Iec 61131-3 Second edition 2003-01 Programmable controllers – Part 3: Programming languages

order
Number of
variables
In Structured Text (ST) language
- see 3.3
1
formal
yes
any
any
A := LIMIT(EN:=COND, IN:=B,
MX:=5, ENO=>TEMPL);
2 
a
non-formal
no
fixed
fixed
A := LIMIT(1, B, 5);
a
Feature #2 is required for invocation of any of the standard functions defined in subclause
2.5.1.5 without formal names for one or more input variables, but feature #1 shall be used if
EN/ENO is necessary in function invocations.
NOTE 1 In the example given in feature #1, the MN variable will have the default value 0
(zero).
NOTE 2 The example given in feature #2 is semantically equivalent to the following
invocation with formal variable assignments (feature #1):
A := LIMIT(EN := TRUE,MN := 1, IN := B, MX := 5);
2.5.1.2 Execution control
As shown in table 20, an additional Boolean EN (Enable) input or ENO (Enable Out) output, or both,
can be provided by the manufacturer or user according to the declarations
VAR_INPUT EN: BOOL := 1; END_VAR
VAR_OUTPUT ENO: BOOL; END_VAR
When these variables are used, the execution of the operations defined by the function shall be
controlled according to the following rules:
1) If the value of EN is FALSE (0) when the function is invoked, the operations defined by the
function body shall not be executed and the value of ENO shall be reset to FALSE (0) by the
programmable controller system.
2) Otherwise, the value of ENO shall be set to TRUE (1) by the programmable controller system,
and the operations defined by the function body shall be executed. These operations can
include the assignment of a Boolean value to ENO.
3) If any of the errors defined in table E.1 for subclauses of 2.5.1.5 occurs during the execution of
one of the standard functions defined in 2.5.1.5, the ENO output of that function shall be reset to
FALSE (0) by the programmable controller system, or the manufacturer shall specify other
disposition of such an error according to the provisions of 1.5.1.
4) If 
the 
ENO output is evaluated to FALSE (0), the values of all function outputs (VAR_OUTPUT,
VAR_IN_OUT and function result) shall be considered to be implementation-dependent.
Copyright International Electrotechnical Commission 
Provided by IHS under license with IEC
Not for Resale
No reproduction or networking permitted without license from IHS
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61131-3 
 IEC:2003(E)
NOTE It is a consequence of these rules that the ENO output of a function must be explicitly
examined by the invoking entity if necessary to account for possible error conditions.