An auto null sequence begins with the muting of the 4-wire input and output signal paths associated with the channel being nulled. This is followed by a short period of 24 kHz signal that is sent out the 2-wire party-line intercom interface channel. This will turn off microphones on those connected user devices that are compatible with the RTS TW-series “mic kill” protocol. The actual auto null- ing process is performed next. A series of tones will be sent out the 2-wire interface. Other Model 45 circuitry, under software control, will rapidly perform adjustments to achieve the best null possible. After the adjustments have been made the results are stored in nonvolatile memory. Once the process is complete the 4-wire input and output paths are again activated.
Configuration DIP switch 3 allows an independent auto null button mode to be selected. If switch 3 is in its on (up) position, the front-panel pushbutton will function in quite a different manner. In the independent mode, a single “tap” to a switch will cause channel 1 to auto null. Two “taps” will cause channel 2 to auto null. By observing the operation of the two auto null status LEDs it will become readily apparent which of the button modes has been selected.
If possible, prior to performing an auto null it’s polite to warn all personnel who are ac- tively using the connected intercom devic- es. The tones sent to the 2-wire intercom circuit during the nulling process are not excessively loud or obnoxious, but most users might want to remove their headsets during the process. In addition to warning users, it might be a good time to ask them to mute any active microphones. While the automatic “mic kill” signal will apply
to many user devices it may not apply to all. Muting microphones is significant as obtaining a “deep” null requires that no extraneous signals be present on the inter- com circuit.
Auto Terminate Disable
As previously discussed in this user guide, the auto terminate function can come into play when the 2-wire party-line interface is configured to not supply power. Using a combination of hardware and software, pins 2 and 3 of the 2-wire (PL) intercom connector are monitored for the presence of DC voltage. If a level of greater than ap- proximately 18 volts DC is not detected on either pin, 200 ohm terminating networks are applied to those same pins. This en- sures that the Model 45’s hybrid circuitry remains stable, preventing objectionable audio signals from being sent to the 4-wire output connectors. One caveat does ap- ply: there may be a few seconds of severe noise whenever an interface moves from a DC present condition to a DC not present condition. But other than that period, the audio paths will remain “clean.”
As a visual aid, LEDs on the front panel will display the DC power status of pins
2 and 3. But when auto terminate disable mode is active (configuration DIP switch 4 set to on) the LEDs will no longer indicate the intercom circuit’s termination status.
For special applications the auto terminate function can be disabled. This will primar- ily come into play when “splitting” the Model 45’s resources into two separate single-channel interfaces. Moving con- figuration DIP switch 4 to its on (up) posi- tion disables the auto terminate function. To someone observing only the Model 45’s front panel this change would not be
