LIFO

 Function Block A function block groups an algorithm and a set of private data. It has inputs and outputs. - Manages a last in / first out list.

Inputs

Inputs	Data Type	Range	Unit	Default	Description
BUFFER	ANY				Array for storing values.
NEXTOUT	ANY				Value at the top of the stack. Updated after call.
NEXTIN	ANY				Value to be pushed.
POP	BOOL				Pop a new value on the rising edge A rising edge is the transition of a digital signal from low to high. It is also called positive edge.
PUSH	BOOL				Push a new value on the rising edge.
RST	BOOL				Reset the list.

Outputs

Outputs	Data Type	Range	Unit	Description
EMPTY	BOOL			TRUE if the list is empty.
OFLO	BOOL			TRUE if the overflow is on a PUSH command.
Count	DINT			Number of values in the list.
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.
  • It cannot be a STRING.
• The NEXTOUT argument specifies a variable 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 stores 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.

FBD Language Example

FFLD Language Example

• In the FFLD "Free Form Ladder Diagram" Language, the input rung is the PUSH input.
  • The output rung is the EMPTY output.

IL Language Example

(* 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

ST Language Example

(* 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;

See Also

FIFO