Schlage PIB300 User Manual page 221

HandPunch 3000/4000 Manual
elevator motors or other large-current drawing machines) will have no effect on
the communication network if this configuration is used.
However, the power supplies must be truly floating, with no hidden paths back
to the high-voltage side of the transformers, or to earth ground. Since this
is difficult to achieve (there is always some parasitic capacitance between
the primary and secondary in any transformer), this method may be more
susceptible to high-frequency transients in the high-voltage side of the power
lines than the earth-grounded method.
The master unit's ground establishes the ground for the entire system. The
main board ground points are connected to the shield ground at each unit,
but are not connected to earth ground. The ground point on the master can
be the data converter power supply negative terminal, or the GND pin on the
RS-232 cable. If the master is an HGU, its main board ground can be used.
This configuration should only be used if the power supplies to the units are
truly floating, otherwise ground loops will be created, and differences in local
grounds may cause large currents to flow through the cable shield.
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