Page 312 - Programmable Logic Controllers, Fifth Edition - Mobile version

LED indicator
not illuminated Input module • When input hardware is suspected to be the source
of a problem, the first check is to see if the status
indicator on the input module illuminates when it is
Input pushbutton receiving power from its corresponding input device
contacts closed
(e.g., pushbutton, limit switch).
+24 V DC IN 0 • If the status indicator on the input module does not
IN 1 illuminate when the input device is on, take a volt-
IN 2 age measurement across the input terminal to check
for the proper voltage level.
Check input ? IN 3
voltage level – + • If the voltage level is correct, then the input module
IN 4
should be replaced.
IN 5
• If the voltage level is not correct, power supply, wir-
IN 6 ing, or input device may be faulty.
IN 7
If the programming device monitor does not show the
DC COM correct status indication for a condition instruction, the
Common
DC COM input module may not be converting the input signal prop-
erly to the logic level voltage required by the processor
module. In this case, the input module should be replaced.
Figure 13-20 Checking for input malfunctions.
If a replacement module does not eliminate the problem
and wiring is assumed to be correct, then the I/O rack,
communication cable, or processor should be suspected.
The circuit of Figure 13-20 illustrates how to check for Figure 13-21 shows a typical input device troubleshooting
discrete input malfunctions. The steps taken can be sum- guide. This guide reviews condition instructions and how
marized as follows: their true/false status relates to external input devices.

Input device troubleshooting guide
Monitor display
status indicator
Input Input module Possible
device condition status indicator fault(s)

True False None - correct indications
Closed — ON ON
24 V DC input
False True
Open — OFF OFF None - correct indications
0 V DC input

False True Sensor condition, input voltage, status indicator
Closed — ON ON are correct. Ladder instructions have incorrect
24 V DC input indications. Input module or processor fault.
False True Status indicator and instructions agree but not
Closed — ON OFF with the sensor condition. Open field device
0 V DC input or wiring.

True False Sensor condition, input voltage, status indicator
Open — OFF OFF are correct. Ladder instructions have incorrect
0 V DC input indications. Input module or processor fault.
True False Input voltage, status indicator, and ladder
Open — OFF ON instructions agree but not with sensor condition.
24 V DC input Short circuit in the field device or wiring.

Figure 13-21 Input troubleshooting guide.

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