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11. TROUBLESHOOTING
11 - 16
MELSEC-A
11.4 I/O Connection Troubleshooting
This section explains possible problems with I/O circuits.
11.4.1 Input circuit troubleshooting
This section describes possible problems with input circuits, and corrective action.
Table 11.2 Input circuit problems and corrective action
Condition Cause Corrective action
Example
1
Input signal
does not turn
OFF
Leakage current of input switch (e.g. drive by non-
contact switch).
C
R
Leakage current
Power supply
AC input
Input module
•
Connect an appropriate resistor which will
make the voltage across the terminals of the
input module lower than the OFF voltage value.
C
R
AC input
Input
module
It is recommended to use 0.1 to 0.47
µ
F + 47 to
120
Ω
(1/2 W) for the CR constant.
Example
2
Input signal
does not turn
OFF
Drive by a limit switch with neon lamp.
Leakage
current
AC input
Input module
Power supply
•
Same as Example 1.
•
Or make up another independent display
circuit.
Example
3
Input signal
does not turn
OFF
Leakage current due to line capacity of wiring cable.
(Line capacity C of twisted pair wire is approx. 100
PF/m).
Leakage
current
AC input
Input module
Power supply
•
Same as Example 1.
•
However, leakage current is not generated
when the power supply is located in the input
equipment side as shown below.
AC input
Input
module
Power supply
Example
4
Input signal
does not turn
OFF
Drive by switch with LED indicator.
Leakage
current
DC input (sink)
Input module
•
Connect a register which will make the voltage
between the input module terminal and
common higher than the OFF voltage, as
shown below.
Resistor Input
module
DC input (sink)
* An example calculation of a value for a
connected resistor is given on the following
page.