Chapter 6 PLC Programming 121
Run mode:
Mode in which the PLC
runs (executes) its ladder
logic diagram program.
Online monitor mode:
Mode in which operating
PLC input/output
instructions can be
monitored on the PC
screen.
6.8 Loading and Troubleshooting PLC
Ladder Diagrams in Run Mode
In this section, the PLC ladder logic diagram for the relay logic
diagram displayed in Figure 6-26 will be created. The PLC ladder logic
diagram will be loaded into the PLC. Then, the PLC will be placed in
the run mode and online monitor mode. Placing the PLC on the run
mode allows the PLC to start controlling the input/output devices.
Placing the PLC on the online monitor mode allows the programmer
to see the operation of a running PLC control system on the computer
screen. This mode is used by the programmer to troubleshoot the PLC
program while it is actually controlling the input/output devices.
Figure 6-27 displays the input/output connections of an Allen-
Bradley fixed SLC 500 PLC device. Table 6-5 illustrates the input/out-
put ports’ assignments.
Place the PLC in the offline mode and create the ladder logic
diagram displayed in Figure 6-28. Offline mode is the mode when
the PLC ladder logic diagram can be created. Online mode is when the
PLC program is downloaded into the PC. Next, follow the instructions
provided by the RSLogix 500 software to download the program into
the PLC. (Note that the
Upload
command will transfer the program
from the PLC to the PC.) Finally, place the PLC in the run and online
monitor mode.
When the PLC is in the run and online monitor mode, the run
mode icon on the top of the screen will start rotating along its vertical
axis. Also, when the PLC is in the run and online monitor mode, the
two vertical rails in the ladder logic diagram will be highlighted.
Stop Start
CR
L1 L2
CR-1
CR-2
Jog Motor
Bell
Figure 6-26. Relay logic diagram.
Offline mode:
Mode in which a PLC
ladder logic diagram can
be created.
Online mode:
Mode in which the PLC
program is downloaded
into the PC.