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Digital I/O digital output with pull-up disabled:
•
When switch 1 closes, the port is on and the front panel LED (IPL EXP RIO8
•
only) is on.
•
When switch 1 opens, the port is off and the front panel LED (IPL EXP RIO8
only) is off.
•
Example application, digital output without pull-up: connecting an LED
and an external +5 VDC power source
This application often requires a current-limiting resistor, as shown in the
diagram below. Many button switches that contain LEDs have a resistor built in.
See the guide for the lighted switch or stand-alone LED for details.
NOTE: Each I/O pin is capable of sinking a maximum of 250 mA.
+5.0 V
1k ohms
SW 2
CTL
SW 1
Figure 14.
Digital I/O Digital Output Application:
LED and External +5 VDC Power Source Without Pull-up
To determine the value of the current limiting resistor in the circuit shown above,
you need to know the values of three variables:
i = LED forward current in amps (found in the data sheet for the LED)
V
= LED forward voltage drop in volts (found in the data sheet for the LED)
f
= supply voltage of the external voltage source
V
s
Insert those values into the following equation to determine the resistor value:
V
- V
s
f
R =
i
Example calculation:
}
i = 5 mA (0.005 A)
V
= 2 V
f
V
= 5 V
s
NOTE: If the value calculated for the current limiting resistor is not a
standard resistor value, you can round up the number to the next highest
common resistance value.
The connected LED is off when the port and switch 1 are open.
The connected LED is on when the port and switch 1 are closed.
IPL EXP I/O Series • Hardware Features and Installation
Digital
I/O
Voltage
Protection
GND
V
- V
5 V - 2 V
s
f
R =
=
= 600 ohms
i
0.005 A
Resistor
(R)
LED
+
-
External
+5 VDC
Power
Source
25
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