Partyline: What is the purpose of "Termination" on a PL channel?
INFORMATION
The purpose of a bridging termination on a Clear-Com party-line channel is to minimize signal drops across the channel line when multiple main stations, remote station and/or party-line beltpacks are connected to the channel.
On some main stations, up to 20 remote stations can be connected in a series string or up to 40 beltpacks can be connected on a single string.
The termination prevents drastic changes in the impedance of the PL channel if remote stations or beltpacks were added or removed from the PL line.
The termination can be found on the main station or power supply.
The fundamental concept of the Clear-Com Party-Line intercom is that all channels are terminated in one location, preferably at a Main Station or Power Supply.
All intercom lines must be terminated.
All unused intercom lines must also be terminated.
The termination switches on a Main Station should be set to the OFF position only if the channel is terminated by another Main Station or Power Supply in the system.
If there are no other Main Stations or Power Supplies terminating the line, the termination switch on each channel of the Main Station should be switched to ON.
Care must be taken NOT to "double-terminate" a line.
CHECKING TERMINATION ON THE CLEAR-COM PARTY-LINE SYSTEM
There should be one and only one termination for each channel in the system. This termination is usually set to ON at the Main Station or Power Supply.
The location of the termination switch varies with model.
On some products, the termination switches are on the rear panel; on others they are jumpers inside the unit.
Please consult the unit's user manual for more information.
To ascertain that only one termination is present on the channel, please perform the following test:
Using a multimeter, measure the resistance between pins 1 and 3 on one of the Channel A XLR connectors at the rear of the unit.
If the channel is terminated properly, then the resistance should measure approximately 4,800-5,000 Ohms. A very high channel resistance means the channel is not terminated. Channel resistance of 2,000 Ohms indicates a double-termination. If a double-termination is indicated, locate the other terminated Power Supply or Main Station and set its termination to OFF.
Repeat for the other channels.
Check resistance between Chassis Ground and pin 1. Using an Ohmmeter, measure the resistance from pin 1 on the Main Station or Power Supply to chassis ground. The measurement should read 10 Ohms. A high reading (over 100 Ohms) indicates that the 10 Ohm resistor in the unit has failed and requires replacement.
A reading of less than 10 Ohms (or a short) typically indicates that the shell and pin 1 of one of the interconnect cables are shorted together. Test the individual cables until the culprit is located and repair or replace the cable.
Failure to perform the replacement will result in an audible "buzz" in the system.
Pin 1 and the shell of the XLR plug on the interconnect cables should NOT be connected together.
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