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The 6A Key System
Part III - Off-Premise Service

By Joel Eves

From the September 2000 Switchers' Quarterly

With key system intercoms, whether selector only, single- or two-link, came the occasional need to provide service to a station located some distance away from the host system. Such a station would often be served over a leased telco cable pair, the resulting loop resistance being too high to allow direct hookup of a set to the intercom. In keeping with the differences in design we have seen so far, Western Electric and Automatic Electric took different approaches toward the solving of this problem in the 6A and 16A Key Systems.


1. The 225A KTU

Figure 1 provides a schematic diagram of the Bell System's 225A KTU. Strapping and connections to the 207C KTU for selector only service are shown. The 225A KTU connects to the host 6A system as a station, except that it is rung with a spurt of ground instead of audible signal supply. This SW-lead signaling means the remote station must have a single-digit code.

Figure 1

Lifting the receiver at the remote station allows current to flow from the 225A KTU talk supply, via windings of relay P and non-operated contacts on relay R, energizing P. As the loop has its own talk battery connections, the voltage may be chosen to provide optimal current for the loop in question.

Relay P operates, placing the windings of relay C across the talk path of the 207C KTU. C operates, as does relay A in the 207C KTU. When the remote station dials, relay P releases and re-operates on each pulse, in turn pulsing the 207C KTU. The selector responds as described in Part I, signaling the called station. Capacitors T and R provide a transmission bridge allowing remote and local parties to communicate once the call is answered.

When the code of the remote station is dialed, a spurt of ground from level 1 of the selector is applied to the winding of relay R. If the system has only single-digit codes (no transfer circuits), the B-lead of the 225A KTU is connected directly to relay battery and R will operate. In mixed one- and two-digit systems, the B-lead is connected to lead CE from the last transfer circuit. This lead provides battery unless a transfer circuit is operated, thus when the remote station code is dialed after seizing the intercom, relay R will operate. If, however, a transfer digit has been dialed and the remote station code is dialed as the second digit of a two-digit code, battery will not be present on lead CE and relay R in the 225A will not operate.

When relay R operates, ringing current is applied to leads T and R of the distant station, causing it to ring. Operated relay R disconnects supervisory battery and the transmission bridge from the remote station during ringing. When the remote station answers, relays P and C operate as described above.

Provision of a system busy lamp at the remote location was not incorporated into the 225A KTU to permit operation via a single pair. Where off-premise busy indication was required on a selector only arrangement, a circuit such as that shown in figure 2 was used. In this application, a power plant would be needed at the remote location, to provide "B" battery for the sensing relay and 10-volt AC for the lamp.

Figure 2

2. The H883002-25 Equipment Unit

Schematically illustrated in figure 3, Automatic Electric's H883002-25 long line intercom circuit operates somewhat differently from the 225A KTU. When the remote station seizes the circuit, current flows through two windings of the repeating coil, non-operated relay C contact and the windings of relay A, energizing this relay. Operated A closes a path from lead J of the selector circuit to relay B. If the selector circuit is already in use, there will be no ground on lead J and B will not operate. Operated A closes a voice path through the other two windings of the repeating coil and capacitor C2 to the selector talking path. The remote station may listen and talk, but it cannot hold the path or dial.

Figure 3

If the selector is idle when the remote station seizes the circuit, relay B will operate via ground on lead J. Operated B locks up under control of A, prepares a pulsing path to the selector over lead K, and closes a dc path through two repeating coil windings, resistor R1 and external strapping, seizing the selector talk path. Operation of selector relay B, ungrounding the J lead, does not affect relay B in the long line circuit, now locked up on its own ground.

When the remote station dials, opening and closing the remote loop pulses relay A in the long line circuit. Relay B remains held due to its slow release characteristic, passing pulses from contacts on A directly to the drive magnet of the H883002-70 selector via lead K, advancing the selector. Since relay B is maintaining a constant termination across the selector talking path, relay A of the selector circuit does not pulse.

Operated relay B in the long line circuit also applies high resistance ground via resistor R2 to one winding of relay A. While this does not provide sufficient current to cause A to operate, it speeds the relay's operate time at the end of each pulse, thus increasing the circuit's dialing sensitivity.

Signaling of the called station and conversation are carried out in the same manner as previously described. Transmission between the remote station and the main loop are provided through the repeating coil, functioning as an audio transformer. When the call is completed, restoring the remote station's handset allows relay A of the long line circuit to release, releasing relay B a short time later. Release of B opens the loop to the selector circuit, allowing it to restore.

When another station seizes the selector circuit and calls the remote station, ground on lead C from the selector or associated transfer circuit operates relay C in the long line circuit. Relay C grounds the tip side of the long line and applies ringing supply to the ring side, ringing the remote station. Relay C restores when the single-spurt signaling ground is removed from lead C. When the remote station answers, relay A in the long line circuit operates but B does not, allowing transmission but no dialing or loop control by the remote station.