The next step was to identify resources that would improve the installation process and make operation more reliable. This led to the use of standard 3-pin XLR-type audio connectors, enabling rapid installa- tion and troubleshooting in any locale. The inclusion of LED level meters allowed con- tinuous monitoring of the input and output signals. Additional status LEDs were also deemed to be valuable. Configuration DIP switches would allow crucial operating modes to be selected.
A subtle but critical requirement was the need for the Model 45 to receive its oper- ating power from either an external 24 volt DC source or the connected 2-wire party- line intercom circuit. It also turned out that in many applications only a small number of user devices, such as belt packs, were typically connected to a 2-wire party-line circuit. So by adding a DC power source to the Model 45’s 2-wire interface the need for an external intercom power supply could often be eliminated.
The final step was to create a physical package that would provide significant resources in a format that allowed simple and reliable integration with other equip- ment. This was accomplished by speci- fying a convenient half-rack form factor which would be excellent for “thrown- down” use. And by creating two “1U” rack-mount options, it would be possible to install one or two Model 45 units in one space of a 19-inch rack.
Installation
In this section you will be preparing the Model 45 for use. Since the standard unit is housed in a self-contained enclosure,
getting it ready for use in portable appli- cations requires only making a few inter- connections. These include 4-wire audio input, 4-wire audio output, and 2-wire party-line intercom connections that use 3-pin XLR-type connectors. In many ap- plications an external source of 24 volts DC will be connected using the supplied power adapter.
For permanent installations one or two Model 45 units can be mounted in one of the optional rack panel adapters. The resulting one-rack-space unit will be mounted in an equipment rack.
Shipping Carton Contents
The shipping carton contains a Model
45 Interface, a universal input/24 volt DC output power supply, one set of self-stick protective bumpers, and a user guide. A North-American standard mains power cord will be provided for use with the power supply. If necessary, the dealer or distributor will provide an AC mains cord that is appropriate for your location. If a rack-mount front panel is going to be used as part of the installation it will typically be shipped in a separate carton.
Locating the Model 45
The location of the Model 45 will dictate the length of the cable runs needed to link the unit with the other required intercom devices. This factor really only relates to the 2-wire party-line intercom circuit. These circuits carry unbalanced audio which can be susceptible to interference and crosstalk issues. And since party-line intercom circuits typically carry DC power a voltage drop due to resistive loss can become an issue. In general, minimizing the length of the 2-wire party-line intercom
