3 Layout Planning and Design
3.1 Reading Drawings and Diagrams
3.1.1 Block Diagram
All electronic equipments can be considered as systems comprising a set of interacting elements responding to inputs to produce outputs. It is quite possible that a system may be too complex to be analysed in detail. It is therefore, necessary to divide it into sub-systems and then integrate them.
Each sub-system would then represent a functional block, and the combination of all the blocks would constitute the functional ‘Block Diagram’ of the equipment. A block is only a ‘black box’
with certain inputs and outputs, but performing a definite function. The lines interconnecting these blocks indicate the signal flow from block to block or circuit to circuit. Understanding of the circuit function becomes easy with a block diagram. Figure 3.1 shows a typical block diagram representation of a simple recorder.
Input stage amplifier
Signal conditioner
Recording Electrical part
signal
Fig. 3.1 Concept of a block diagram. It shows various sub-systems in an equipment
The integrated circuits such as microprocessors, counters, etc. are represented as individual blocks.
These blocks are labelled with pin numbers, signals and associated interconnecting wires.
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Layout Planning and Design 105
3.1.2 Schematic Diagram
A schematic diagram is a graphical representation of interconnections of various electronic, electrical and electromechanical components of an equipment. The schematic is the first step in an electronic circuit design because it displays and identifies the components that make up the equipment. Further, the first step in designing a printed circuit is to convert the schematic diagram in to an art master.
Therefore, for any printed circuit designer, it is important to learn to read and interpret the schematic diagram. However, the schematic diagram does not show any of the mechanical details of the printed circuit board.
The schematic provides the most broadly used view of the design and includes all components.
In addition (Mentor Graphics, 2002):
a It gives visibility into the status of all parts of the design process;
a Schematics are the primary source for developing deliverables to product design and manufacturing groups;
a Design variants are built around slightly differing schematics;
a Test departments rely on schematics;
a Field service relies on schematics; and
a Bills-of-materials are generated from schematics.
In short, a schematic is the focal point for a product’s electronic data and can be viewed as a set of crucial business documents that capture the decisions affecting all aspects of the product.
Usually, every assembly in an equipment is assigned an assembly number which appears on the schematic diagram. The schematic diagram shows various components by means of symbols which are so arranged that they show the working of the circuit in a clear way. The component symbols are usually governed by various standards, which do vary widely. Therefore, it is advisable to first find out which standard has been followed before attempting to read a schematic diagram. The schematic diagram is also called the ‘circuit diagram’.
In a schematic diagram, the symbol represents either what the component does in the circuit or how it is physically constructed. For example, a capacitor can be charged to store electricity similar to a battery. A picture of two parallel plates of equal length and separated by a given space has thus been adopted as the symbol of capacitor. In most cases, electronic symbols have been evolved logically from their circuit application, their construction, or from a combination of both.
All electronic components have been designated when represented on a schematic diagram. The common classification from ANSI (American National Standards Institute), IEEE (Institute of Electrical and Electronic Engineers) and IEC (International Electrotechnical Commission) is given in Table 3.1.
Table 3.1 Reference Designators (Reference: Component Identification, IPC-DRM-18 F, Desk Reference Manual
Component ANSI/IEEE IE C Component ANSI/IEEE IEC
Amplifier AR A Jumper W,P or R
Capacitor C Microprocessor U
Pack C G
network C
Polarized C Oscillator Y (crystal or
Variable C G (other)
Relay K
Connector J or P Resistor R
Crystal Y B Pack R
network R
Delay Line DL Potentiometer R
Variable R
Diode D or CR V
Light Emitting DS (display) E Thermistor RT
Diode
Voltage Rectifier D or CR V Varistor
Zener Diode D or VR V Asymmetrical D or CR
Symmetrical RV
Filter FL Z Socket X, XAR, XU,
XQ, etc.
Fuse F
Header J or P Switch S
Inductor, Choke L Test Point or Pin TP V
Integrated Circuit U, IC
Transistor Q
Insulated Jumper W or P
Transformer T
Battery BT Voltage VR
Regulator Meter, Instrument M
Antenna, Spark E
Plug, Connector Male P Gap, Shield
Attenuator AT
Power Supply PS Motor, Fan, B
Test Point TP Synchro
Layout Planning and Design 107
BAL P15
39 R15 50K 2
3 90
<
<
–12 V +12 V
R14 100 R13 1K
Q14
–12 V PartialA1 vert.board AMPL
R10
100K –10.5 V TP 12 g +
J13 P13 J14
VERT OUT
Internal screwdriver adjustment Testvoltage Plug to E.C.board Panel adjustment Wire colour code Plug index
Modified component—see parts list
Refer towaveform Coaxial connector Refer to diagram number Panel connector
Assemblynumber Board name Etched circuitboard outlined inblue +
2
Schematic name and number
R17 RP15
Diode
Capacitor 0.1mF C8
Connector and pin identification Signal
names ALE (PG 3)
PPB1 To find the source
of this signal,go to page3
Crystal
D6
1 2
4 5
14 13
11 10
Inverted signal Transistor Q15
Resistor R6 1K
Lines cross, but not connected Lines connected Z6 LSO4
ICSequence number
10 ICpin number 11
ICpin numbers
PPB (PG 5)2 To followthis
signal go to page5 GAB
673 (a)
(b)
TO DIAG +12V
Fig. 3.2 Schematic circuit with symbols as per the American National Standard Institute (a) Circuit with discrete components (b) Circuit with integrated circuit
Figure 3.2 shows a typical schematic diagram and demonstrates its basic parts. Each component is represented as a symbol in the diagram along with its reference designation. Electronic components shown on the circuit diagram are generally in the following units unless mentioned otherwise:
Capacitors = Values one or greater are in the picofarads (pF)
= Values less than one are in microfarads (mF) Resistors = Ohms ( W)
Guidelines have been developed over the years for drawing schematic diagrams. The main features of these guidelines are:
a Signal flow moves from left to right across the page with inputs on the left and outputs on the right.
a Electronic potentials (voltages) should increase as you move from the bottom to the top of a page. For example, in the figure, +12V supply is shown upwards while the –12V is indicated downwards.
a Use the ‘unit number’convention for assigning a unique IC package identification. For example, U1 with its internal gates identified by letter suffixes; U1A, U1B, etc. Only one of the common gates need show the power connections. Power connections are often omitted, but it is better to include as a reminder as well as to make your schematic complete.