RDL ST-TC1 Telephone System Coupler / CO Line Simulator / 600 Ohm Balanced Audio / Relay Closure

Description

The ST-TC1 is a telephone system coupler that simulates a central office (CO) line for connection to on-site telephone switching units (KSU). It presents itself to the KSU as an incoming line and provides a relay closure when the line is seized. The unit is designed to interface with telephone systems, not directly to live CO lines, and it does not accept ringer voltage.

The ST-TC1 provides a transformer-isolated 600 Ohm balanced audio feed for input or output and will pass DTMF tones generated by handset keypads or tone generators. A double-pole, double-throw relay provides contact closure maintained while the line is off-hook. Maximum switching capability supports up to 60 W and the relay contacts are suitable for control of external paging or radio systems.

Key technical attributes include a simulated telco CO line connection, 600 Ohm balanced audio impedance, and double-pole double-throw switching contacts. Power is ground-referenced 24 Vdc at approximately 50 mA and the ambient operating range is 0° C to 55° C. The ST-TC1 will not tolerate telephone lines with ringer voltage present and must be installed on KSU ports configured for non-ringing simulated lines.

Connectivity is via the telephone switching unit so the ST-TC1 is wired as if it were an incoming trunk line to the KSU. The relay closure can key external equipment such as paging systems or radio console inputs and the audio feed can be used for speech transmission to those systems. The unit can also be combined with RDL ST-VOX1 modules to provide push-to-talk keying for two-way radio or other simplex systems.

Common applications include adding paging zones through a telephone system, interfacing telephone-initiated control to radio systems, and enabling broadcast of handset audio over external systems. The ST-TC1 is used when a KSU-controlled closure and audio path are required without connection to an actual CO line. It is suitable for installations where DTMF signaling needs to pass through the interface.

Installation is straightforward and intended for technicians familiar with telephone KSUs and audio interfacing. The compact unit ships in a simple package and requires a ground-referenced 24 Vdc supply at 50 mA. Build quality supports stable relay operation and transformer isolation for the audio path to minimize ground-loop issues and preserve audio integrity.