105
When
drawing a ladder diagram, be careful not to use TR
bits unless necessary
.
Often the number of instructions required for a program can be reduced and
ease
of understanding a program increased by redrawing a diagram that would
otherwise
required TR
bits. In both of the following pairs of diagrams, the bottom
versions
require fewer
instructions and do not require TR bits. In the first exam
-
ple,
this is achieved by reorganizing the parts of the instruction
block: the bottom
one, by separating the
second
OUTPUT instruction and using another LOAD in
-
struction to create the proper execution condition for it.
Note Although
simplifying programs is always a concern, the order of execution of in
-
structions
is sometimes important. For example, a MOVE instruction may be re
-
quired before the execution of a BINARY ADD instruction to place the proper
data
in the required operand word.
Be sure that you have considered execution
order before reorganizing a program to simplify it.
Instruction
1
00000
Instruction 2
00001
TR 0
Instruction 2
00000
Instruction 1
00001
Instruction 1
00000
Instruction 2
00003
TR 0
00001
00004
00002
00001 00003
00000
00004
00002
00001
Instruction 1
Instruction 2
Note TR
bits are only used when programming using mnemonic code.
They are not
necessary
when inputting ladder diagrams directly
. The above limitations on
the
number of branching points requiring TR bits, and considerations on methods to
reduce the number of programming instructions, still hold.
Interlocks The problem of storing execution conditions at branching points can also be
handled
by using the INTERLOCK (IL(02)) and INTERLOCK CLEAR (ILC(03))
instructions
to eliminate the branching point completely while
allowing a specific
execution
condition to control a
group of instructions. The INTERLOCK and IN
-
TERLOCK CLEAR instructions are always used together.
Inputting, Modifying, and Checking the Program Section 4-7
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