Page 308 - Programmable Logic Controllers, Fifth Edition - Mobile version

13.7 Programming and Monitoring
An online programming mode permits the user to
change the program during machine operation. As the
When you program a PLC, several instruction entry PLC controls its equipment or process, the user can add,
modes are available, depending on the manufacturer change, or delete control instructions and data values as
and the model of the unit. A personal computer, with desired. Any modification made is executed immediately
appropriate software, is generally used to program and on entry of the instruction. Therefore, the user should
monitor the program in the PLC. Additionally, it makes assess in advance all possible sequences of machine
possible offline programming, which involves writing operation that will result from the change. Online pro-
and storing the program in the personal computer with- gramming should be done only by experienced person-
out its being connected to the PLC and later download- nel who understand fully the operation of the PLC they
ing it to the PLC. Figure 13-14 illustrates how programs are dealing with and the machinery being controlled. If
are downloaded and uploaded from and to the computer. at all possible, changes should be made offline to pro-
With online programming the program can be modi- vide a safe transition from existing programming to new
fied, the modifications can be tested, and finally they can programming.
be accepted or rejected while the PLC is running. How- Two useful monitoring tools provided with PLC pro-
ever, offline programming is the safest manner in which to gramming packages are data monitor and cross reference.
edit a program because additions, changes, and deletions Data monitoring functions allow you to monitor and/or
do not affect the operation of the system until downloaded modify specified program variables. The cross reference
to the PLC. function allows you to search each instance of a particular
Many manufacturers provide a continuous test mode address.
that causes the processor to operate from the user pro- The data monitor feature allows you to display data
gram without energizing any outputs. This mode allows from any place in the data table. Depending on the
the control program to be executed and debugged while PLC, the data monitor function can be used to do the
the outputs are disabled. A check of each rung can be following:
done by monitoring the corresponding output rung on the
programming device. A single-scan test mode may also • View data within an instruction
be available for debugging the control logic. This mode • Store data or values for an instruction prior to use
causes the processor to complete a single scan of the user • Set or reset values and/or bits during a debug opera-
program each time the single-scan key is pressed with no tion for control purposes
outputs being energized. • Change the radix or data format

Unsaved program in the
computer is lost.

Program from the computer
is loaded into the PLC.

Program from the PLC
is loaded into the computer.

Program that was in the Program that was in the
PLC is lost. PLC remains unchanged.
(a) Downloading a program (b) Uploading a program
Figure 13-14 Downloading and uploading PLC program.

PLC Installation Practices, Editing, and Troubleshooting Chapter 13 289

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