Chapter 8: Installation and Wiring
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Do-more T1H Series PLC User Manual, 1st Edition, Rev. C
8-30
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 Terminator I/O 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 Terminator I/O system output by using a pull-up
resistor. In the circuit below, an R
pull-up
is connected from the output to the DC power input
circuit.
NOTE 1
: DO NOT attempt to drive a heavy load (>25mA) 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 Terminator I/O 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 pull-up. In order to do so, you need to know
the nominal input current to the field device (I input) when the input is energized. If this
value is not known, it can be calculated as shown (a typical value is 15 mA). Then use I
input and the voltage of the external supply to compute R
pull-up
. Then calculate the power
P
pull-up
(in watts), in order to size R
pull-up
properly.
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.
Field Device
Output
Ground
Input
Common
Do-more T1H PLC System DC Output
+DC pwr
+
–
(sourcing)
(sinking)
Power
(sinking)
pull-up
Supply
R
input
R
pull-up
R
input
R
=
supply
V
– 0.7 –
input
I
input
I
=
input (turn–on)
V
input
R
pull-up
P
=
supply
V
2
pullup
R
Field Device
Output
Ground
Input
Common
Do-more T1H PLC DC Sourcing Output
+DC pwr
+
–
(sourcing)
(sinking)
Supply
input
R
