Ground Reference Problem - Allen-Bradley 2711P Technical Data Manual

32 Wiring and Grounding Guidelines for PanelView Plus Terminals
Nonisolated Communication
DC Power Supply
+24V DC
Power Common
DC - DC +
Comms
Circuitry
>
PanelView Plus
Nonisolated
Comms Port

Ground Reference Problem

A ground reference problem can occur if the equipment communicating with the
PanelView Plus terminal, using nonisolated communication lines on both ends, is
referenced away from earth ground by its power supply. Some systems have
voltages other than +24V, such as -24V DC. The communication equipment
might use these connections:
• +24V DC input is connected to the power supply common.
• Common is connected to the -24V DC power supply.
These connections provide +24V DC across the communication equipment
power supply and do not present a problem with the communication equipment
alone. However, when the communication equipment is connected to the
PanelView Plus terminal, the -24V DC power supply is shorted to the device's
DC- and earth ground. High or sustained current in the PanelView Plus terminal
can damage the device. Use communication lines that are isolated on at least
one end.
User -24V Power Supply
24V DC
- 24V DC
OV
+24V
Comms
Circuitry
> >
> >
Communication Equipment
Nonisolated Comms Port
Rockwell Automation Publication 2711P-TD001D-EN-P - February 2011
Isolated Communication
DC Power Supply
+24V DC
Power Common
DC -
DC +
Circuitry
Isolated
PanelView Plus
Communication
Terminal
User -24V Power Supply
24V DC
OV - 24V DC
+24V
Circuitry
Comms
Communication Equipment