Page 287 - Programmable Logic Controllers, Fifth Edition - Mobile version

timer accumulated time to zero and shift the logic
L1 Inputs LS Ladder logic program • After 3 seconds, the timer done bit is set to reset the
Limit switch BSR EN bit 1 to the left to B3:0/1.
BIT SHIFT RIGHT
LS File #B3:50 • This causes PL1 to turn off and PL2 to turn on.
Control R6:1 DN
Sensor • After another 3 seconds, the timer done bit is set
Bit address Ι:3/5
Length 24 once again.
Ι:3/5 • The BSL instruction shifts the bits to the left
once more and causes PL2 to turn off and PL3
to turn on.
B3: Table - Before limit switch clock pulse • The process continues with each of the pilot lights

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 turned on in sequence for 3 seconds.
B3:50 1 0 1 1 0 0 0 1 1 0 0 1 0 1 1 0 A shift register is often used in material handling pro-
B3:51 0 0 0 0 0 0 0 0 1 0 1 1 0 0 1 1
cesses where some form of binary information must be
synchronized with a moving part on a conveyor. The
Unload bit binary information refers to any two conditions that can
R6:1/UL 1 be assigned to the moving product—for example, the
Shift direction
15 0 presence or absence of a part. As the part moves along
1 0 1 1 0 00 1 1 0 0 1 0 1 1 0 B3:50 the conveyer, some form of sensing device will determine
Invalid 1 0 1 1 0 0 1 1 B3:51 which of these two categories the passing product falls
24 into. Figure 12-27 illustrates cartons traveling on a con-
0 veyor being detected by a photoelectric sensor. The sen-
Ι:3/5
Bit address sor that drives the data line on a shift register is fixed such
that the beam detects the presence or absence of a carton.
B3: Table - After limit switch clock pulse A logic 1 sensor condition state can indicate the presence
of a carton, and a 0 the absence.
15 14 13 12 11 10 98 76 5 4 3 21 0
B3:50 1 1 0 1 1 0 00 11 0 0 1 01 1 The process of Figure 12-28 illustrates a spray-painting
B3:51 0 0 0 0 0 0 00 01 0 1 1 00 1 operation controlled by a shift left register. As the parts
pass along the production line, the shift register bit pat-
terns represent the items on the conveyor hangers to be
Unload bit painted. Each file bit location represents a station on the
R6:1/UL 0
Shift direction line, and the status of the bit indicates whether or not a
part is present at that station.
1 1 0 1 1 0 0 0 1 1 0 0 1 0 1 1 B3:50 The program for the spray-painting operation is shown
Invalid 0 1 0 1 1 0 0 1 B3:51
in Figure 12-29. Its operation can be summarized as
24 follows:
0
Ι:3/5 • Limit switch LS1 is used to detect the hanger and
Bit address
limit switch LS2 the part.
Figure 12-25 Bit shift right (BSR) instruction program.
• The pulse generated by the hanger-operated limit
switch LS1 shifts the status of the data provided by
part-detection limit switch LS2.

on-delay timer T4:0. The operation of the program can be • The logic of this operation is such that when a part
summarized as follows: to be painted and a part hanger occur together at
station 1 (indicated by simultaneous closing of LS2
• Go to the data tables and set bit addresses B3:0/0, and by LS1), logic 1 is input into the shift register at
B3:0/1, B3:0/2 to logic 0 and bit address R6:0/UL B3:0/0.
to logic 1. • This causes the SOL 1 rung to be true and the
• When the PLC is then placed in run, bit B3:0/0 is undercoat spray gun to energize.
set to logic 1 causing PL1 to turn on. • At station 5, a 1 appears in bit B3:0/5 of the shift
• Closing input switch SW starts timer T4:0 register to make the SOL 2 rung true and topcoat
timing. spray gun energize.

268 Chapter 12 Sequencer and Shift Register Instructions

pet73842_ch12_252-280.indd 268 03/11/15 7:20 PM