SECTION 484-400-400 ISSUE 2 10. DIAL INTERCOM SELECTOR CIRCUIT (FIGURE 9, H-88390-70, H-85973-1) 10.01 Seizure. Lifting of the handset at an intercom station telephone and operation of the intercom system push-button key at that telephone, when the intercom system is idle (busy lamps dark), closes a loop circuit through the telephone over the T and R leads to the windings of relay A to talking (A) battery and ground. Relay A operates, closes a circuit to relay B, and relay B operates after a brief delay due to its sleeve. Relay B, in operating, closes a circuit to light the busy lamps associated with the intercom system at all intercom stations. This also removes a ground from lead J, closes a ground to lead H, and completes a circuit from ground on terminal 20B (strap "L"), through,strap "A", to the two windings of relay D in series, aiding, causing relay D to operate. Relay B also prepares a pulsing path to the rotary switch MM. 10.02 Dialing. As the calling party dials, opening and closing the loop, relay A follows the pulses, alternately restoring and reoperating; relay B is alternately energized and de-energized, but remains operated due to its sleeve. When relay A restores, a circuit is closed from ground through contacts of relay B and strap "D" to the motor magnet of the rotary switch MM. The rotary switch MM armature operates, but the wipers do not step at this time since this is an indirect drive switch. The interrupter contacts, however, do operate closing a circuit from the same ground, through rectifier MRI and operated off-normal contacts 2 and 3, to the winding of relay T and relay C and. in parallel, to capacitor Cl through resistor, R1 and operated contacts of relay D. Relay T operates, relay C also operates after a brief delay due to its heel-end slug, and closes a multiple holding ground from make contacts 2T and 3T of relay B to the winding of relay T. Relay C also completes a circuit to charge capacitor C2; and opens the signaling and group­selecting circuits. When relay A reoperates, relay B is re-energized, relay C is de-energized but holds due to its heel-end slug, and maintains a holding ground to relay T. Rotary switch MM is deenergized, allowing its armature to restore and move the wipers one rotary step. Relay A follows the remaining pulses of the digit dialed by the calling party. Rotary switch MM follows the pulses of relay A and steps its wipers to a position corresponding to the digit dialed. 10.03 Signaling. When dialing is completed, relay A remains operated, relay B is energized and stays operated as do relays D and T. Relay C is de­energized, but it is slow to release due to its slug. When relay C does restore, a circuit is closed from the audible signal power supply through contacts of operated relays D and T, contacts of relay C, restored to level A of the rotary switch and to the signaling lead (R) to the dialed station. Over this path, and a common return, the audible signal of the called station is operated. Relay C, restored, removes the holding ground from relay T and from capacitor C2; capacitor C2 discharges through the winding of relay T, holding relay T operated for a period of 1.5 to 3 seconds. During this time, the called station is audibly signaled. When capacitor C2 has discharged sufficiently, relay T will restore, opening the signaling circuit to the called station. No further signaling will take place unless the calling station re-dials. 10.04 Homing. When, relay T restores at the end of signaling, a ground is closed from terminal 14B (strap "C") to the rotary switch motor magnet MM through the interrupter and off-normal contacts; the rotary switch steps, self-interrupted, to its home position where its off-normal contacts operate and open the stepping (homing) circuit. This circuit is now back to the condition just preceding dialing. 10.05 Re-dialing. The calling party may re-dial the called number one or more times for additional 1.5 to 3 seconds audible signaling periods. 10.06 Answer and talking. If the called party answers, both telephones are connected in parallel across the same talking (A) battery source and relay A. They can now converse without difficulty. Other stations may also listen and talk by raising their handsets and, if necessary, operating their intercom system push-button keys. The seven party or seven station limit for participants in one conversation is desirable to insure that there is adequate current and voltage for each station. During conversation, relays A, B, and D remain energized and operated. 10.07 Disconnect. When both (or all) parties disconnect, the loop to relay A is opened, and relay A restores. As during dialing, relay B holds, relays C and T, and switch magnet MM are energized and operate. After a sufficient delay, relay B restores; this de-energizes relays D, C, and the rotary switch motor magnet MM. When relay D restores, the rotary switch wipers step onto the first position, and relay C restores after a brief delay. No signaling takes place because relay D is restored and rotary position 1 on the switch is not used. When relay C restores, relay T is de-energized and since relay D has already restored and capacitor C2 is not connected, relay T will restore at once. When relay T restores, the rotary switch will home as before. When the rotary switch MM is in its home position, and all relays are restored and de-energized, this circuit is ready to handle another call.