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Complicated Diagrams When determining what logic block instructions will be required to code a dia-
gram, it is sometimes necessary to break the diagram into large blocks and then
continue breaking the large blocks down until logic blocks that can be coded
without logic block instructions have been formed. These blocks are then coded,
combining the small blocks first, and then combining the larger blocks. Either
AND LOAD or OR LOAD is used to combine the blocks, i.e., AND LOAD or OR
LOAD always combines the last two execution conditions that existed, regard-
less of whether the execution conditions resulted from a single condition, from
logic blocks, or from previous logic block instructions.
When working with complicated diagrams, blocks will ultimately be coded start-
ing at the top left and moving down before moving across. This will generally
mean that, when there might be a choice, OR LOAD will be coded before AND
LOAD.
The following diagram must be broken down into two blocks and each of these
then broken into two blocks before it can be coded. As shown below, blocks a
and b require an AND LOAD. Before AND LOAD can be used, however, OR
LOAD must be used to combine the top and bottom blocks on both sides, i.e., to
combine a1 and a2; b1 and b2.
00000 00001 00004 00005
00503
Block
a
Block
b
00006 00007
Block
b2
Block
b1
00002 00003
Block
a2
Block
a1
Blocks a1 and a2
Blocks b1 and b2
Blocks a and b
Address Instruction Operands
00000 LD 00000
00001 AND NOT 00001
00002 LD NOT 00002
00003 AND 00003
00004 OR LD —
00005 LD 00004
00006 AND 00005
00007 LD 00006
00008 AND 00007
00009 OR LD —
00010 AND LD —
00011 OUT 00503
The following type of diagram can be coded easily if each block is coded in order:
first top to bottom and then left to right. In the following diagram, blocks a and b
would be combined using AND LOAD as shown above, and then block c would be
coded and a second AND LOAD would be used to combined it with the execution
condition from the first AND LOAD. Then block d would be coded, a third AND
LOAD would be used to combine the execution condition from block d with the
execution condition from the second AND LOAD, and so on through to block n.
Block
a
Block
b
00500
Block
n
Block
c
Basic Ladder Diagrams Section 4-4