external 2-wire power mode the Model 46 is simply an observer; whether intercom power is present on pin 2, pin 3, or both pins 2 and 3 is not significant.
So that the Model 46’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 so long as neither of the 2-wire power sta- tus 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 connect- ed. In this case the 200 ohm termination is expected to be provided elsewhere, typi- cally as part of the external power supply, and the Model 46’s termination is “lifted.”
Internal Power Source
When an interface’s front-panel DIP switch is set for internal 2-wire power the Model 46’s 2-wire party-line interface supplies 30 volt DC on pin 2 of the output con- nector. 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 easiest figure to calculate but a web search will generally find specifications for all com- monly 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 46 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 46’s intercom power supply circuits operate under software control. This allows detection of fault conditions and protection of both the Model 46’s circuitry and connected intercom user devices. Upon initial Model 46 power up no monitoring of the intercom power output takes place for a period of five seconds. This allows the Model 46’s circuitry and the connected intercom user devices to stabilize. The LED associated 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 pin
2 falls below 24 volts for a continuous one second interval. The hardware and soft- ware responds to this condition by turning off the power source to pin 2 and flashing the pin 2 LED as a warning. After a five second “cool down” interval the output re- turns to the same condition as upon initial power up: power is again applied to pin 2, the pin 2 status LED will light, and moni- toring won’t begin for another five sec- onds. A full short-circuit condition applied to the Model 46’s 2-wire connectors will result in a continuous cycle of 6 seconds on (5 seconds for start up and one second for detection) and 5 seconds off.
As a diagnostic aid the 2-wire power status LED associated with pin 3 remains active in the internal power mode. When- ever DC in excess of approximately
18 volts is present on pin 3 the LED will light. This condition will normally never exist but could prove useful in special circumstances.
