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22
TROUBLE EXAMPLES of DC INPUT/OUTPUT
22.1
Troubles and Countermeasures for DC Input
Circuits
Examples of troubles in the DC input circuit and countermeasures are shown below.
An input signal does not turn off No.1
■Cause
Drive by a switch with LED indicator
Leakage current
■Action
Connect an appropriate resistor as shown below so that a current through the DC input circuit may become lower than the
OFF current.
DC input (sink)
Resistance
For the calculation example of a resistor to be connected, refer to the following.
■Calculation example
If the switch with LED indicator with maximum leakage current of 3mA when 24VDC external power supply is provided is
connected
Leakage current 3mA
24VDC
1.
The OFF current through the DC input circuit is not 1.5mA or less. Therefore, connect a resistor as shown below.
3mA
I
=1.5mA
R
24VDC
2.
To satisfy the condition that the OFF current of the DC input circuit is 1.5mA or lower, the current through the connected
resistor should be 1.5mA or higher. From the formula below, the connected resistor (R) is lower than 1.6k.
I
: I
= Z (input impedance): R
R
Z
I
1.5
Z
R ≤
× Z (input impedance) =
I
1.5
R
DC input (sink)
FPGA
module
FPGA
module
FPGA module
I
=1.5mA
Z
Input impedance
R
1.6kΩ
× 1.6 = 1.6 [kΩ]
22 TROUBLE EXAMPLES of DC INPUT/OUTPUT
22.1 Troubles and Countermeasures for DC Input Circuits
22
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Troubleshooting
