Introduction; Instruction Terminology; The Ladder Diagram - Omron SYSMAC C20P Operation Manual

The Ladder Diagram

4–1

Introduction

1, 2, 3...
4–2

Instruction Terminology

4–3

The Ladder Diagram

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This section explains the basic steps and concepts involved in programming
and introduces the instructions used to build the basic structure of the ladder
diagram and control its execution. The entire set of instructions used in pro-
gramming is described in Section 5 Instruction Set .
There are several basic steps involved in writing a program.
1. Obtain a list of all I/O devices and the I/O points that have been as-
signed to them and prepare a table that shows the I/O bit allocated to
each I/O device.
2. If the PC has any Units, i.e. Analog Timer Units, Host Link Units , and
I/O Link Units that are allocated words in data areas other than the IR
area or are allocated IR words in which the function of each bit is speci-
fied by the Unit, prepare similar tables to show what words are used for
which Units and what function is served by each bit within the words.
3. Determine what words are available for work bits and prepare a table in
which you can allocate these as you use them.
4. Also prepare tables of TC numbers so that you can allocate these as
you use them. Remember, the function of a TC number can be defined
only once within the program. (TC number are described in 5–11 Timer
and Counter Instructions .)
5. Draw the ladder diagram.
6. Input the program into the CPU. When using the Programming Console,
this will involve converting the program to mnemonic form.
7. Check the program for syntax errors and correct these.
8. Execute the program to check for execution errors and correct these.
9. After the entire Control System has been installed and is ready for use,
execute the program and fine tune it if required.
The basics of ladder diagramming are described in the rest of this section.
Converting the program to mnemonic form and debugging are described in
Section 7 Program Input, Debugging, and Execution . Section 8 Troubleshoot-
ing also provides information required for debugging.
There are basically two types of instructions used in ladder-diagram pro-
gramming: instructions that correspond to conditions on the ladder diagram
and are used in instruction form only when converting a program to mne-
monic code and instructions that are used on the right side of the ladder dia-
gram and are executed according to the conditions on the instruction lines
leading to them.
Most instructions have at least one or more operands associated with them.
Operands indicate or provide the data on which an instruction is to be per-
formed. These are sometimes input as the actual numeric values, but are
usually the addresses of data area words or bits that contain the data to be
used. For instance, a Move instruction that has IR 00 designated as the
source operand will move the contents of IR 00 to some other location. The
other location is also designated as an operand. A bit whose address is des-
ignated as an operand is called an operand bit; a word whose address is
designated as an operand is called an operand word.
Other terms used in describing instructions are introduced in Section 5 In-
struction Set .
A ladder diagram consists of one line running down the left side with lines
branching off to the right. The line on the left is called the bus bar; the
Section 4–3