Conventions for S7-200 Instructions
8-4
S7-200 Programmable Controller System Manual
A5E00066097-02
Table 8-1 EN/ENO Operands and Data Types for LAD and FBD
Language
Editor
Inputs/Outputs Operands Data Types
LAD EN Power Flow BOOL
ENO Power Flow BOOL
FBD EN I, Q, M, S, SM, T, C, V, L, Power Flow BOOL
ENO I, Q, M, S, SM, T, C, V, L, Power Flow BOOL
General Conventions of Programming
Network: In LAD the program is divided into segments called networks. A network
is an ordered arrangement of contacts, coils, and boxes that are all connected to
form a complete circuit between the left power rail and the right power rail (no short
circuits, no open circuits, and no reverse power flow conditions exist).
STEP 7-Micro/WIN 32 allows you to create comments for your LAD program on a
network by network basis.
FBD programming uses the network concept for segmenting and commenting your
program. STL programs do not use networks; however, you can use the
NETWORK keyword to segment your program. If you do this, your program can be
converted to either LAD or to FBD.
Execution Subsections: In LAD, FBD, or STL a program consists of at least one
mandatory section and other optional sections. The mandatory section is the Main
Program. Optional sections can include one or more subroutines and/or interrupt
routines. You can easily move between subsections of the program by selecting or
clicking on the subsection tabs displayed by STEP 7-Micro/WIN 32.
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 will pass 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 SIMATIC 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 STL instruction AENO (AND ENO), and may be used to
generate the same effect as the ENO bit of a box.
Conditional/Unconditional Inputs: In LAD and FBD, a box or a coil that is
dependent upon power flow is shown without 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. Both conditional and unconditional inputs are shown
in Figure 8-1.
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