Page 313 - Programmable Logic Controllers, Fifth Edition - Mobile version

Output Malfunctions
an output device is commanded to turn on but the
In addition to the logic indicator, some output modules • If, according to the programming device monitor,
incorporate either a blown fuse indicator or a power indi- status indicator is off, then the output module or
cator or both. A blown fuse indicator indicates the status processors may be at fault.
of the protective fuse in the output circuit, while a power • Check voltage at output; if incorrect, power supply,
indicator shows that power is being applied to the load. wiring, or output device may be faulty.
Electronic protection, as shown in Figure 13-22, is also
used to provide protection for the modules from short- Ladder Logic Program
circuit and overload current conditions. The protection
is based on a thermal cut-out principle. In the event of Many PLC software programs offer various software
a short-circuit or overload current condition on an out- checks used to verify program logic. Figure 13-24 shows
put channel, that channel will limit current within mil- a sample of verifying program errors using RSLogix 500
liseconds after its thermal cut-out temperature has been software. Selecting edit then verify project will check
reached. All other channels continue to operate as di- the program for errors. The sample shows what the error
rected by the processor. message might look like.
When an output does not energize as expected, first The ladder logic program itself is not likely to fail, as-
check the output module blown fuse indicator. Many out- suming that the program was at one time working cor-
put modules have each output fused. This indicator will rectly. A hardware fault in the memory IC that holds the
normally illuminate only when the output circuit corre- ladder logic program could alter the program, but this is
sponding to the blown fuse is energized. If this indicator a PLC hardware failure. If all other possible sources of
is illuminated, correct the cause of the malfunction and trouble have been eliminated, the ladder logic program
replace the blown fuse in the module. should be reloaded into the PLC from the master copy of
Figure 13-23 shows a typical discrete output module the program. Make sure the master copy of the program is
troubleshooting guide. In general, the following items up to date before you download it to the PLC.
should be noted when troubleshooting discrete output Start program troubleshooting by identifying which
modules: outputs operate properly and which outputs do not. Then
trace back from the output on the nonfunctioning rung and
• If the blown fuse indicator is not illuminated (fuse examine the logic to determine what may be preventing
OK), then check to see if the output device is re- the output from energizing. Common logic errors include:
sponding to the LED status indicator.
• An output module’s logic status indicator func- • Programming an examine if closed instruction
tions similarly to an input module’s status indica- instead of an examine if open (or vice versa)
tor. When it is on, the status LED indicates that the • Using an incorrect address in the program
module’s logic circuitry has recognized a command Although the ladder logic program is not likely to fail,
from the processor to turn on. the process may be in a state that was unaccounted for in
• If an output rung is energized, the module status the original program and thus is not controlled properly.
indicator is on, and the output device is not respond- In this case, the program needs to be modified to include
ing, then the wiring to the output device or the out- this new state. A careful examination of the description of
put device itself should be suspected. the control system and the ladder logic program can help
identify this type of fault.
The force on and force off instructions allow you to turn
Output specific bits on or off for testing purposes. Figure 13-25
Electronic
protection illustrates how forces are identified as being enabled or dis-
Status E LED
indicators F abled in RSLogix 500 software. Forcing lets you simulate
U operation or control an output device. For example, forcing
S
E a solenoid valve on will tell you immediately whether the
solenoid is functional when the program is bypassed. If it
is, the problem must be related to the software and not the
hardware. If the output fails to respond when forced, either
the actual output module is causing the problem or the so-
lenoid itself is malfunctioning. Take all necessary precau-
Figure 13-22 Electronic output module protection. tions to protect personnel and equipment during forcing.

294 Chapter 13 PLC Installation Practices, Editing, and Troubleshooting

pet73842_ch13_281-304.indd 294 03/11/15 7:22 PM