Automationdirect.com DirectLOGIC DL205 Series User Manual page 59

Chapter 2: Installation, Wiring and Specifications
In the next example a PLC sinking DC output point is connected to the sinking input of a
field device. This is a little tricky, because both the PLC output and field device input are
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sinking type. Since the circuit must have one sourcing and one sinking device, a sourcing
capability needs to be added to the PLC output by using a pull-up resistor. In the circuit
below, a R
2
3
4
5
6
NOTE 1: DO NOT attempt to drive a heavy load (>25 mA) with this pull-up method
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NOTE 2: Using the pull-up resistor to implement a sourcing output has the effect of inverting the output
point logic. In other words, the field device input is energized when the PLC output is OFF, from a ladder
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logic point of view. Your ladder program must comprehend this and generate an inverted output. Or, you
may choose to cancel the effect of the inversion elsewhere, such as in the field device.
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It is important to choose the correct value of R
the nominal input current to the field device (I
is not known, it can be calculated as shown (a typical value is 15 mA). Then use I
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the voltage of the external supply to compute R
watts), in order to size R
11
12
13
Of course, the easiest way to drive a sinking input field device as shown below is to use a DC
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sourcing output module. The Darlington NPN stage will have about 1.5 V ON-state
saturation, but this is not a problem with low-current solid-state loads.
A
PLC DC Sourcing Output
B
C
D
2–20
DL205 User Manual, 4th Edition, Rev. B
is connected from the output to the DC output circuit power input.
pull-up
PLC DC Output
Power
+DC pwr
R
pull-up
(sourcing)
(sinking)
Output
Common
properly.
pull-up
V
input (turn–on)
I
=
input
R
input
V
– 0.7
supply
R
=
pull-up
I
input
+DC pwr Common
Output (sourcing)
Input
(sinking)
+
Supply
–
Ground
. In order to do so, you need to know
pull-up
) when the input is energized. If this value
input
. Then calculate the power P
pull-up
R P
–
input pull-up
Input
(sinking)
+
Supply
–
Ground
Field Device
R
input
and
input
pull-up
2
V
supply
=
R
pullup
Field Device
R
input
(in