Using this method, an intercom station is connected to the intercom line just like any
phantom-powered intercom station, except that a local power supply is also con-
nected. This local power supply is located with the intercom station and provides
power for that station only. Since power loss on the intercom lines is no longer an is-
sue, the operating range is now limited only by the audio transmission range, which
is several miles. Another advantage to this method is that more stations can be con-
nected to the intercom channels. When local power is supplied to an intercom sta-
tion, the station detects this and automatically disconnects from the system's
phantom power supply.
Figure
6
illustrates an intercom system with both phantom powered and locally pow-
ered "U" boxes. As long as a system power supply (PS2000L, SPS2000A, etc.) is lo-
cated somewhere in the intercom system, the proper terminating impedance will still be
supplied for all stations. However, if all stations are locally powered, and there is no
system power supply, a special line termination must be installed as described below.
All Locally Powered Intercom Stations (Dry Lines)
If intercom stations are widely distributed, you can dispense with a system power
supply (PS2000L, SPS2000A, etc.) and use local power for each station. When no
power is delivered on the intercom channels, this is known as dry-line operation.
Since a system power supply is not used, a line termination must be inserted in each
intercom channel for proper operation. Again, note that only the "U" box has provi-
sions for local power. The 3-pin connectors on the "S" box and "P" box do not permit
this type of connection. Figure
7
illustrates an intercom system using all locally pow-
ered "U" boxes.
1 2
Au d i o c o m ®
