Siemens SIMATIC S7-200 System Manual page 73

General Conventions of Programming for an S7-200
EN/ENO Definition
EN (Enable IN) is a Boolean input for boxes in LAD and FBD. Power flow must be present at this
input for the box instruction to be executed. In STL, the instructions do not have an EN input, but
the top of stack value must be a logic "1" for the corresponding STL instruction to be executed.
ENO (Enable Out) is a Boolean output for boxes in LAD and FBD. If the box has power flow at the
EN input and the box executes its function without error, then the ENO output passes power flow
to the next element. If an error is detected in the execution of the box, then power flow is
terminated at the box that generated the error.
In STL, there is no ENO output, but the STL instructions that correspond to the LAD and FBD
instructions with ENO outputs do set a special ENO bit. This bit is accessible with the AND ENO
(AENO) instruction and can be used to generate the same effect as the ENO bit of a box.
Tip
The EN/ENO operands and data types are not shown in the valid operands table for each
instruction because the operands are the same for all LAD and FBD instructions. Table 5-1 lists
these operands and data types for LAD and FBD. These operands apply to all LAD and FBD
instructions shown in this manual.
Table 5-1 EN/ENO Operands and Data Types for LAD and FBD
Program Editor Inputs/Outputs
LAD EN, ENO
FBD EN, ENO
Conditional/Unconditional Inputs
In LAD and FBD, a box or a coil that is dependent upon power flow is shown with a connection to
any element on the left side. A coil or box that is independent of power flow is shown with a
connection directly to the left power rail. Table 5-2 shows an example of both a conditional and an
unconditional input.
Table 5-2 Representation of Conditional and Unconditional Inputs
Power Flow
Instruction that is dependent on power flow (conditional)
Instruction that is independent of power flow (unconditional)
Instructions without Outputs
Boxes that cannot cascade are drawn with no Boolean outputs. These include the Subroutine
Call, Jump, and Conditional Return instructions. There are also ladder coils that can only be
placed on the left power rail. These include the Label, Next, Load SCR, Conditional SCR End, and
SCR End instructions. These are shown in FBD as boxes and are distinguished with unlabeled
power inputs and no outputs.
Compare Instructions
The compare instruction is executed regardless of the state of power flow. If power flow is false,
the output is false. If power flow is true, the output is set depending upon the result of the
compare. SIMATIC FBD, IEC Ladder, and IEC FBD compare instructions are shown as boxes,
although the operation is performed as a contact.
Programming Concepts, Conventions, and Features
Operands
Power Flow
I, Q, V, M, SM, S, T, C, L
LAD
Chapter 5
Data Types
BOOL
BOOL
FBD
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