National Instruments FP-3000 - Manual (Page 29)

Chapter 2 Installation and Configuration
© National Instruments Corporation 2-11 FieldPoint FP-3000 User Manual
The Vsupply and Common of input modules do not need to be wired. If the
inputs you are measuring should have a common ground, you could wire
the Common terminal of the module to the Common of the incoming
signals.
Supplying Power for Outputs
There are three ways to wire power for the outputs:
• Power the FP-3000 and each output module’s output circuitry with a
separate power supply. Do not cascade any of the Vsupply and
Common terminals. This option provides the most isolation.
• Power all the output circuitry by connecting a second power supply,
leaving the network module on its own power supply, and cascading
power from one set of connections to the next. This establishes two
independent grounds—one for the FP-3000. The backplane through
the terminal bases and the I/O modules themselves allows for some
isolation without using a dedicated supply for each output module.
• Power the outputs with the same supply as the FP-3000 and FieldPoint
modules by cascading the Vsupply and Common from the network
module to the first terminal base and then cascading power from one
set of connectors to the next. This allows outputs to be powered by the
same supply (assuming the supply can provide enough power to meet
the demands of the modules AND any output currents). The major
drawback of this scheme is that a single ground will be established for
both the backplane and the input/output circuitry.
Caution This method defeats the isolation of the modules.
Isolation
The Isolation rating is the maximum voltage differential that can occur
between the common terminal on the terminal base (the input circuit’s
ground level) and the ground in the backplane of the module (the network
module’s ground level) without causing damage to the circuitry.
Safety Isolation (or working voltage) is the maximum voltage differential
(per safety isolation specifications) that can be sustained between the
common terminal on the terminal base (the input circuit’s ground level) and
the ground in the backplane of the module (the network module’s ground
level) while still allowing accurate measurements and safe working
conditions for human operators.