interface to be auto nulled at approxi- mately the same time. The dual auto null mode allows a single “tap” of the button to initiate nulling of both hybrid circuits.
When configuration DIP switch 3 is in its on (up) position the independent auto null mode is selected. This is appropriate for applications in which the two audio paths associated with the Model 45’s intercom interface are used with independent party- line intercom circuits. This situation might arise when two Clear-Com single-chan- nel intercom circuits are connected to the Model 45. The hybrid circuit associated with each channel can be auto nulled as desired. Another example would be in an RTS TW-series application that uses source assignment panels. The “SAP” panels would be used to route multiple intercom channels to various sets of user devices as desired. In this case, the two channels associated with the Model 45’s interface will often end up not routed to the same user device. Independent auto nulling is certainly desired.
Auto Terminate Disable
The auto terminate function is designed to ensure that each 2-wire-to-4-wire in- terface circuit remains stable under most operating conditions. It can only be active when the Model 45 is not serving as the intercom power source. The auto termi- nate function applies 200 ohm imped- ances to both pins 2 and 3 of the 2-wire party-line interface when no source of DC power is detected on either of those pins. This function is directly compatible with RTS TW-series intercom applications in which the three conductors of a cable support both DC power and two channels of audio. It’s also compatible in situations where all three conductors of a Clear-Com
single-channel intercom circuit are con- nected to the Model 45. In this latter case only one of the Model 45’s audio channels will be used.
There may be situations where it’s neces- sary for the two channels associated with the Model 45’s interface to be used with separate 2-wire party-line circuits. An ex- ample of this might be where two “loops” of single-channel belt packs, such as units from Clear-Com, need to be connected. In this case the intercom circuit’s DC power will not be connected to the Model 45, but only to the user devices. In this scenario, a DC voltage won’t be present on pin 2 or pin 3. Another example of this might be where adapter boxes with DC blocking are used to interconnect intercom circuits with the Model 45. In both these cases a DC voltage won’t be present on either pin 2 or 3 and the Model 45 will automati- cally apply 200 ohm terminations. These terminations would be in error, resulting in incorrect audio levels due to a double- termination condition.
To prevent this problem the auto terminate function can be disabled. When configura- tion DIP switch 4 is in its off (down) posi- tion the auto terminate function is active. When switch 4 is in its on (up) position the auto terminate function associated with each interface is disabled.
The auto terminate disable function should be used only when absolutely necessary; it’s possible that a significant downside could be experienced. With auto terminate disabled it’s important that properly termi- nated 2-wire party-line intercom circuits be connected to pins 2 or 3 of the Model 45’s 2-wire (PL) interface. If they are not con- nected, it’s likely that audio oscillations, noise, and distortion will be generated in
