Conventions for S7-200 Instructions
8-3
S7-200 Programmable Controller System Manual
A5E00066097-02
In LAD, which is patterned after Relay Ladder Logic Electrical Drawings, there is a
left power rail that is energized. In LAD, contacts that are closed allow energy to
flow through them to the next element and contacts that are open block that
energy flow. Any LAD element that can be connected to the left or right power rail
or to a contact has a power flow input and/or output.
In SIMATIC FBD, which does not use the concept of left and right power rails, the
term “power flow” is used to express the analogous concept of control flow through
the FBD logic blocks. The logic “1” path through FBD elements is called power
flow.
In LAD a power flow input or output is always exclusively power flow and cannot be
assigned to an operand. In FBD the origin of a power flow input and the destination
of a power flow output can be assigned directly to an operand.
In addition to power flow many, but not all, instructions have one or more input and
output operands. The allowed parameters for the operand inputs and outputs are
provided in the Inputs/Outputs table beneath the LAD/FBD/STL figure.
CPU Type: The checked boxes at the bottom of the instruction figure show the
CPUs that support the instruction. In this example, the instruction is supported by
the CPU 221, CPU 222, CPU 224, and CPU 226.
Instruction Description: The text to the right of the instruction figure on
page 8-2 describes the operation of the instruction(s). In some cases there is a
description of the operation of the instruction for each language and in other cases
there will be a single description that applies to all three programming languages.
Note that IEC terminology is slightly different from SIMATIC terminology. For
example, a SIMATIC Count Up (CTU) is called an instruction; an IEC CTU is called
a function block.
Error Conditions that Set ENO = 0: If the LAD/FBD instructions have an ENO
output, this section lists the error conditions that result in ENO being set to a zero.
SM Bits Affected: If the instruction affects SM bits as a normal part of executing
the instruction, the bits affected and how they are affected are listed in this section.
Operand Table: Beneath the LAD/FBD/STL figure is a table that lists the allowed
operands for each of the inputs and outputs, along with the data types of each
operand. The memory ranges of the operands for each CPU are shown in
Table 8-3.
EN/ENO operands and data types are not shown in the instruction operand table
because the operands are the same for all LAD and FBD instructions. Table 8-1
lists these operands and data types for LAD and FBD. These operands apply to all
LAD and FBD instructions shown in this manual.
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