Logix5000 Controllers General Instructions - SLAC Confluence

Array (File)/Shift Instructions (BSL, BSR, FFL, FFU, LFL, LFU) 415CONTROL StructureMnemonic Data Type: Description.EU BOOL The enable unload bit indicates that the LFU instruction is enabled. The .EU bit is set topreset a false unload when the program scan begins..DN BOOL The done bit is set to indicate that the LIFO is full (.POS = .LEN)..EM BOOL The empty bit indicates that the LIFO is empty. If .LEN ≤ 0 or .POS < 0, both the .EM bit and.DN bit are set..LEN DINT The length specifies the maximum number of elements the LIFO can hold at one time..POS DINT The position identifies the end of the data that has been loaded into the LIFO.Description:Use the LFU instruction with the LFL instruction to store and retrievedata in a last-in/first-out order.When enabled, the LFU instruction unloads the value at .POS of theLIFO and places that value in the Destination. The instruction unloadsone value and replaces it with 0 each time the instruction is enabled,until the LIFO is empty. If the LIFO is empty, the LFU returns 0 to theDestination.IMPORTANTYou must test and confirm that the instruction doesn’t change data that you don’twant it to change.The LFU instruction operates on contiguous memory. In some cases, the instructionunloads data from other members of the tag. This happens if the length is too bigand the tag is a user-defined data type.Arithmetic Status Flags:not affectedFault Conditions:A Major Fault Will Occur If Fault Type Fault CodeLength > LIFO array size 4 20Publication 1756-RM003I-EN-P - January 2007