(Issue 1, 2004)
How to connect 3-wire sinking and sourcing
devices to PLC input modules
by Keri Schieber,
When connecting a 3-wire field device to your
PLC system, it is important to have a solid understanding of
sinking (NPN) and sourcing (PNP) concepts.
First, sinking and sourcing are only associated with DC circuits.
DC circuits can conduct current in one direction only. This
means that it is possible to connect the external supply and
field device to an I/O point with current trying to flow in
the wrong direction. By understanding sinking and sourcing
concepts, the supply and field devices can be connected correctly
Be sure to pay close attention to what field device you are using. If a field
device is a sinking (NPN) device, then the I/O module must have a sourcing input.
There are some I/O modules that are sink/source I/O circuits. This means that
the circuitry will allow current to flow in either direction. When using a sink/source
I/O module, you should wire all I/O points with a shared common as only sinking
or only sourcing.
the figures shown below, you will notice that the sinking PLC
input module is connected so the input provides a path to supply
common (-). So, starting at the PLC input terminal, follow
through the input sensing circuit, exit at the common terminal,
and connect the supply (-) to the common terminal. By adding
the switch between the supply source (+) and the PLC input,
the circuit is completed. Current flows in the direction of
the arrow when the switch is closed.
Sinking provides a path to supply common (-).
Sourcing supplies a path to supply source (+).
Sinking PLC Input Module
PLC Input Module
devices work in the same manner. The difference is that the
3-wire device has a supply source (+), supply common (-) and
signal (output). In the diagrams below, for a PNP (sourcing)
device, the supply common (-) is connected to the field device common (-).
The supply source (+) passes through the field device output into the PLC input
(sinking), through the input sensing circuit, out the common terminal and to
the supply common (-).
By following these guidelines, you should have a working circuit every time.
device examples - 3 wire connections
Links to the current issue of Automation Notebook are found below.
Click the Back Issues link to visit the archives.