Page 112 - Programmable Logic Controllers, Fifth Edition

Program Mode The program mode is used to enter
a new program, edit or update an existing program,
upload files, download files, document (print out) pro-
grams, or change any software configuration file in the
program. When the PLC is switched into the program
mode, all outputs from the PLC are forced off regard-
less of their rung logic status, and the ladder I/O scan
sequence is halted.
Run Mode The run mode is used to execute the user
Highlighted rungs indicate program. Input devices are monitored and output de-
the instruction is true.
vices are energized accordingly. After all instructions
Figure 5-51 Monitoring a ladder logic program. have been entered in a new program or all changes
made to an existing program, the processor is put in
the run mode.
for PLC control. Not only is it reasonably intuitive, es- Test Mode The test mode is used to operate or
pecially for users with relay experience, but it is also monitor the user program without energizing any
particularly effective in an online mode when the PLC outputs. The processor still reads inputs, executes
is actually performing control. Operation of the logic is the ladder program, and updates the output status
apparent from the highlighting of rungs of the various table files, but without energizing the output cir-
instructions on-screen, which identifies the logic state of cuits. This feature is often used after developing
contacts in real time (Figure 5-51) and which rungs have or editing a program to test the program execu-
logic continuity. tion before allowing the PLC to operate real-world
For most PLC systems, each Examine If Closed and outputs. Variations of the test mode can include
Examine If Open contact, each output, and each branch the single-step test mode, which directs the proces-
Start/End instruction requires one word of user memory. sor to execute a selected single rung or group of
You can refer to the SLC 500 Controller Properties to see rungs; the single-scan test mode, which executes
the number of instruction words used and the number left a single processor operating scan or cycle; and the
as the program is being developed. continuous-scan test mode, which directs the pro-
cessor to continuously run the program for checking
5.10 Modes of Operation or troubleshooting.
Remote Mode Some processors have a three-
A processor has basically two modes of operation: the position switch to change the processor operating
program mode and some variation of the run mode. The mode. In the Run position, all logic is solved and the
number of different operating modes and the method of I/O is enabled. In the Program position, all logic solv-
accessing them varies with the manufacturer. Figure 5-52 ing is stopped and the I/O is disabled. The Remote
shows a typical three-position keyswitch used to select position allows the PLC to be remotely changed
different processor modes of operation. between program and run mode by a personal com-
Some common operating modes are explained in the puter connected to the PLC processor. The remote
following paragraphs. mode may be beneficial when the controller is in a
location that is not easily accessible.

5.11 Connecting with Analog Devices

Electrical devices and signals can be divided into two cat-
RUN REM PROG egories: analog and digital. Digital devices operate using
discrete ON or OFF signals that have only two possible
values. Analog signals can take any shape and represent
an infinite number of possible values, as illustrated in
Figure 5-53. Analog circuits are usually much more sus-
Figure 5-52 Three-position keyswitch used to select ceptible to noise (small, undesired variations in voltage).
different processor modes of operation. Small changes in the voltage level of an analog signal

Basics of PLC Programming Chapter 5 93

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