Reference Designations

A reference designation is a combination of letters and numbers used to identify the various parts and components on electronic drawings, diagrams, parts lists, and so on. The prints you work with will have one of two systems of reference designations. The old one is called a block numbering system and is no longer in use. The current one is called a unit numbering system. We will discuss both in the following paragraphs.

Figure 6-14.-Sample wiring diagram.

Figure 6-15.-Wiring color code for electronic equipment.

BLOCK NUMBERING SYSTEM.-Parts designations in figures 6-11, 6-12, and 6-13 were made according to the block numbering system, which is no longer in use. In that system, a letter identifies the class to which a part belongs, such as R for resistor, C for capacitor, V for electron tube, and so on. A number identifies the specific part and in which unit of the system the part is located. Parts within each class in the first unit of a system are numbered consecutively from 1 through 199, parts in the second unit from 201 through 299, and so on.

UNIT NUMBERING SYSTEM. In this currently used reference designation system, electronic systems are broken into sets, units, assemblies, subassemblies, and parts. A system is defined as two or more sets and other assemblies, subassemblies, and parts necessary to perform an operational function or functions. A set (fig. 6-16) is defined as one or more units

Figure 6-16.-A five-unit set.

and the necessary assemblies, subassemblies, and parts connected or associated together to perform an operational function.

Reference designations are assigned beginning with the unit and continuing down to the lowest level (parts). Units are assigned a number beginning with 1 and continuing with consecutive numbers for all units of the set. This number is the complete reference designation for the unit. If there is only one unit, the unit number is omitted.

Assemblies and subassemblies are assigned reference designations consisting of the unit number that identifies the unit of which the assembly or subassembly is a part, the letter A indicating an assembly or subassembly, and a number identifying the specific assembly or subassembly as shown in figure 6-17.

Parts are assigned reference designations that consist of the unit and assembly or subassembly designation, plus a letter or letters identifying the class to which the part belongs (as in the block numbering system), and a number identifying the specific part. For each additional subassembly, an additional letter A and number are added to the part reference designation. For example, if the resistor shown in figure 6-16 is the number 1 resistor in the subassembly, its complete reference designation would be 4A13A5AIR1. This number identifies the number 1 resistor on the number card of rack number 5 in assembly 13 of unit 4. On electronic diagrams, the usual procedure is to use partial (abbreviated) reference designations. In this procedure, only the letter and number identifying the part is shown on the part itself, while the reference designation prefix appears at some other place on the diagram as shown in figure 6-14. For the complete reference designation, the designation prefix precedes the partial designation.

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