DETACHED CONTACT SCHEMATICS

[Music] Heat. Heat. [Music] For many years, SD drawings have been prepared in what may be called attached form. On these drawings, the individual parts such as relays, lamps, jacks, and so forth were shown as complete physical units. It was necessary to show the circuit paths going back and forth between the separate parts of the apparatus items. This method of presenting the circuit was generally satisfactory for the types of circuits than in use. However, as the switching systems became more complex, it became increasingly difficult to present the circuit information. Understanding the circuit was often difficult and circuit tracing was complicated by having to follow the action of the circuit through a large number of paralleling and crossing lines. After extensive investigation, the detached contact type SD has been devised. It is intended that the detached contact type of drawing will be used for all new systems and for major new circuits added to existing systems. The information contained in a detached contact type schematic drawing is separated into class groups, each serving to portray information of a similar character. Cross referencing between the groups is employed to coordinate the information. The circuit paths are shown in a series of functional schematics constituting one of the groups of the drawing. In order to achieve the objectives of the functional schematic, parts that perform separate operations are shown in a detached manner whenever this is helpful. For example, relay contacts are shown directly associated with the functional path of which they are a part and not in their physical relation to the relay coil that operates them. In order to associate the detached parts readily into their physical relation to each other, they are shown in another group of the drawing called apparatus figures. In addition to showing the physical arrangement of an item, the apparatus figures serve as an index as to where each part of a piece of apparatus is shown on the functional schematics of the circuit. To aid in locating the circuit components in the drawing, sheet coordinates are furnished on most sheets. Since the individual parts of a given component in a circuit may perform different functions, they may be shown on several sheets of a series. As an aid to understanding the circuit, another group of information known as sequence charts is usually included as part of the drawing. These sequence charts show the sequence of operation and release of the circuit components. In addition, other information required to complete the circuit story is contained in other groups of the drawing. Complete information is described in the Bell system practices. It would be impractical to cover in this film all the information and since the bell system practices are available for reference only a few of the symbols used in detached contact schematics will be discussed in detail. the negative side of a 48vt positive grounded battery and ground connection. A single wound relay or coil. SG is the functional designation. 2500 is the resistance of the winding in ohms. 5T and 5B identify the winding terminals. S O indicates that the relay is slow operating. A multi-wound relay or coil. HM is the functional designation. P and S are primary and secondary windings of 2500 ohms each. Make contact one and two T. Identify the number one and two springs in the top pileup. Brake contact of a relay. Close when the relay is unoperated. Open when the relay is operated. A single figure usually indicates a wire spring relay. This illustrates a 206 type selector. For simplicity, the terminals are arranged in a vertical line and one terminal symbol may represent a number of connected terminals. The principal purpose of a sequence chart is to serve as a guide to the understanding of a circuit as a whole and as a key to the order of operation of the circuit. These symbols indicate that the A relay operates and the AL lamp lights. Here is indicated the fact that the A relay releases and that the TWW tube output decreases. In this simple sequential cause and effect relation, relay A operates and causes the operation of relay B, which in turn causes the operation of relay C. Relay C then releases relay D. A multiple effect from a single cause. Relay A operates and causes the operation of both relays B and C and the release of relay D. In this case, both relays A and B must operate before C operates. An alternative cause in which either relay A or relay B will cause operation of relay C. a time delay condition. The SR relay is slow to release following the operation of relay A in order to apply what we have covered on detached contact schematics. This simplified drawing of a step-by-step selector circuit will be used. The sequence chart indicates that the T and R loop is closed when the selector is seized. This causes the operation of the A relay which operates the B relay grounding the incoming S lead. The A relay operates from battery through its own winding through the normally closed contacts of the D relay and the loop back through the contacts of the D relay and the other winding of the A relay to ground in the trouble alarm circuit. The B relay operates from battery in the trouble alarm circuit through the closed two and three contacts of the A relay through the normally closed contacts one and two of the D relay to ground. Ground is then connected through the four and five contacts of the B relay to the incoming S lead. Upon receipt of the first dial pulse, the A relay now releases, causing the operation of the vertical magnet and the C relay. The vertical magnet operates. On the first vertical step, the vertical off-normal springs operate. This plus the operated C relay causes the operation of the E. The A relay releases, but since the B relay is a slowrelease relay, it does not release. The vertical magnet and the C relay operate from battery through their windings through contacts two and three of the B relay, contacts one and two of the released A relay, through normally closed one and two contacts of the D relay to ground. With the C relay and the vertical off-normal springs operated, the E-re will operate from battery through its winding, through the vertical off-normal contacts, through contacts one and three of the C relay, through contacts four and five of the B relay to ground. At the end of the first dial pulse, the A relay again operates and the vertical magnet releases. Intermediate dial pulses cause the same circuit action. As the loop is again placed on the tip and ring, the A relay operates as before. The vertical magnet releases, but the C relay is slow releasing and will stay operated during the succeeding dial pulsing. Notice that detached contacts in a circuit shown thus are similar to older type circuits and indicate normal open or closed contacts. This symbol however represents contacts normally open but which will close when the vertical offnormal springs are operated. And this symbol indicates contacts normally closed but which will open when the D relay operates. After the last dial pulse is received, the A relay reoperates from the loop. This releases the vertical magnet. And after a time delay, the C relay releases, operating the rotary magnet, cutting the wipers into the first terminal. The A relay reoperates and the vertical magnet releases. The C relay releases, operating the rotary magnet. The chart then shows the e-relay releasing which releases the rotary magnet. At this point, we have an option as indicated by this symbol. We can assume terminal one is busy or idle. If we assume it is busy, we follow the sequence chart for trunk hunting. If we assume terminal one is idle, the chart indicates the D relay operates. The operation of the D removes dial tone, releases the A relay, operates the A and B relay of the next selector, which locks the D relay. The operation of the D relay also releases the B relay after a time delay. The D relay operates releasing the A. The B relay releases and the D is locked from the next selector. The outstanding characteristics of the detached contact schematic are that paths may be easily traced and paths may be arranged to promote understanding of the circuit. The ease with which paths can be traced on the detached contact schematic can be seen by comparing them with the older standard type drawings. For instance, a file cabinet would be required to hold the prints for an average central office whereas these books using detached contact schematics contain all the circuits for an office. By studying the symbols used and applying them to the schematic, skill can readily be acquired for securing information from the drawings. [Music]