LIFO

Function A function calculates a result according to the current value of its inputs. A function has no internal data and is not linked to declared instances. block - Manages a "last in / first out" stack.

Inputs

PUSH	BOOL	Push a new value (on rising edge A rising edge is the transition of a digital signal from low to high. It is also called positive edge).
POP	BOOL	Pop a new value (on rising edge).
RST	BOOL	Reset the list.
NEXTIN	ANY	Value to be pushed.
NEXTOUT	ANY	Value at the top of the stack - updated after call!.
BUFFER	ANY	Array for storing values.

Outputs

EMPTY	BOOL	TRUE if the stack is empty.
OFLO	BOOL	TRUE if overflow on a PUSH command.
COUNT	DINT	Number of values in the stack.
PREAD	DINT	Index in the buffer of the top of the stack.
PWRITE	DINT	Index in the buffer of the next push position.

Remarks

NEXTIN, NEXTOUT and BUFFER must have the same data type and cannot be STRING. The NEXTOUT argument specifies a variable which is filled with the value at the top of the stack after the block is called.

Values are stored in the BUFFER array. Data is never shifted or reset. Only read and write pointers and pushed values are updated. The maximum size of the stack is the dimension of the array.

The first time an instance of the LIFO function block is called, that instance will store which array is passed to BUFFER. If a later call to the same instance passes a different array for the BUFFER argument, the call is considered invalid and no action is performed. The EMPTY output returns TRUE in this case.

In FFLD language, input rung is the PUSH input. The output rung is the EMPTY output.

ST Language

(* MyLIFO is a declared instance of LIFO function block. *)
MyLIFO (PUSH, POP, RST, NEXTIN, NEXTOUT, BUFFER);
EMPTY := MyLIFO.EMPTY;
OFLO := MyLIFO.OFLO;
COUNT := MyLIFO.COUNT;
PREAD := MyLIFO.PREAD;
PWRITE := MyLIFO.PWRITE;

FBD Language

FFLD Language

IL Language

(* MyLIFO is a declared instance of LIFO function block *)
Op1: CAL MyLIFO (PUSH, POP, RST, NEXTIN, NEXTOUT, BUFFER)
FFLD  MyLIFO.EMPTY
ST "Structured text"
A high-level language that is block structured and syntactically resembles Pascal  EMPTY
FFLD  MyLIFO.OFLO
ST  OFLO
FFLD  MyLIFO.COUNT
ST  COUNT
FFLD  MyLIFO.PREAD
ST  PREAD
FFLD  MyLIFO.PWRITE
ST  PWRITE

See Also

FIFO