readily apparent. The front-panel LEDs will continue to display the DC power status of pins 2 and 3. But whether or not either or both LEDs are lit, the Model 45 will never apply 200 ohm terminations to pins 2 or
3.For the hybrid circuits to remain stable termination impedances must be provided by the connected circuits. If these are not present one might find the hybrid circuits generating very unpleasant audible noise. This condition, caused by the 2-wire out- put circuitry being in an “unloaded” state, will not damage the Model 45’s circuitry. However, without the auto terminate func- tion a disconnected cable or other real- world wiring problem could present users with a rude surprise!
In conclusion, it’s important that technical personnel working with the Model 45 be informed when the auto termination func- tion has been disabled. They will then be aware of the potential noise issues and be ready to make corrections should a prob- lem arise.
4-Wire Mute During Auto Null
Normally, the 4-wire outputs are muted whenever the auto null process is taking place. This ensures that the test tones generated as part of the nulling process will not be heard by users of the signals associated with the 4-wire outputs. For special applications configuration DIP switch 5 allows the automatic muting mode to be disabled. This mode is pro- vided primarily for use by factory person- nel or during demonstration use. During normal operation switch 5 should remain in its off (down) position. When switch 5 is in its on (up) position 4-wire muting dur- ing auto null will not take place. While not appropriate during actual Model 45 use, it
is interesting to “hear” the nulling process take place. But unless there’s a really good reason, configuration DIP switch 5 should always remain in its off (down) position.
Technical Notes
Software Version Display
A special Model 45 power-up sequence allows the unit’s software version number to be displayed. This is useful when work- ing with factory personnel on application support and troubleshooting situations. The five “from” 4-wire LEDs are used to display the major release number with a range of
1 through 5. The five LEDs associated with “to” 4-wire channel 1 are used to display the release sub-number which ranges from 1 through 5. Refer to Figure 8 for a detailed view of the LEDs and the corresponding software version numbering scheme. The Model 45’s initial software release is ver- sion 1.1 which is represented by the bot- tom LED of each column being lit.
To display the Model 45’s software version is very simple. From the powered-down state, press and hold the auto null button. While continuing to press the auto null but- ton, apply power either by connecting an external 24 volt DC power source or a pow- ered intercom circuit. The normal power-up sequence will not occur but instead one LED in the status column will be lit and one LED in the column associated with “from” 4-wire channel 1 will also be lit. As previ- ously described, these two LEDs represent the unit’s current software version. After the software version number has been “read” the auto null button can be released. At this time the unit will begin its normal power-up sequence.
