GAI-Tronics LE200-RM Rack-Mount Page/Party Line Extender - Manual (Page 36)

Pub. 42004-392G
M ODEL LE200-RM R ACK -M OUNT P AGE /P ARTY
®
L INE E XTENDER P AGE 34 of 56
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Verifying the Proper Line Balance Resistance
For proper Line Extender operation, the line balance resistance, for both page and party lines, should be set
close to 33 ohms. This can be done by enabling and adjusting the line balance potentiometers on the
69441-xxx terminal board using an ohmmeter.
When the line balance potentiometer is enabled, it is possible to measure the selected dc resistance
directly. One side of the ohmmeter should connect to the exposed header pin on the header used to enable
or disable the line balance potentiometer. The other side of the ohmmeter should connect to the side of the
associated 15-ohm series resistor closest to the edge of the printed circuit board. Ensure the 15-ohm
resistor is part of the measurement before adjusting the potentiometer for the correct resistance. All six
circuits are symmetric and their signals have minimal overlap. If the incorrect series resistor is chosen,
then the ohmmeter should read an open circuit.
Distributing Line Balance Resistance
As an increasing length of cable is added from the Line Extender the effects of cable resistance should be
taken into consideration. The farther a Page/Party
® station is connected to the system cable from the line
balance resistance the greater the chance deficiencies in system behavior may occur. The station farthest
away from the line balance will exhibit increased sidetone audio in the handset earpiece and will place
higher than nominal levels of audio onto the system cable. Stations receiving this audio nearby will have
louder than expected outputs. These same stations receiving audio from other stations near the line
balance resistance will have lower than expected outputs due to the line loss of the cable resistance. A
system showing these characteristics should have distributed line balancing.
For reference, if a line balance is installed at both ends of one mile of 18 gauge cable, then both line
balances should be set to nominally 46.7 ohms. The one mile of cable provides approximately 33 ohms of
resistance in each leg; the resulting series-parallel combination will result in the devices installed at both
ends seeing a line impedance of approximately 33 ohms.
SmartSeries systems utilize FSK for communication on the page line between the stations and the Line
Extender. Since FSK frequencies range between 31 kHz and 33 kHz instead of audio frequencies, the
effects of longer cable lengths will present itself sooner. The higher frequency will lend to a greater
probability that standing waves of the FSK signal will be present. The cable length, in addition to the
number of cable branches present, will combine to determine the severity of these standing waves. Severe
standing waves present on the line can cause a portion of the system devices to be unable to properly
communicate with the Line Extender giving the need for distributing the line balance resistance.
Determining the cable resistance with any accuracy in installed systems is too difficult. Fortunately, the
resistance values to be installed can be determined without using the cable resistance in the calculation, as
precise accuracy is not necessary to achieve acceptable system functionality. The resistance values
installed should always calculate to a nominal parallel resistance of 33 ohms. One of the line balance
resistances can be the resistance included on the 69441-xxx terminal board, but it is not a requirement. It
will provide a resistance up to 115 ohms determined by the position of the potentiometer. Examples of
line balance resistances that could be installed are 51║100 ohms, 68║68 ohms or 100║100║100 ohms. It
is best to install the line balance resistors as far apart from one another in cable distance as possible.