Page 293 - Programmable Logic Controllers, Fifth Edition - Mobile version

Destination—Address to which the FFU unloads data.
The program of Figure 12-36 is an example of how
Control—R data-table type. It is a three-word ele- data are indexed in and out of a FIFO file using the FFL
ment that consists of the status word, the length, and and FFU instruction pair. The operation of the program
the position. When it is paired with the FFL, the con- can be summarized as follows:
trol addresses are the same. • The FIFO load and FIFO unload instructions share
Length—File length in words. Specifies the maxi- the same control element, R6:0, which may not be
mum number of words in the stack. used to control any other instructions.
Position—Next location from which data are • FIFO, #N7:12, is the address of the stack. The
unloaded when the instruction goes from false same address is programmed for the FFL and FFU
to true. instructions.

The status bits of the control word are the enable (EN), • Data enter the FIFO file from the source address,
the done (DN), and the empty (EM) bits. The enable bit N7:10, on a false-to-true transition of input A.
follows the instruction’s status, the done bit is set when • Data are placed at the position indicated in the
the instruction’s position equals the length, and the empty instruction on a false-to-true transition of the
bit is set when all the data have been unloaded from the FFL instruction, after which the position indicates the
FIFO file. current number of data entries in the FIFO file.

Inputs Ladder logic program
L1 Input A
FFL
FIFO LOAD EN Integer Tabl e
Source N7:10
Input A FIFO #N7:12 DN Value
Control R6:0 N7:10 23
Length 10 EM N7:11 16
Position 8
N7:12 31
Input B N7:13 53
FFU
FIFO UNLOAD EU N7:14 146
FIFO #N7:12 N7:15 9875
Input B Dest N7:11 DN N7:16 125
Control R6:0 N7:17 867
Length 10 EM N7:18 5
Position 8 N7:19 11
N7:20 0
N7:21 0
Radix: Decimal

FIFO file
Destination #N7:12
N7:11 16 0 31 N7:12 Data index
1 53
Data exit from position 0 of the 2 146 toward the
FIFO file on a false-to-true 3 9875 starting
transition of the FFU and write 4 125 address of
over current data in the 5 867 the file, one
destination. 6 5 word with
Source 7 11 each false-to-true
N7:10 23 8 0 transition of the
FFU.
9 0 N7:21
Data enter the FIFO file on a
false-to-true transition of the FFL Position
at the position indicated in the
instruction.
Figure 12-36 How data are indexed in and out of a FIFO file.

274 Chapter 12 Sequencer and Shift Register Instructions

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