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7
I/O Sentinel 4 G2 Installation and Operation Manual
INSTALLATION
Status Input
Each optically isolated input has a terminal labelled “xA” and a terminal
labelled “xB”. Inputs can be configured for either wet or dry operation via internal
jumpers. The factory default configuration is dry, where the “A” side of the input is
ground (GND) and the “B” side of the input is the cathode (-) of the opto-isolator.
In this configuration 5V is applied internally to the anode (+) of the opto-isolator.
This configuration is best for interfacing with external dry contact relay outputs,
switches, and open collector outputs.
In the “wet” configuration an external voltage must be applied to the input
to activate the opto-isolator. When configured for wet operation the “A” side of the
input is the anode (+) and the “B” side of the input is the cathode (-). This configu-
ration is best where full isolation is preferred or when interfacing with external volt-
age/logic level outputs.
Each optically isolated input is connected through an internal 2.2k ohm
series current-limiting resistor directly to an opto-coupler circuit so no external
resistor is necessary if the input voltage is between 5 and 24 VDC. Higher DC volt-
ages, from 25 to 48 VDC, can be used but must be reduced with an additional exter-
nal resistor of the appropriate value and power rating to limit the input current.
Here is how to calculate the value and power rating of an external current
limiting resistor for DC voltages up to 48 VDC: Each opto-isolated input has an
internal 2.2K ohm series resistor. The opto-isolator works well with an input current
of 9 mA and has a voltage drop of around 1.2V. With this information we can deter-
mine the correct external series current limiting resistor value needed for other volt-
ages using the equation:
R = ((Vin-1.2)/0.009)-2200
Where:
R = External resistor value required
Vin = Desired input voltage
1.2 V = Forward voltage drop of the LED in the opto-isolator
0.009 A = Nominal LED current
2200 ohms = Internal resistor
For example:
To connect a 48 VDC signal voltage to an input on the I/O Sentinel 4 G2 in “wet”
configuration the completed equation for the external resistor value would be:
R = ((48-1.2)/0.009)-2200 = 3000 ohms
To calculate the power dissipated by the external resistor, the equation would be:
P = I x I x R, so the resistor must be at least .009 x .009 x 2200 = 0.243 Watts,
use a 1/2 Watt rated resistor.
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