FIFO

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

Inputs

Inputs	Data Type	Range	Unit	Default	Description
@Tail	ANY				Value of the oldest pushed value - updated after call!
Buf[]	ANY				Array for storing values.
IN	ANY				Value to be pushed.
POP	BOOL				Pop 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).
PUSH	BOOL				Push a new value (on 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 overflow on a PUSH command.
Count	DINT			Number of values in the list.
pRead	DINT			Index in the buffer of the oldest pushed value.
pWrite	DINT			Index in the buffer of the next push position.

Remarks

• IN, @Tail and Buf[] must have the same data type.
  • It cannot be STRING.
• The @Tail argument specifies a variable filled with the oldest push value after the block is called.
• Values are stored in the Buf[] array.
  • Data is arranged as a roll over buffer and is never shifted or reset.
  • Only read and write pointers and pushed values are updated.
  • The maximum size of the list is the dimension of the array.
• The first time an instance of the FIFO function block is called, that instance stores which array is passed to BUF[].
  • If a later call to the same instance passes a different array for the BUF[] argument, the call is considered invalid and no action is performed.
  • In this instance, the EMPTY output returns TRUE.

FBD Language

FFLD Language

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

IL Language

(* MyFIFO is a declared instance of FIFO function block *)
Op1: CAL MyFIFO (PUSH, POP, RST, IN, @Tail , BUFF[])
FFLD  MyFIFO.EMPTY
ST "Structured text"
A high-level language that is block structured and syntactically resembles Pascal  EMPTY
FFLD  MyFIFO.OFLO
ST  OFLO
FFLD  MyFIFO.COUNT
ST  COUNT
FFLD  MyFIFO.PREAD
ST  PREAD
FFLD  MyFIFO.PWRITE
ST  PWRITE

ST Language

(* MyFIFO is a declared instance of FIFO function block: *)
MyFIFO (PUSH, POP, RST, IN, @Tail , BUFFER);
EMPTY := MyFIFO.EMPTY;
OFLO := MyFIFO.OFLO;
COUNT := MyFIFO.COUNT;
PREAD := MyFIFO.PREAD;
PWRITE := MyFIFO.PWRITE;

See Also

LIFO