Page 316 - Programmable Logic Controllers, Fifth Edition - Mobile version

happens when the same address is used for two coils. The
resulting problem scenario can be summarized as follows: could be causing the problem rather than arbitrarily check-
ing every connection, switch, motor, sensor, I/O module,
and so on. First, observe the system in operation and try
• The problem is turning input switch I:1/1 on will to describe the problem. Using these observations and the
not turn on PL output O:2/1 as it appears to be description of the control system, you should identify the
programmed. possible sources of trouble. Compare the logic status of
• The root of the problem lies in the fact that the PLC the hardwired inputs and outputs to their actual state, as
scans the program from left to right and top to bottom. illustrated in Figure 13-29. Any disagreements indicate
• Whenever input switch I:1/1 is true (closed) and malfunctions as well as their approximate location.
input switch I:1/2 is false (open) output O:2/1 will Some of your troubleshooting can be accomplished by
be off. interpreting the status indicators on the I/O modules. The
• This is because when the PLC updates the outputs it key is to know whether the status indicators are telling
does so based on the status of input I:1/2. you that there is a fault or that the system is normal. Often
• Regardless of whether input I:1/1 is open or closed PLC manufacturers supply a troubleshooting guide, map,
the output reacts only to the status of input switch or tree that presents a list of observed problems and their
I:1/2. possible sources. Figure 13-30 shows a sample trouble-
shooting tree for a discrete output module. Figures 13-31
When a problem occurs, the best way to proceed is to and 13-32 are samples of input and output troubleshoot-
try to logically identify the devices or connections that ing guides.

Memory
Output image table

Input Output
module module

Status Status

o Input image table On L1
L1
Input
device
Load
energized
on
Open-o
User program
L2 L2

Hardware comparison test—compare
state of I/O as stored in memory to
actual state

True True
Logic observation—determine validity of decisions made by processor

Figure 13-29 General methods of troubleshooting.
Source: Courtesy of TheLearningPit.

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