AutomationDirect DL105 - System Wiring Strategies; PLC Isolation Boundaries

Installation, Wiring,
and Specifications
2--14
Installation, Wiring, and Specifications
DL105 PLC User Manual, 3rd Edition
System Wiring Strategies
The DL105 Micro PLC is very flexible and will work in many different wiring
configurations. By studying this section before actual installation, you can probably
find the best wiring strategy for your application . This will help to lower system cost,
wiring errors, and avoid safety problems.
PLC circuitry is divided into three main regions separated by isolation boundaries,
shown in the drawing below. Electrical isolation provides safety, so that a fault in one
area does not damage another. A transformer in the power supply provides
magnetic isolation between the 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. Note that 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 avoid
making external connections that connect logic side circuits to any other.
CPU
Input
Circuit
Main
Power
Supply
Discrete Inputs
Discrete Outputs
Power Input
Output
Circuit
Primary Side Secondary, or
Logic side
Field Side
PLC
Programming Device or
Operator Interface
Isolation
Boundary
Isolation
Boundary
The next figure shows the internal layout of DL105 PLCs, as viewed from the front
panel. In addition to the basic circuits covered above, it includes an auxiliary
+24VDC power supply with its own i solation boundary. Since the supply output is
isolated from the other three circuits, it can power input and/or output circuits!
Output Circuit
Input Circuit
CPU
Comm.
Port
Main
Power
Supply
Auxiliary
+24VDC
Supply
To Programming
Device, or
10 Discrete Inputs Commons
Commons8 Discrete Outputs
+24VDC Out
Power Input
Power Input
(DC Out versions only)
PLC
DL105
(AC--powered versions only)
Operator
Interface
PLC Isolation
Boundaries