Ladder Logic Editor; Example Of Lad Program - Siemens SIMATIC S7-200 System Manual

Basic Concepts for Programming an S7-200 CPU

Ladder Logic Editor

The STEP 7-Micro/WIN 32 Ladder Logic (LAD) editor allows you to build programs
that resemble the equivalent of an electrical wiring diagram. Ladder programming
is probably the method of choice for many PLC programmers and maintenance
personnel. Basically, the ladder programs allow the CPU to emulate the flow of
electric current from a power source, through a series of logical input conditions
that in turn enable logical output conditions. The logic is usually separated into
small, easy-to-understand pieces that are often called "rungs" or "networks." The
program is executed one network at a time, from left to right and then top to
bottom as dictated by the program. Once the CPU has reached the end of the
program, it starts over again at the top of the program.
Figure 4-5 shows an example of a ladder logic program.
Network 1
I0.0
Network 2
I2.1
Figure 4-5
The various instructions are represented by graphic symbols and include three
basic forms. As shown in Figure 4-5, you can connect multiple box instructions in
series.
Contacts - represent logic "input" conditions analogous to switches, buttons,
internal conditions and so on.
Coils - usually represent logic "output" results analogous to lamps, motor
starters, interposing relays, internal output conditions and so on.
Boxes - represent additional instructions such as timers, counters, or math
instructions.
The main points to consider when you select the LAD editor are:
Ladder logic is easy for beginning programmers to use.
Graphical representation is often easy to understand, and is popular around the
world.
The LAD editor can be used with both the SIMATIC and IEC 1131-3 instruction
sets.
You can always use the STL editor to display a program created with the
SIMATIC LAD editor.
4-8
I0.1
I2.0 I2.1
MOV_B
EN ENO
VB50 IN OUT AC0

Example of LAD Program

Q5.0
SWAP
EN ENO
AC0 IN
S7-200 Programmable Controller System Manual
A5E00066097-02