44
something to happen when a bit is ON, and a normally closed condition when
you want something to happen when a bit is OFF.
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
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.
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
ladder diagram can be determined by I/O bits, flags, work bits, timers/count-
ers, etc. LOAD and OUTPUT instructions can also use TR area bits, but they
do so only in special applications. Refer to
4-6-7 Branching Instruction Lines
for details.
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
analyzing individual logic blocks, understanding logic blocks is necessary for
efficient programming and is essential when programs are to be input in mne-
monic code.
4-3-2
Mnemonic Code
The ladder diagram cannot be directly input into the PC via a Programming
Console; a GPC, a FIT, or LSS is required. To input from a Programming
Console, it is necessary to convert the ladder diagram to mnemonic code.
The mnemonic code provides exactly the same information as the ladder dia-
gram, but in a form that can be typed directly into the PC. Actually you can
program directly in mnemonic code, although it in not recommended for be-
ginners or for complex programs. Also, regardless of the Programming De-
vice used, the program is stored in memory in mnemonic form, making it im-
portant to understand mnemonic code.
Because of the importance of the Programming Console as a peripheral de-
vice 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 a GPC, a FIT, or LSS (although you can
use it with these devices too, if you prefer).
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 (de-
scribed in more detail later) required for that instruction. Because some in-
structions require no operands, while others require up to three operands,
Program Memory addresses can be from one to four words long.
Execution Conditions
Operand Bits
Logic Blocks
Program Memory Structure
Basic Ladder Diagrams
Section 4-3
