are fully isolated from the 2-wire intercom circuit. In this situation the Model 45’s only function is to route, by way of the hybrids, the 2-wire interface’s audio signals to and from the 4-wire outputs and inputs. In this mode of operation the Model 45 acts as a typical user station on the connected inter- com circuit.
As a connection confirmation, hardware and software in the Model 45 monitors pins 2 and 3 of the 2-wire (PL) intercom connec- tors for the presence of DC voltage. A level greater than approximately 18 volts DC on pin 2 will cause the status LED labeled PIN 2 to light. A level greater than 18 volts DC on pin 3 will cause the status LED labeled PIN 3 to light.
The author is aware that user intercom devices almost always draw power from pin 2 rather than pin 3. However, in many broadcast applications power is provided on all intercom paths so that flexible chan- nel assignments can be made. In the ex- ternal 2-wire power mode the Model 45 is simply an observer; whether intercom power is present on pin 2, pin 3, or both pins 2 and 3 is not significant.
In order that the Model 45’s 2-wire-to-4-wire interface circuits remain stable and don’t generate audio artifacts an auto terminate feature is implemented. This maintains a 200 ohm termination on both pins 2 and
3 as long as neither of the 2-wire power status LEDs is lit. To clarify, if either or both the LEDs are lit then both terminations are removed. It was felt that this method would provide a fairly accurate indication that an active intercom circuit is connected. In this case the 200 ohm termination is expected to be provided elsewhere, typically as part of the external power supply, and the Mod- el 45’s termination is “lifted.”
When the Model 45 is Serving as the 2-Wire Intercom Power Source
When configuration DIP switch 2 is set so that the Model 45 provides 2-wire (PL) power its interface supplies 30 volts DC on pin 2 of the 2-wire (PL) intercom con- nectors. A maximum current draw of 300 milliamperes is available. This current is sufficient to power various intercom user devices such as small user stations and belt packs. A common broadcast appli- cation would be to use RTS BP325 belt packs. Select the connected devices so that their total current doesn’t exceed 300 milliamperes. That’s not always the easi- est figure to calculate but a web search will generally find specifications for all commonly used devices. For example, a search finds that the original version of the BP325 consumes a maximum of 85 milliamperes of current. According to this figure up to three of these units can be connected to each Model 45 Interface. Newer versions of the BP325 use surface mount component technology and have a lower maximum current draw of 65 mil- liamperes. Four of these “modern” BP325 units can easily be supported.
The Model 45’s intercom power supply circuit operates under software control. This allows detection of fault conditions and protection of both the Model 45’s circuitry and connected intercom user devices. Upon initial Model 45 2-wire inter- com power up no monitoring of the inter- com power output takes place for a period of five seconds. This allows the Model 45’s circuitry and the connected intercom user devices to stabilize. The LED associ- ated with pin 2 will light to indicate that the output is active. After this initial 5-second period monitoring becomes active. A fault condition is detected if the power on
