Troubleshooting For The Built-In I/O Function; Troubleshooting For Input Circuit - Mitsubishi MELSEC L Series User Manual

15.4

Troubleshooting for the Built-In I/O Function

This section describes possible problems with I/O circuits and their corrective actions.
For troubleshooting for I/O modules, refer to the following.
MELSEC-L I/O Module User's Manual
15.4.1

Troubleshooting for input circuit

(1) An input signal does not turn off.
(a) Case 1
Cause
A current exceeding the off current of the module leaks even after a switch with LED indicator is turned off.
Connect an appropriate resistor so that a current across the module may become lower than the off current.
Action
The resistance value of a connected resistor is calculated by the following formula.
Ex.
A switch with LED indicator that generates a current leakage of 2.33mA when 24VDC is supplied is connected to the CPU module set
to high-speed input.
Check the following with the specifications of the module.
• Off current: 1.5mA
• Input resistance: 3.8kΩ
To hold the off current equal to or lower than 1.5mA, connect a resistor (R) so that a current across the resistor may be greater than or
equal to 0.83mA. Calculate the resistance value (R) of a connected resistor as follows.
Calculation
→ The resistance value R < 6.87kΩ must be met.
example
<Checking a connected resistor by calculating the power capacity>
When the resistor (R) is 6.2kΩ, for example, the power capacity (W) of the resistor (R) is calculated as follows.
Since the resistor requires the power capacity of 3 to 5 times as large as the actual power consumption, the resistor connected to the
terminal should be 6.2kΩ and 1/2 to 1W.
Off voltage when the resistance (R) is connected is calculated as follows.
This meets the condition: less than or equal to the off voltage of the CPU module, 8V.
120
Iz=1.5mA
Input impedance
3.8k
I (Leakage current) = I (Off current of the CPU module) + I
Z
I = I - I = 2.33 - 1.5 = 0.83 [mA]
R Z
I : I = Z (Input impedance): R
R Z
I
Z
Z (Input impedance)=
R <
I
0.83
R
2
2
(Input voltage) 26.4
W = = = 0.112 [W]
6200
R
1
2.33 [mA] = 5.49 [V]
1 1
6.2 [k ] 3.8 [k ]
2.33mA
R
I =0.83mA
R
24VDC
(Current through the connected resistor)
R
1.5
3.8 = 6.87 [k ]