Illegal Power Flow

Figure 3-8 Power Flow in Reverse Direction (Illegal)

Overview

Ending a Ladder

Network

Power Flow

Creating Logic Blocks

1 2 ... 34 35 36 37 38 39 40 41 42 43 44 ... 317 318

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Siemens NAN 67 Series Manual de servicio Pagina 39