THE STEP BY STEP SWITCH 197 TYPE SELECTORS Part 1

[Music] the electrical part of the switch operation is shown in the circuit schematic this tells a simple story to the experienced Telephone Man but for those unfamiliar with the selector circuit we have a simplified circuit drawing in fact we have rearranged it in order to show clearly just how the switch operates here in symbolic form are the a relay the B relay the C the D and the E relay we have also shown the shaft with its wipers rotary ratchet and vertical ratchet the vertical magnets that step the shaft up and the rotary magnets that rotate the shaft but first let's go back to the customer in this symbolic diagram the dial represents the calling customer's telephone in the demonstration we saw that when the customer raised the receiver the line finder hunted for the line and connected the circuit to the first selector to simplify the drawing we will show the dial connected directly to the selector which is now under the control of the customer as he dials to show how the selector operates let's go back to the circuit diagram the moment the selector is connected to the linefinder a circuit is completed through the station instrument the customer hears dial tone and the a relay operates this completes a circuit through the wi ing of the B relay the B relay operates preparing the circuit for the C relay and the vertical magnet but these do not operate because the circuit is open at the a relay now the selector is ready to receive the customer's dial pulses in this case the customer will dial digit three as the the dial moves off normal the circuit remains closed but as the dial returns to normal it will momentarily open the circuit each time the dial clicks these signals or pulses will cause the selector to operate let's watch as the first Interruption occurs the a relay releases this opens the circuit through the B relay one ings the B relay however is a slow release relay and does not release the release of the a relay also completes the circuit through the vertical magnet and the C relay the C relay operates preparing a circuit through the E relay at the same time the vertical magnet operates as it raises the shaft One Step note that contacts located near the top of the shaft close these contacts are called vertical off normal because they are closed only when the shaft is above its normal release position the shaft is held in its raised position by a mechanical device called a vertical dog which is the upper part of the double dog a the circuit is now completed through the vertical offn normal contacts and the C relay the e- relay operates this relay comes into use later on Note however that another circuit has been established through the contacts of the e- relay to hold the relay operated as the dial continues to return the circuit through the a relay winding is again closed the a relay operates opening the circuit through the C relay and the vertical magnet the vertical magnet releases the C relay remains operated because it is a slow release relay the circuit through the B relay is closed the dial now opens the circuit for the second pulse the a relay releases the vertical magnet re operates raising the shaft up one more step as the dial continues to return the circuit through the a relay winding is again closed the vertical magnet releases the cycle repeats itself as the last dial pulse is received the vertical magnet operates as the dial returns to normal the a relay operates and releases the vertical magnet the customer dialed three and the shaft has stepped up three levels remember that all the foregoing operations took place during the short interval that the dial was returning to normal if this were shown at normal speed there would now be a pause as the customer dials the next digit during this interval the selector completes its rotary operation automatically the slow release ser relay whose circuit is open at the a relay finally releases this completes a circuit through the rotary magnet and the e- relay contacts the rotary magnet operates and rotates the shaft One Step this brings the shaft wipers in contact with the first trunk terminals when the rotary magnet operated the rotary interruptor contacts opened thus opening the circuit through the winding of the e- relay which releases the release of the e- relay opens the Circuit of the rotary magnet which releases closing its contacts if the trunk with which the wipers made contact is Idle a circuit is closed through the windings of the e- relay and the D relay although the circuit goes through the winding of the e- relay this relay does not operate because the current is limited by the high resistance of the D relay winding the D relay operates and opens the circuits to the A and B relays the a relay releases the B relay however is a slow release relay and stays operated for a moment the D relay extends the tip and ring talking path from the customer's dial through to the next switch relays in the next switch operate and return ground to the sleeve wiper this ground holds the D relay operated when the B relay releases the D relay remains operated until the calling customer hangs up now let's watch this entire operation again the a relay operates the B relay operates preparing the circuit for the C relay and vertical magnet now the customer dials the digit three the dial starts to return to normal we'll stop the dial as its contacts open the circuit to the a relay the release of the a relay closes the circuit through the vertical magnet and the C relay the C relay operates and the vertical magnet moves the shaft up one step the e- relay operates and establishes another path to its own winding through its contacts now we'll let the dial return until its contacts close the a relay operates the vertical magnet releases as the dial continues to return its contacts open the a relay releases and again the vertical magnet is energized the shaft moves up another step as the dial continues to return the a relay reoperate the vertical magnet releases the dial contacts open the vertical magnet steps the shaft up another step the dial returns to normal its contacts closed the a relay operates the vertical magnet releases vertical stepping is now complete and the switch is ready for rotary action the slow release C relay releases the rotary magnet is energized the e- relay is held operated by the path through its own contacts the shaft rotates One Step the rotary interruptor Springs open the circuit to the e- relay the e- relay releases the rotary magnet releases the Springs close the e- relay and D relay are energized in series the e- relay will not operate due to the current limiting effect of the D relay the D relay operates the A and B relay circuits are opened the talking path is extended to the next switch the next switch places ground on the sleeve terminal this holds the D relay operated when the slow release B relay releases the customer's dial now is extended to the next switch the next train of dial pulses will operate that switch and no further action takes place in this one but now suppose that when the switch rotated to the first trunk the wipers found that trunk busy let's start at the point where the switch completed its vertical stepping and see what happens during the interval after the first series of dial pulses was received the slow release C relay released the rad R magnet operated and the shaft rotated One Step the rotary magnet opened the circuit to the e- relay which released and in turn released the rotary magnet but now a new chain of events occurs when the switch wipers hit the first trunk they find ground on the sleeve terminal because the trunk is busy this ground reoperate the E relay the D relay does not operate because there is ground on both sides of its windings a circuit is now completed through the rotary magnet the rotary magnet again operates stepping the wipers into the next terminal operation of the rotary magnet opens the circuit to the e- relay which releases and releases the rotary magnet if the second trunk is Idle No ground is found on the sleeve and the switch operation is the same as Illustrated before when the rotary interruptor contacts close the D relay operates the D relay opens the circuits to the A and B relays and extends the talking path to the next switch the next switch places ground on the sleeve terminals to hold the D- relay operated now let's watch the complete operation of the switch again at a little faster speed we'll start as the selector is connected to the linefinder and we'll assume that the first two trunks are busy the customer dials digit three the dial contacts open the shaft steps up the e- relay operates and another path through its winding is establish established the dial contacts close the vertical magnet releases the dial contact open again the vertical magnet raises the shaft another step the dial contacts close the vertical magnet releases the dial contacts open the vertical magnet steps the shaft again the dial returns to normal the vertical magnet releases and vertical stepping is complete now the C relay releases the rotary magnet rotates the shaft One Step opening the e- relay circuit the e- relay releases the rotary magnet releases ground on the sleeve of the busy trunk reoperate the e- relay the rotary magnet steps the shaft in to the second trunk the e- relay releases the rotary magnet releases the second trunk is also busy the e- relay operates and the rotary magnet steps the shaft in once more the e- relay releases and the rotary magnet releases the third trunk is Idle the D relay operates the A and B relay circuits are opened the talking path is extended to the next switch which returns ground on the sleeve to hold the D relay operated the switch has stepped up to the third level and cut into the third terminal which was idle the operation we have just seen is basic in most step-by-step equipment you probably noticed the definite rhythm with which the electrical and mechanical operations occurred this is the result of careful engineering precise manufacturer and accurate adjustment [Music]