Page 97 - Programmable Logic Controllers, Fifth Edition

L1 L2 Discrete Inputs L1
L2

AC I/0 I/1 I/2 I/3 AC I/4 I/5 I/6 I/7 I/8 I/9 I/10 I/11 I/12 I/13 I/14 I/15 I/16 I/17 I/18 I/19
COM COM
VAC O/0 VAC O/4 VAC O/2 O/3 VAC O/4 O/5 O/6 O/7 VAC O/8 O/9 O/10 O/11
VDC VDC VDC VDC VDC

CR CR CR CR CR CR CR CR CR CR

VAC 2 VDC 1 VDC 2 VDC 3 VDC 3
L1 VAC 2 VDC 1 VDC 2 COM
COM COM COM
L2
Discrete Outputs
Figure 5-7 Typical micro PLC with predefined addresses.
Source: Image Courtesy of Rockwell Automation, Inc.

• If the program calls for a specific output to be ON, 5.2 Program Scan
its corresponding bit in the table is set to 1.
• If the program calls for the output to be OFF, its When a PLC executes a program, it must know—in real
time—when external devices controlling a process are
corresponding bit in the table is set to 0. changing. During each operating cycle, the processor reads
• The processor continually activates or deactivates the all the inputs, takes these values, and energizes or de-
output status according to the output table file status. energizes the outputs according to the user program. This
Typically, micro PLCs have a fixed number of inputs process is known as a program scan cycle. Figure 5-8 illus-
and outputs. Figure 5-7 shows the MicroLogix controller trates a single PLC operating cycle consisting of the input
from the Allen-Bradley MicroLogix 1000 family of con- scan, program scan, output scan, and housekeeping duties.
trollers. The controller has 20 discrete inputs with pre- Because the inputs can change at any time, it constantly
defined addresses I/0 through I/19 and 12 discrete outputs repeats this cycle as long as the PLC is in the RUN mode.
with predefined addresses O/0 through O/11. Some units The time it takes to complete a scan cycle is called the
also contain analog inputs and outputs embedded into the scan cycle time and indicates how fast the controller can
base unit or available through add-on modules. react to changes in inputs. The time required to make a

The status of
external inputs
Internal checks START INPUT is written to the
on memory, speed, SCAN input image table
and operation. (file or register).
Service any HOUSE-
communication KEEPING
requests.
Each ladder rung
PROGRAM is scanned and solved
SCAN using the date in the
OUTPUT
SCAN input file. The resulting
logic is written to the
The output image output image table
date is transferred (file or register).
to the external output
circuits, turning the
output device
ON or OFF.
Figure 5-8 PLC program scan cycle.

78 Chapter 5 Basics of PLC Programming

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