Page 276 - Programmable Logic Controllers, Fifth Edition - Mobile version

O:2/0 N/S E/W O:2/3 N/S E/W
O:2/1 O:2/4
O:2/2 O:2/5
Step 1 Step 2
N/S E/W N/S E/W

Step 3 Step 4

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Output
word O:2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Positions
B3:0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Start
B3:1 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 1 Step 1
Sequencer
file B3:2 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 1 Step 2
B3:3 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 Step 3
B3:4 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 Step 4

Figure 12-8 Four-step sequencer.

• The arrows in the figure indicate the unmasked bits • The position of the sequencer instruction is incre-
that are passed through the mask and into the desti- mented by one for each false-to-true transition of
nation address. the sequencer rung.
• The dashes in the bits of the designation address • Whenever PB is momentarily closed, the
indicate that those bits remain unchanged in the des- sequencer is both enabled and advanced to
ignation location during the sequencing. the next position.
• These unchanged bits therefore can be used inde- • When the sequencer is at step 1, the binary informa-
pendently of the sequencer. tion in word B3:1 (100001) of the sequencer file is
transferred into word O:2 of the output.
The sequencer output instruction requires preceding
logic on the rung where it is located. When this logic goes • As a result output O:2/0 and O:2/5 will be on and
from false to true, it triggers the sequencer to perform its all the rest will be off.
functions. Only when the logic preceding the sequencer • Advancing the sequencer to step 2 will transfer the
instruction makes the transition from false to true will it data from word B3:2 (010001) into word O:2.
go through its functions of reading the data file, applying • As a result output O:2/0 and O:2/4 will be on and
the mask, and transferring the masked data file to the out- all the rest will be off.
put destination. After this cycle, it waits for another false- • Advancing the sequencer to step 3 will transfer the
to-true occurrence of the preceding logic to increment to data from word B3:3 (001100) into word O:2.
the next step. • As a result output O:2/2 and O:2/3 will be on and
Figure 12-10 illustrates how the sequencer moves data all the rest will be off.
from a file to an output. The operation of the logic rung • Advancing the sequencer to step 4 will transfer the
can be summarized as follows: data from word B3:4 (001010) into word O:2.
• Pushbutton PB is used to send false-to-true trigger • As a result output O:2/1 and O:2/3 will be on and
signals to the sequencer output instruction. all the rest will be off.

Sequencer and Shift Register Instructions Chapter 12 257

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