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

Table 32 - Examples of function block I/O variable usage
Usage
Inside function block
Outside function block
Input read
IF IN1 THEN ...
Not allowed (Notes 1 and 2)
Input assignment
Not allowed (Note 1)
FB_INST(IN1:=A,IN2:=B);
Output read
OUT := OUT AND NOT IN2;
C := FB_INST.OUT;
Output assignment
OUT := 1;
Not Allowed (Note 1)
In-out read
IF INOUT THEN ...
IF FB1.INOUT THEN...
In-out assignment
INOUT := OUT OR IN1; (Note 3)
FB_INST(INOUT:=D);
NOTE 1
Those usages listed as “not allowed” in this table could lead to implementation-
dependent, unpredictable side effects.
NOTE 2
Reading and writing of input, output and internal variables of a function block may
be performed by the “communication function”, “operator interface function”, or the
“programming, testing, and monitoring functions” defined in IEC 61131-1.
NOTE 3
As illustrated in 2.5.2.2, modification within the function block of a variable declared
in a VAR_IN_OUT block is permitted.
2.5.2.1a) Use of EN and ENO in function blocks
As shown in table 20 for functions, for function blocks an additional Boolean EN (Enable) input or ENO
(Enable Out) output, or both, can also 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 block shall be
controlled according to the following rules:
1) If the value of EN is FALSE (0) when the function block instance is invoked, the assignments
of actual values to the function block inputs may or may not be made in an implementation-
dependent
fashion, the operations defined by the function block 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,
the assignments of actual values to the function block inputs shall be made and the operations
defined by the function block body shall be executed. These operations can include the
assignment of a Boolean value to ENO.
3) If the ENO output is evaluated to FALSE (0), the values of the function block outputs
(VAR_OUTPUT) keep their states from the previous invocation.
NOTE It is a consequence of these rules that the ENO output of a function block must be explicitly
examined by the invoking entity if necessary to account for possible error conditions.
EXAMPLES The figures below illustrate the use of EN and ENO in association with the standard
TP, TON and TOF blocks (represented by T**) defined in subclause 2.5.2.3.4, and the
CTU and CTD blocks (represented by CT*) defined in subclause 2.5.2.3.3. In accordance
with the above rules, a FALSE value of the EN input may be used to “freeze” the operation
of the associated function block; that is, the output values do not change irrespective of
changes in any of the other input values. When the EN input value becomes TRUE, normal
Copyright International Electrotechnical Commission 
Provided by IHS under license with IEC
Not for Resale
No reproduction or networking permitted without license from IHS
--``````-`-`,,`,,`,`,,`---

---

61131-3 
 IEC:2003(E)
– 69 –
operation of the function block may resume. The value of the ENO output is FALSE after
each evaluation of the function block for which the EN input is FALSE. When EN is TRUE, a
TRUE value of ENO reflects a normal evaluation of the block, and a FALSE value of ENO
may be used to indicate an implementation-dependent error condition.
+-------+ +-------+
| T** | | CT* |
BOOL---|EN ENO|---BOOL BOOL---|EN ENO|---BOOL
BOOL---|IN Q|---BOOL BOOL--->CU Q|---BOOL
TIME---|PT ET|---TIME BOOL---|R CV|---INT
+-------+ INT---|PV |
+-------+