SCHEMATIC AND LOGIC SYMBOLOGY

Logic and schematic diagrams are used to convey the electrical
description of analog and digital circuit functions. A logic
diagram is the drawing that depicts the multi-state device
implementation of logic functions with logic symbols and
supplementary notations that show the details of signal flow and
control, but not necessarily the point to point wiring. A schematic
diagram is the drawing that shows, by means of graphic symbols, the
electrical connections, components and functions of a specific
circuit.

Both type of diagrams use reference designators to indicate the
type of component and which specific one it is within the circuit.
The reference designator consists of one or two letters which are
followed by a specific number that identifies that unique component
in the circuit. Some examples are resistor (R), capacitor (C),
diode (CR), plug (P), socket (X) delay line (DL). The letter U is
usually used to signify the Integrated Circuit. The logic reference
designator is also followed by a number however since the U
represents an I/C assembly there may be more than one of a
particular function in a single I/C Package.

A two input NAND gate consists of two signals as the input with one
signal as the output. The logic symbol for this gate type can be
drawn as a graphic element that describes the logic function.
Examples are AND, NAND, NOR, OR, INVERTER, FLIP FLOP etc.;each has
their own unique graphic symbol. There is usually a truth table
associated with the logic symbol to indicate the state of the
inputs that are necessary to achieve an output. The standard Dual-
Inline-Package, as an example, contains four (4) two input gates.
Each gate has the same characteristics and the share the same
voltage and ground in the I/C package. This characteristic allows
the designer to swap gates within the package in order to more
efficiently interconnect the circuit.

Gate swapping is permitted when the logic functions are identical
and no other performance criteria must be maintained. The original
logic reference designator, as paired by the circuit design
engineer, is then modified during the component interconnection
process. This requires that the reference designators be
redistributed in order to properly pair the logic that is in a
particular I/C package with its companions. This practice is
referred to as  back-annotation  and is accomplished once the
layout has been completed. The final logic diagram must reflect the
board component positions so that R12 is a specific part in the
parts list, just as U12 is a specific I/C package. It is important
to maintain the correct gate segmentation within an I/C package
exactly to the way the gates are interconnected on the final
assembly.

The assembly drawing shows the position of each part by its
reference designator; the bill of materials indicates which
components relate to which designator. Usually schematic and logic
designators are assigned on the schematic/logic diagram in a left
to right, top to bottom sequence. Sometimes the board assembly
controls the number assignment in a similar manner or in a
coordinate matrix fashion where parts within a coordinate zone of
the board assembly are assigned the number of that zone. In either
case the intent is to make finding the correct gate or part easier
during trouble shooting or maintenance. 

Resources: IPC-T-50 Specific Terms and IPC-D-275 3.2.1; 6.6; 6.7
and IPC-D-325 para 10.6; 10.8; 10.16; 10.19; 10.24 and ANSI Y32.
2A.2