Reference / Paper · 2025
The step-by-step telephone switching system: The line finder switch
Line finder operation in detail.
Reference / Paper · 2025
Line finder operation in detail.
The step-by-step telephone switching system: The line finder switch Douglas A. Kerr Issue 7 July 22, 2025 FOREWORD The step-by-step telephone switching system (as it is known in the Bell Telephone System; the “Strowger” system elsewhere) was the earliest “mechanized” telephone switching system to receive broad acceptance, and it remained important for many decades. This is one of a series of articles on this system, and it describes the line finder switch, which is used in the first stage of the switching network. The basic functions of the switch are summarized, its basic operation is described, and an illustrative circuit schematic drawing is used as the basis for a detailed description of its operation. 1 GENERAL 1.1 The series of articles This article is one of a series. The “master” article, “The step-by-step telephone switching system: Overview”, by the same author, gives background on the historical development of the system, and then describes its overall architecture, scheme of operation, and the technical details of the unique type of switch used in the system. It also gives background on such telephone concepts as battery and ground; tip, ring, and sleeve; and the like. The other articles (including this one) describe in detail (including at the circuit level) the different switches used in the step by step system. In some cases, information given in the master article is repeated here for continuity. All the articles are indexed on, and available at, my site, The Pumpkin: http://dougkerr.net/pumpkin 1.2 Types of switches and their roles The step by step switching system in its most widely-used form uses three kinds of switch, all with essentially the same base mechanism but varying substantially in their complement of relays, function, and operation. The three types are the line finder, the selector, and the connector. The line finder serves to provide a connection from a subscriber line requesting service (the user lifts the handset) into the switching network itself, in the person of a selector switch (this one Copyright © 2018-2025 Douglas A. Kerr. May be reproduced and/or distributed but only intact, including this notice. Brief excerpts may be reproduced with credit. Step by step: The line finder switch Page 2 being a so-called 1st selector). The selectors serve to advance the connection stage by stage through the “interior” of the switching network, each one in response to a successive digit of the dialed number, not including the last two. The connectors constitute the final stage of the overall switching network. After all but the last two digits are dialed, the connection has been extended to a connector switch, one which can access 100 line, including of course the one whose number has been dictated by the earlier dialed digits. The last two dialed digits move the connector to the corresponding line. The connector tests the line to determine if it is busy (on an existing connection). If not, a connection is made to the line and the ringing signal applied. When the called station answers, the ringing signal is removed and a transmission path is completed between the calling station and the called station. Figure 1. Typical line finder switch Step by step: The line finder switch 2 THE SWITCH ITSELF 2.1 General Page 3 Figure 1 shows a typical “200-point” switch (in particular a line finder), with an unwired bank assembly attached, in a test stand, used to hold the switch while it is being adjusted or tested. Like all the switches we see in this series, this is a two-motion switch. It can move to any of 100 terminals positions arrayed in a curved contact bank in 10 “levels” of 10 terminals each. It reaches a certain terminal by first moving its shaft (which carries the contact making “wiper”) up in steps to the appropriate level, and then rotating the shaft in steps to the appropriate terminal on that level. 2.2 The “200-point” line finder bank assembly But in a line finder, in the most-common “200 point” form, at each terminal position the switch makes contact with the leads of two (unrelated) lines, so that a total of 100 lines can be accessed by a given line finder. A relay in the switch determines which of the two lines at the terminal position to which the switch has moved is actually accessed. At the bottom we see the bank, or to be more accurate, banks. Plural? Yes. Three leads (conductors) have to be carried through the switching network for each line, the ring and tip (which carry the line itself) and the sleeve1, which is used for various control purposes. And because this is a 200-point switch, there are two sets of those three leads at each terminal position, six leads altogether. It would be easy to imagine that each “terminal” of the bank had six contacts (for tip, ring and sleeve) and that the wiper had six contact-making members to connect to them. But in reality, we can only readily have two contacts at a bank position. So in fact the “bank” of the switch is actually an assembly of three banks, as we see in some detail in Figure 2. On each bank, at each terminal position are two contacts, rather thin, lying opposite one another on a thin insulating phenolic sheet. There is a thicker phenolic sheet, not shown in the drawing, between these “sandwiches” at the various levels. 1 These names all come from the designations of the three contact members of the plugs used in manual telephone switching systems. Step by step: The line finder switch Page 4 On the lower bank, at each position, these contact pairs carry the ring and tip leads of one of the two lines. On the middle bank, at each position, these contact pairs carry the ring and tip leads of the other of the two lines. On the top bank, at each position, these contact pairs carry the sleeve leads of the two lines. The bank assembly is fastened to the switch, in rather precise alignment, by way of the threaded studs seen at the tips of the bank rods (with the nuts seen there). Figure 2. 200-point line finder bank assembly Accordingly, what we have thought of as “the wiper” of the switch is actually three wipers, one running on each bank. Each wiper has an upper and lower contact “leaf”, insulated from one another (since they will touch separate contacts). In Figure 3 we see such a wiper. As we see, the wiper (which of course moves around quite a bit) is connected to the switch circuitry with two very flexible cords. Their actual conductors use what is called “tinsel” construction. A very thin ribbon of conducting material is wound around a small fabric core, this whole thing then covered by a durable but flexible woven cover. This style of cord is able to withstand literally millions of cycles of flexure and twisting as the switch goes through its motions. Step by step: The line finder switch Page 5 Figure 3. Two-contact wiper Toward the top of the switch “chassis” we see six relays. The different kinds of switch (line finder, selector, and connector, whose roles will be described shortly) have different numbers of relays, and their functions vary between the type of switch. These relays are of a basic design long associated with the step by step system. Although, for example, the switching systems designed by the Bell System and made by Western Electric used, from the outset, new types of relay, more compact and less costly to manufacture, step by step switches made by Western electric (as well as those made by Automatic Electric) continued to use the relay design we see here. Just above the bank assembly is the switch mechanism. Its heart is three electromagnets (generally just called “magnets”). One steps the switch in the vertical direction, one steps the switch in the rotary direction, and the third releases the “dogs” (retaining pawls) that hold the switch in place after it has stepped up and around, allowing a spring to rotate the shaft back to its home angular position, and then allowing the shaft to drop by gravity to its home position. There are also various contact assemblies that do things like detect if the switch is at its home position or not. These play various roles in the logic executed by the relays in controlling the switch movement and otherwise managing the emerging connection. In use, the switches (not including the banks and wipers) are each covered by the iconic “mailbox-shaped” sheet metal cover. 2.3 The vertical commutator When a line finder is started on behalf of a line requesting service, and we look to it to “find” that line, it is not practical for the line finder to somehow scan over (potentially) over all 100 terminal positions. Step by step: The line finder switch Page 6 Rather, we give it “hint” by telling it on which level that target terminal lies. This utilizes what is called the vertical commutator. We see it in Figure 4. It is a little vertical strip of 11 terminals, against which a special wiper runs as the switch steps vertically. The lowest terminal, against which the wiper rests when the switch shaft is in its home position, is typically not connected to anything, but merely serves to properly align the wiper so that, as the shaft begins to rise, the wiper can slide onto terminal 1. Figure 4. Line finder vertical commutator and its wiper When the line finder is started, ground is placed on the commutator terminal for the level on which the calling line is located. As the switch steps vertically (autonomously), a relay in the line finder looks for that ground. When it finds it, the vertical stepping stops and the switch begins to step in the rotary direction. The calling line has battery on its sleeve terminal, and now the switch looks for that. 3 SWITCHING NETWORK ARCHITECTURE AND OPERATION In these articles, “switching network” means the portion of a switching system through which the connection is extended inside the switching system from the calling line to the called line (as distinguished from the use of the term to mean a number of interconnected switching systems). It us sometimes called the “switching fabric” To set a specific context for discussion of the role of the line finder, we will consider a city in which there are several central offices, and in which a 5-digit numbering plan is used. The first digit identifies the central office in which the line with that number is served. Step by step: The line finder switch Page 7 Figure 5 shows the entire “train” through which the call is handled from one end to another. It shows a completed connection from a certain subscriber’s line to the line whose number is 5-2368. (Actually, the number dialed is of no consequence to the line finder.) We see a line finder, three selectors (in successive stages), and a connector. I note here that in fact there are two “schemes” for connecting a line wanting to make a call to a 1st selector, the line switch scheme and the line finder scheme. This situation is discussed in some detail in section 4 of this article. But of course the central scope of this article is the line finder, so for a while we will assume the use of that scheme. Figure 5. 5-digit switch train Here, since our concern is with the line finder, I will not follow the way in which the entirety of the connection is built up. That topic is covered thoroughly in the “master article”, which also presents a broader outlook on network architecture. Imagine now that 200 lines are connected through their line circuits to the 200 “two-line” terminals of several line finders (all in parallel). The little diagonal line (called a multiple symbol) reminds us of this, even though we only see one line finder. Each line has associated with it a line circuit, comprising two relays. Battery is fed to the ring of the line through the winding of one, the line relay; ground is fed directly to the tip. When the subscriber lifts the handset to place a call (“requests service”), current flows in the line. and that operates the line relay. This causes the next one of the group of line finders that is not already busy on a connection to “start”. That line finder, by first stepping vertically, and then in rotation, goes to the position at which the line (with another line) is connected, and then, either operating or not the “which of the two lines” relay, connects to the line of interest. Step by step: The line finder switch Page 8 The line finder then grounds the sleeve lead to the line circuit, which operates the second relay there (the cutoff relay). This frees the line from the battery and ground applied at the line circuit. Each line finder is permanently (more or less) connected to a 1st selector. They work together at all times. The 1st selector feeds battery and ground (through two windings of its battery feed relay) to the leads where a line will show up when the line finder has connected to it. When that happens, the presence of the calling station allows current to flow in the line, operating that relay, ”awakening” the 1st selector; it now has a “client”. This 1st selector is now “holding the baby”, and will be, for a short while, responsible for managing the connection (nascent as it is at this point). As part of that, it grounds the sleeve lead going back to the line finder. This tells the line finder that the connection is proceeding as expected. The continued presence of this ground (which will come back from later and later switches in the connection as it unfolds, ultimately from the connector) tells all the intermediate selectors, and the line finder, “don’t release–this connection is still live”. 4 TWO APPROACHES AT THE “FRONT END” 4.1 Introduction As I mentioned a while ago, over the range of step by step systems, there are two “schemes” for connecting a line desiring to place a call to a 1st selector so that a connection can start to be built up. These are called the line switch and line finder schemes. Not surprisingly, given the title of this article, all the discussion above has been predicated on the line finder scheme. But here, we will, for completeness, briefly look into the other scheme, which in fact had two quite different executions. 4.2 Review By way of review, Figure 6 shows in “single-line” form almost the entire path from a calling line to the line with number 5-2368, Step by step: The line finder switch Page 9 Figure 6. Switch train from the 1st selector This figure for the moment leaves as a mystery how the calling line gets connected to this particular 1st selector. In the earliest “demonstration” Strowger systems, serving a very small number of lines, every line had its own 1st selector, which it could use without further ado any time the subscriber wanted to make a call. Figure 7 in fact shows this arrangement applied to our hypothetical central office. Figure 7. Switch train with individual 1st selectors The operation of this arrangement should be self-evident, and it would work just fine. But selector switches are complicated, and bulky, and costly, and involve a gigantic amount of connecting wiring from their banks. If in fact we were to consider a central office using this scheme and serving 10,000 lines, we would have to have 10,000 1st selectors. Yet perhaps only 1000 1st selectors would be adequate to handle the amount of traffic (at the first stage of the switching network) from those 1,000 lines at “busy hour”. Step by step: The line finder switch Page 10 So this scheme is, not surprisingly, not found in any “serious” step by step systems. 4.3 The line switch scheme The earliest “serious” Strowger systems used a scheme called the line switch scheme to allow a line requesting service to get connected to an 1st selector as its “doorway” to the switching network proper, rather than the line finder scheme to which this article is primarily devoted. Figure 8 shows this scheme, in our familiar context, in one common implementation. Figure 8. Switch train with line switches Here, each line is equipped with its own switch, a line switch, but this is a much simpler switch than a line finder, using a totally different structure. (Actually, there are two dramatically different kinds of line switches; I will at this point describe the “most obvious” of them.) These switches are sometimes called uniselector2s, and they are single-motion stepping switches (that leading to the name). That is, each time their electromagnet operates (and releases; in the kind of interest, it actually does its work when it releases). a set of wiper arms is stepped one further position over a group of terminals arranged in a curved bank. But there is no motion in a second direction. Figure 9 shows one of these little beauties (a Western Electric 200-type, or in a later design, 206-type, selector). 2 Although this terms is essentially never used in the Bell System Step by step: The line finder switch Page 11 Figure 9. Uniselector The terminals (it is typical for a Western Electric switch of this type to have 22 terminals, as in the one shown) covering a span of rotation of the switch of 180°. Each wiper arm is double ended. As the collection of wiper arms is stepped off the last terminal in the bank, the opposite end of each arm comes onto the first terminal in the bank. Thus the wipers are always in contact with one terminal of the bank or another. A 23rd set of lugs on the bank connected to the feeder brushes, which made the “common” connection to the wipers. We also see in Figure 8 for the first time a line circuit, a small collection of relays. Each line has one. The up to 22 terminals of the line switch (each one having three contacts, for the tip, ring, and sleeve of the line; only the tip and ring actually go out to the station) are wired to the “inputs” of separate 1st selectors (typically up to 10 of them). The terminals of the line switch banks for other subscriber lines in a group are all connected “in multiple” to that collection of 1st selectors. If a selector is busy (participating in an existing connection), its sleeve lead carries ground. With the line idle, battery is fed through the winding of one of the relays in its line circuit (the line relay) to the ring of the line, and ground to the tip (just as with the line finder scheme). When the subscriber lifts the handset, the resulting flow of current operates the line relay, and this sets into motion a chain of events that results in the line’s line switch starting to autonomously step over its bank terminals (that is, over the “candidate” 1st selectors). At each terminal, the state of the sleeve lead is examined by a relay in teh line circuit and if it shows ground, the switch steps on. But at the Step by step: The line finder switch Page 12 first terminal encountered whose sleeve does not show ground (and is thus “idle”), the switch stops its stepping, and another relay connects the line through to the line switch wipers and thus to the lucky 1st selector. It is not ideal from a traffic efficiency standpoint that each line can only have access to a pool of up to 22 1st selectors. Thus in some installations there is a second stage of line switch (“secondary line switches”), again implemented with uniselectors, allowing a larger group of lines access to a larger pool of 1st selectors. The further details of this and of the uniselector line switch scheme overall are beyond the scope of this article. 4.4 Another kind of line switch Another rather different implementation of the line switch architecture uses what are called plunger switches rather than uniselectors. There, all the line switches for perhaps 25 or 50 lines are consolidated into a single mechanical assembly, with a common drive element. We can see the principle in Figure 10. Figure 10. Plunger-type line switch Step by step: The line finder switch Page 13 The connection between the line and the chosen 1st selector is not made with wipers touching contacts, but rather by a plunger on the unit for the line spreading a set of contact springs that close the path. There are 10 positions in the most common construction of this kind of unit. Thus every line in the group can be connected to any of 10 1st selectors. The assembly includes a common drive element (think “motor”) that, through a shaft with a fin, couples to the line switch plungers of all idle lines to move them until they align with (and are ready to “plunge into”) the springs that would connect the line to the first currently idle 1st selector. Oddly enough, this common drive element is called the “master switch”. Of course, it is not a switch at all. Perhaps “switch master” would have been more apt. It is driven by a powerful spring in much the form of a bicycle pants clip (seen in the figure), with a small flyball governor (not shown) to control its speed. When it has come to the end of its travel, a contact closes and a solenoid resets it to its starting position. When a line comes off hook (the handset being lifted) to request service, the line relay operates, which energizes a magnet in that line’s line switch that makes the plunger “plunge”. making a connection from that line to the currently first idle 1st selector. The “master switch”, detecting that the “current switch” for all the lines is now busy, then moves all the remaining plungers until they are positioned to connect their lines to a now idle 1st selector One advantage of this scheme is that the per-line cost of this assembly may be less than that of an equivalent group of uniselectors. This is a fascinating mechanism3. Figure 11 shows a typical plunger switch assembly, this for 50 lines. The shaft is actually vertical; I have rotated the picture to save space. (This assembly is in fact from an office in New Zealand.) 3 The design of this mechanism is attributed (ca. 1906) to Alexander E. Keith, one of the most prolific and influential inventors in the Strowger company (and its successor, Automatic Electric Company). Especially outside the Bell Telephone System, this mechanism is often called a “Keith switch”. Step by step: The line finder switch Page 14 Figure 11. 50-line plunger line switch assembly On either side, there is a row of 13 switches and a facing row of 12 more, interleaved. In each switch, there is the plunger magnet and the line relay. By means of a second armature on the plunger magnet, it also serves as the cutoff relay. In the center, there is the “master switch” and 5 magnets and relays for controlling it. In an alternate configuration of this 50-line assembly, there would be two master switches, each associated with 25 line switches. Notwithstanding the greatly different mechanical arrangement, the role of the plunger type line switch in the network architecture is essentially identical to what we see in Figure 4. As with the uniselector line switch configuration, it is very common to have a second stage of plunger type switches (“secondary line switches”) in the path to the pool of 1st selectors. This allows any given line to potentially gain access to more than 10 1st selectors, for an improvement in the efficiency of utilization of the line finders. The details of the plunger type line switch configuration are beyond the scope of this article. 4.5 The line finder scheme The scheme for the “front end” of the switching network on which we have concentrated in this article uses a switch called a line finder. For recollection, Figure 12 shows it in the same context we have seen before. Step by step: The line finder switch Page 15 Figure 12. Switch train with line finders The line finder uses the same basic mechanism as the other switches in the system. But as a system element, it “faces the other way”. This scheme, at the system level, was already discussed in detail in section 3 of this article. 4.6 Usage preferences In the Bell Telephone System, during the early days of years of use of the step-by-step switching system, that system used the line switch scheme, mainly using the plunger type line switch, Later the line finder scheme became the norm. But for Strowger systems in the U.K. for instance, the line switch scheme, with uniselectors, was the most common in larger central offices, over the entire era of the system. 5 CIRCUIT SCHEMATIC DRAWING CONVENTIONS The circuit schematic drawings in this article utilize my adaptation of a system introduced in the Bell Telephone System in the late 1950s, called detached contact schematic notation. In the prior drawing style (now called the attached contact style), the relay contacts were shown in a form evocative of actual physical contacts, with all the contacts on a certain relay adjacent on the drawing, much as they are in real life. A major disadvantage of this system for circuits of any complexity was that the many circuit paths, often involving contacts on several relays, were a nightmare on the drawing. Following any particular circuit path was arduous. Rather, in this newer system, simple (and easily drawn!) geometric symbols are used for the basic relay elements: the coil, which actuates the contacts, and the contacts themselves. The contacts on a certain relay are not gathered together on the drawing, but rather are placed so as to allow the most clear portrayal of the circuit paths (they are “detached” from the symbol for the relay coil, thus the name of this style). The possibly-many contacts of a Step by step: The line finder switch Page 16 relay, and its activating coil, are related by each being marked with the same symbol (which, by the way, in real equipment would likely also be marked on the relay itself). Figure 13 shows the principles of the detached contact style. Figure 13. Detached contact schematic symbols—relays In panel (a) we see a relay, M, under the older attached contact convention. This relay has a coil with a single winding and four contact “spring sets”, each of a different type. Each “spring” is identified by a number (from 1-10). The dashed line we see between the three spring sets shown above the coil emphasizes that the “moving springs” of all these spring sets move together (toward the “coil”). Of course, spring 10 on the spring set shown below the coil moves at the same time (toward the “core”), but we are expected to know that. Spring set 1-2 is a make contact (what would be called in other electrical work a “normally open” or “form A” contact). Spring set 3-4 is a break contact (a “normally closed” or “form B” contact). Spring set 5-6-7 (what would be called in other electrical work a “form C” contact) is called a transfer contact; It implies a break-before-make operation (so there is never, even momentarily, a path from spring 6 to spring 7. Spring set 8-9-10 is also called a transfer contact. It however implies a make-before-break operation. There is never, even momentarily, the loss of a path to one of springs 9 and 10. This is often in fact spoken of in other electrical work as a “make before break”, or “form D”, contact. It is sometimes called a continuity transfer contact. Step by step: The line finder switch Page 17 In panel (b) we see this same relay portrayed under the detached contact system. There I have purposely shown the spring sets “scattered” to remind us that they would not ordinarily be shown adjacent to the relay coil but would be placed on the drawing wherever the circuit paths through them would be easiest to follow. (In larger drawings, they may well appear on separate sheets.) We see that the coil (called here the relay “core”) has a simpler symbol, one not graphically evocative of its winding. As we see for contact 1-2, the symbol for a make contact is a simple cross, centered in the line representing the circuit path. For contact 3-4 we see the symbol for a break contact, a simple line across the circuit path. For the basic transfer contact (break-make) (5-6-7), we use a combination of those two symbols, usually adjacent, as we see here. (But if needed for clarity of the circuit paths, the two parts may be drawn un different places.) For the make-before-break contact (8-9-10), the portrayal is the same as for the break-before--make contact. There is nothing in the graphic representation that distinguishes the two forms of a transfer contact. In formal Bell Telephone Laboratories drawings, that distinction was provided in tabular form in a separate “apparatus figure”. But in the adaptation of the system I use, I use the symbol “#” to indicate that this is a “make-before-break” contact. In some cases, there are two (in some cases even more) windings on the coil. We must generally be aware of the relative “polarity” of the windings, so the current through the two windings produces adding, or opposing, magnetic fields (whichever we need for the intended circuit operation). In panel (c) we see a two-winding relay coil shown under the attached contact system. In panel (d) we see that same coil under the detached contact system. In both cases, the little half-moon marks show “corresponding” ends of the two windings. Especially in the case of more modern relays whose physical construction is not that suggested by the symbol shown in panel (a), the contacts (rather than individual springs) are identified by number. In panel (a),I have shown these contact designations in brackets. In panel (e), we see the contact whose springs might be numbered 6-7-8, but as a contact is numbered 3. Step by step: The line finder switch 5.1 5.1.1 Page 18 References By spring or winding terminal In the detailed circuit description in Appendix A, the reference “A:1-2m” refers to, on relay A, contact spring pair 1-2, which is a make contact. The reference “D:3-4b” refers to, on relay D, contact spring pair 3-4, which is a break contact. The reference “D:wT-B” refers to, on relay D, the coil winding connected to terminals T and B. In both cases, the order in which the springs or winding terminals are stated matches the direction of the path being described. 5.1.2 By contact number or winding designation In some cases, I only used a single number to designate a contact, in which case a reference to a contact might be “L:1m”, for the contact on L identified as number 1, which is a make contact. Similarly, in some cases I will only identify the windings of a relay with multiple windings as “P” (primary), “S” (secondary, and (if applicable) “T” (tertiary). In the circuit description, the primary winding of L would be spoken of as “L:wP”. If L had only a single winding, that would be referred to just as “L:w”. 6 THE CIRCUIT SCHEMATIC DRAWINGS 6.1 Introduction Figure 22 (at the end of this article) is the circuit schematic drawing for an actual typical line finder switch. It is in fact of the 200-point variety, which is almost universally used in all but the smallest central offices. The drawing also includes a portion of the group circuit, which is responsible for “dispatching” the line finders (its other portions deal with many other functions not pertinent to the scope of this discussion). Figure Error! Reference source not found. gives the notes for this drawing. Figure 14 shows a typical line circuit that would be used with this kind of line finder. Figure Error! Reference source not found. shows a later variant. Figure 20 shows a lower-cost variant. Step by step: The line finder switch 6.2 Page 19 Designations and notation In the schematic drawing in this article, the relays in the line finder switch are identified with the designations (the letters A-F) used in the formal circuit schematic drawings.4 The various coil winding terminals and contact springs of each relay are identified by under the numbering system that would be found on the formal circuit schematic drawings.5 6.3 Relays and their functions This section lists the relays in this switch with simplistic descriptions of their major functions. A–Start B–Calling line found in upper bank C–Vertical/rotary step D–Switch busy E–Marked level found F–Calling line found in upper bank; cut through 7 ON “LINE NUMBERS” 7.1 Introduction Before I launch into this topic, let me remind us that the “line numbers” mentioned in connection with line finder operation are not in any way directly related to the “telephone numbers” of the lines. Regardless of its telephone number, a line is assigned to a certain “position” in a certain line finder group based on making most efficient use of the line finders, thus giving it a “line number”. The “line numbers” of which I speak here are used to identify the line “positions” as they appear on the line finders. 4 Bell System step by step systems use this convention, inherited directly from the practice in systems made by Automatic Electric, in which the relay designations came from their positions on the “chassis”. Other systems, originally designed in the Bell System would typically use mnemonically-based relay designations, perhaps “ST” for the start relay rather than “A”. 5 This system is overly tedious for the purposes of this articles, but I do it this way to facilitate my maintaining in parallel a formal circuit schematic drawing and the drawing in this article, and a formal circuit description document and the circuit description in this article. Step by step: The line finder switch Page 20 That all having been said, next recall that in a connector, the first level is reached by dialing “1” as the tens digit, but the 10th level is reached by dialing “0”, and is thus generally labeled “level 0”. Similarly, the first rotary position is reached by dialing “0” as the units digit, and is thus generally labeled “step 0”. So, considering the 100 positions on the connector bank, on level 1 (the first level) the first position is considered to be position “11”, the ninth position as “19”, and the tenth position as “10”. On level “0” (the 10th level), the first position is considered as 01, the ninth position as “09”, and the tenth position as “00”. 7.2 In a line finder In the line finders used in all but the smallest offices, 200 lines are served by a line finder group. The line finder has only 100 positions, but each position can access either of two lines, one said to be on the “lower bank” and the other said to be on the “upper bank.” We will see more details of that shortly. In actual practice, the lines that appear on a line finder are identified in terms of the positions they occupy, using the notation that would be used on a connector. This numbering plan of course differs from what we would expect from other parts of our experience (not in a step by step office). Thus, for the first subgroup of lines (10 “lower” lines plus 10 “upper” lines), the line numbers would be: For the “lower” lines: 11, 12, 13, 14, 15, 16, 17, 18, 19, 10 For the “upper” lines: 111, 112, 113, 114, . . . 118, 119, 110 Then, for the tenth subgroup, the line numbers would be: For the “lower” lines: 01, 02, 03, 04, 05, 06, 07, 08, 09, 00 For the “upper” lines: 101, 102, 103, 104 . . . 108, 109, 100 8 BASIC OPERATION When the calling line comes off hook, the L (line) relay in its line circuit operates. One contact of the line relay operates a G (for “group”, but it actually relates to a subgroup) relay in the line finder group circuit. A second contact of the L relay connects the winding of the CO relay (which is fed from battery) to the sleeve contact of the line’s appearance on the line finder bank multiple. One contact of the G relay grounds a terminal in the vertical commutator of all line finders in the group. A second contact of the G Step by step: The line finder switch Page 21 relay grounds the start lead of the first available line finder in the line finder group (“first” according to the “batting order” established for the line finder group, starting with the preferred line finder for that line subgroup). That operates the A (start) relay in that line finder. The line finder begins to step in the vertical direction. The commutator wiper looks for ground on each commutator contact, At the level corresponding to the subgroup in which the calling line resides, that contact is grounded, and the E relay operates. That shifts the automatic stepping operation to the rotary direction, As the switch steps, the F and B relays look for battery on the sleeve terminals of the lower and upper bank, respectively. If the calling line is on a terminal in the lower bank, when the switch reaches that terminal the battery on the sleeve contact (from the winding of the CO relay in the line’s line circuit) operates the F relay. That operates the D relay, which halts the rotary stepping. F also cuts the line tip and ring through to the mated 1st selector. D advances the start lead to the next idle line finder in the start sequence for the group so it could respond to a subsequent service request. If the calling line is in the upper bank, when the switch reaches that terminal the battery on the sleeve contact (from the winding of the CO relay in the line’s line circuit) operates the B relay which halts rotary stepping. B switches the “from” part of the line finder’s tip and ring circuit to the wipers of the upper bank. The B relay operated also operates the F relay. the next part of the operation is as described above. In either case, when the tip and ring are cut through to the mated 1st selector, that selector returns ground on the S (sleeve) lead. That holds the B (if applicable), F, and D relays in the line finder operated. When the connection is ended, the selector removes ground from the sleeve lead. All relays release and the selector mechanism is released to normal. When the D relay releases, that puts this line finder back “in the rotation” to possibly be used for another service request. 9 The line circuit 9.1 Introduction Each line is provided with a line circuit. Each line circuit comprises two relays, L (line) and CO (cutoff). We see it in a basic configuration in Figure 14. Step by step: The line finder switch Page 22 Figure 14. Line circuit (typical—unbalanced) With the line idle (the station on hook (“hung up”) and the line not involved in any connection) both these relays are released. Battery is fed through the winding of L and CO1B to the ring of the line, Ground is fed through CO2B to the tip of the line. We will see later that the line circuits are organized in subgroups, each comprising 10 “lower bank” lines and 10 “upper bank” lines. 9.2 A transmission issue I interrupt the main story to note that with the line circuit as shown in Figure 14, with the line idle, we have the impedance of the winding of the L relay in the feed to the ring but no such in the feed to the tip. Hence the line is unbalanced from a transmission standpoint. The result of this is that if the line takes on any induced voltages or currents (perhaps by passing near electrical equipment or a power line), these may result in a spurious voltage from tip to ring, which would produce noise in any telephone set connected to the line (likely a hum or buzz). For most of the life of a line circuit in this state (idle), there is no telephone set actually connected across the line, so the phenomenon is of no consequence. But, as we will see shortly, from the time a station wanting service “comes on the line” until the line finder finds the line and extends it to a 1st selector, such spurious noise could be heard. Step by step: The line finder switch Page 23 Figure 15. Line circuit (typical—balanced) In later designs of most versions of the step by step system, this phenomenon was averted by using a two winding L relay (with one winding in the feed to each line conductor), as we see in Figure 15. Oddly enough, in the panel dial, No. 1 crossbar, and No. 5 crossbar switching systems, over the lives of those systems, the standard line circuit used an unbalanced line relay, as seen for the step by step system in Figure 14. But the No. 1 ESS and 5ESS switching systems used an essentially balanced equivalent of the line relay. I d