DL205 User Manual, 4th Edition, Rev. D
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Chapter 2: Installation, Wiring and Specifications
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Connecting DC I/O to “Solid State” Field Devices
In the previous section on Sourcing and Sinking concepts, the DC I/O circuits were explained
to sometimes only allow current to flow one way. This is also true for many of the field devices
which have solid-state (transistor) interfaces. In other words, field devices can also be sourcing
or sinking. When connecting two devices in a series DC circuit, one must be wired as sourcing and
the other as sinking.
Solid State Input Sensors
Several DL205 DC input modules are flexible because they detect current flow in either
direction, so they can be wired as either sourcing or sinking. In the following circuit, a field
device has an open-collector NPN transistor output. It sinks current from the PLC input
point, which sources current. The power supply can be the +24 auxiliary supply or another
supply (+12VDC or +24VDC), as long as the input specifications are met.
In the next circuit, a field device has an open-collector PNP transistor output. It sources
current to the PLC input point, which sinks the current back to ground. Since the field device
is sourcing current, no additional power supply is required.
Solid State Output Loads
Sometimes an application requires connecting a PLC output point to a solid state input on a
device. This type of connection is usually made to carry a low-level control signal, not to send
DC power to an actuator.
Several of the DL205 DC output modules are the sinking type. This means that each DC
output provides a path to ground when it is energized. In the following circuit, the PLC output
point sinks current to the output common when energized. It is connected to a sourcing input
of a field device input.
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