I/O Wiring Strategies; Plc Isolation Boundaries - Automationdirect.com DirectLOGIC DL205 Series User Manual

Chapter 2: Installation, Wiring and Specifications

I/O Wiring Strategies

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The DL205 PLC system is very flexible and will work in many different wiring
configurations. By studying this section before actual installation, you can probably find the
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best wiring strategy for your application. This will help to lower system cost, wiring errors,
and avoid safety problems.
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PLC Isolation Boundaries

PLC circuitry is divided into three main regions separated by isolation boundaries, shown in
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the drawing below. Electrical isolation provides safety, so that a fault in one area does not
damage another. A powerline filter will provide isolation between the power source and the
power supply. A transformer in the power supply provides magnetic isolation between the
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primary and secondary sides. Opto-couplers provide optical isolation in Input and Output
circuits. This isolates logic circuitry from the field side, where factory machinery connects.
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Note the discrete inputs are isolated from the discrete outputs, because each is isolated from
the logic side. Isolation boundaries protect the operator interface (and the operator) from
power input faults or field wiring faults. When wiring a PLC, it is extremely important to
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avoid making external connections that connect logic side circuits to any other.
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In addition to the basic circuits covered above, AC-powered and 125VDC bases include an
auxiliary +24VDC power supply with its own isolation boundary. Since the supply output is
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isolated from the other three circuits, it can power input and/or output circuits!
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Primary Side
Power
A
Input
B
+24VDC Out
C
D
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DL205 User Manual, 4th Edition, Rev. B
Primary Side
PLC
Power
Input
Main
Filter
Power
Supply
Isolation
Boundary
DL205
PLC
Main
Filter
Power
Supply
Auxiliary
+24VDC
Supply
Secondary, or
Logic side
(backplane)
CPU
(backplane)
Programming Device,
Operator Interface, or Network
Secondary, or
Logic side
Internal
CPU
Comm.
Input Module
Inputs Commons
To Programming
Device, Operator
Field Side
Interface, Network
Field Side
Input
Inputs
Module
Output
Outputs
Module
Isolation
Boundary
Backplane
Output Module
Outputs Commons
Supply for
Output Circuit