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Basic Ladder Diagrams
Section 4-3
Execution Conditions
In ladder diagram programming, the logical combination of ON and OFF con-
ditions before an instruction determines the compound condition under which
the instruction is executed. This condition, which is either ON or OFF, is called
the execution condition for the instruction. All instructions other than LOAD
instructions have execution conditions.
Operand Bits
The operands designated for any of the ladder instructions can be any bit in
the IR, SR, HR, AR, LR, or TC areas. This means that the conditions in a lad-
der diagram can be determined by I/O bits, flags, work bits, timers/counters,
etc. LOAD and OUTPUT instructions can also use TR area bits, but they do
so only in special applications. Refer to
4-3-8 Branching Instruction Lines
for
details.
Logic Blocks
The way that conditions correspond to what instructions is determined by the
relationship between the conditions within the instruction lines that connect
them. Any group of conditions that go together to create a logic result is called
a logic block. Although ladder diagrams can be written without actually ana-
lyzing individual logic blocks, understanding logic blocks is necessary for effi-
cient programming and is essential when programs are to be input in
mnemonic code.
Instruction Block
An instruction block consists of all the instructions that are interconnected
across the ladder diagram. One instruction block thus consists of all the
instructions between where you can draw a horizontal line across the ladder
diagram without intersecting any vertical lines and the next place where you
can draw the same type of horizontal line.
4-3-2
Mnemonic Code
The ladder diagram cannot be directly input into the PC via a Programming
Console; the CX-Programmer is required. To input from a Programming Con-
sole, it is necessary to convert the ladder diagram to mnemonic code. The
mnemonic code provides exactly the same information as the ladder diagram,
but in a form that can be typed directly into the PC. Actually you can program
directly in mnemonic code, although it is not recommended for beginners or
for complex programs. Also, regardless of the Programming Device used, the
program is stored in memory in mnemonic form, making it important to under-
stand mnemonic code.
Because of the importance of the Programming Console as a peripheral
device and because of the importance of mnemonic code in complete under-
standing of a program, we will introduce and describe the mnemonic code
along with the ladder diagram. Remember, you will not need to use the mne-
monic code if you are inputting via the CX-Programmer (although you can use
it with the CX-Programmer if you prefer).
Program Memory
Structure
The program is input into addresses in Program Memory. Addresses in Pro-
gram Memory are slightly different to those in other memory areas because
each address does not necessarily hold the same amount of data. Rather,
each address holds one instruction and all of the definers and operands
(described in more detail later) required for that instruction. Because some
Instruction
Instruction
00000
00000
Instruction is executed
when IR bit 00000 is ON.
Instruction is executed
when IR bit 00000 is OFF.
Normally open
condition
Normally closed
condition
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