Automation Direct DL205 Manual page 32

3–8
I/O Wiring and Specifications
DL205 Installation and I/O Manual, Rev. B
In the next example a sinking DC output point is connected to the sinking input of a
field device. This is a little tricky, because both the DL205 system output and field
device input are sinking type. Since the circuit must have one sourcing and one
sinking device, a sourcing capability needs to be added to the DL205 system output
by using a pull-up resistor. In the circuit below, a R
to the DC output circuit power input.
DL205 System DC Output
Power
+DC pwr
R
(sourcing)
(sinking)
Output
Common
NOTE 1: DO NOT attempt to drive a heavy load (>25 mA) with this pull-up method
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 DL205 system output is OFF, from a ladder 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.
It is important to choose the correct value of R
know the nominal input current to the field device (I
If this value is not known, it can be calculated as shown (a typical value is 15 mA).
Then use I and the voltage of the external supply to compute R
input
calculate the power P
pull-up
V
input (turn–on)
I
=
input
R
input
V
supply
R
=
pull-up
I
input
Of course, the easiest way to drive a sinking input field device as shown below is to
use a DC 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.
DL205 DC Sourcing Output
+DC pwr
pull-up
+
Supply
–
pull-up
(in watts), in order to size R
– 0.7
R
–
input
Common
Output (sourcing)
+
Supply
–
is connected from the output
pull-up
Field Device
R
input
Input
(sinking)
Ground
. In order to do so, you need to
) when the input is energized.
input
pull-up.
properly.
pull-up
V
supply
P
=
pull-up
R
pullup
Field Device
R
input
Input
(sinking)
Ground
Then
2