6-3
Section
Basic Ladder Diagrams
343
To code diagrams with logic block instructions in series, the diagram must be
divided into logic blocks. Each block is coded using a LOAD instruction to code
the first condition, and then AND LOAD or OR LOAD is used to logically combine
the blocks. With both AND LOAD and OR LOAD there are two ways to achieve
this. One is to code the logic block instruction after the first two blocks and then
after each additional block. The other is to code all of the blocks to be combined,
starting each block with LOAD or LOAD NOT, and then to code the logic block
instructions which combine them. In this case, the instructions for the last pair of
blocks should be combined first, and then each preceding block should be com-
bined, working progressively back to the first block. Although either of these
methods will produce exactly the same result, the second method, that of coding
all logic block instructions together, can be used only if eight or fewer blocks are
being combined, i.e., if seven or fewer logic block instructions are required.
The following diagram requires AND LOAD to be converted to mnemonic code
because three pairs of parallel conditions lie in series. The two means of coding
the programs are also shown.
00000
00002
00004
00001
00003
00005
20000
Address
Instruction
Operands
Address
Instruction
Operands
00000
LD
00000
00001
OR NOT
00001
00002
LD NOT
00002
00003
OR
00003
00004
AND LD
—
00005
LD
00004
00006
OR
00005
00007
AND LD
—
00008
OUT
20000
00000
LD
00000
00001
OR NOT
00001
00002
LD NOT
00002
00003
OR
00003
00004
LD
00004
00005
OR
00005
00006
AND LD
—
00007
AND LD
—
00008
OUT
20000
Again, with the method on the right, a maximum of eight blocks can be com-
bined. There is no limit to the number of blocks that can be combined with the
first method.
The following diagram requires OR LOAD instructions to be converted to mne-
monic code because three pairs of conditions in series lie in parallel to each oth-
er.
00000 00001
00002 00003
00004 00005
20001
Logic Block Instructions in
Series