44
tion of the first of the pair. Conditions should not be placed on the instruction
lines leading to these instructions. Refer to
Section 5 Instruction Set
for de-
tails.
When drawing ladder diagrams, it is important to keep in mind the number of
instructions that will be required to input it. In diagram A, below, an OR Load
instruction will be required to combine the top and bottom instruction lines.
This can be avoided by redrawing as shown in diagram B so that no AND
Load or OR Load instructions are required. Refer to
5–5–2
AND Load and
OR Load
for more details and
7–5 Inputting, Modifying and Checking the
Program
for further examples.
0000
0001 0207
0207
0001
0000
0207
0207
Diagram A:
Diagram B:
4–7 Program Execution
When program execution is started, the CPU scans the program from top to
bottom, checking all conditions and executing all instructions accordingly as it
moves down the bus bar. It is important that instructions be placed in the
proper order so that, for example, the desired data is moved to a word before
that word is used as the operand for an instruction. Remember that an in-
struction line is completed to the terminal instruction at the right before exe-
cuting any instruction lines branching from the first instruction line to other
terminal instructions at the right.
Program execution is only one of the tasks carried out by the CPU as part of
the scan time. Refer to
Section 6 Program Execution Timing
for details.
Program Execution Section 4–7
To Next Page
Loading...
Need help?
General
