Chapter 5

Hardware Laboratory

Objectives: To present rules and guidelines for the hardware laboratory.

Topics covered:

Rules and Guidelines

The following rules and guidelines must be followed in order to ensure proper care and longevity of equipment and safety to students.

General Care of Equipment

  1. Computers and electronic equipment are expensive to acquire, maintain and repair. Please help us in keeping all equipment in good condition so that they will be usable for many years to come.
  2. Learn to use all equipment properly. It is inappropriate to be flicking knobs and switches without knowing the proper function and usage of each control. Know what you are doing.
  3. Ensure that all cords, cables and leads are tidy and without twists, kinks and undue stress. Always plug and unplug cables and power cords by firmly grasping the plug. Never pull on the cord.
  4. Always turn off the power switch on equipment before disconnecting or connecting to the AC power outlet.
  5. Report any malfunctioning equipment to the TA or technician immediately.
  6. Do not move equipment from one work area to another.
  7. No equipment is to be removed from the laboratory under any circumstances.
  8. Each group will be issued the following items at the start of each lab period:
  

 

  1. Bread-pan
  2. Needle-nose pliers
  3. Flat screwdriver
  4. Wire-stripper

These are to be returned at the end of each lab period.

 



Working with Breadboards

  • Some power supplies are grounded, i.e. the zero volt reference is at safety ground. Others may be floating, i.e. there is no reference to any ground. In normal usage, connect the COMMON banana-post to the GND post.
  

 

  • Connect +5V and GND to vertical rails on the solderless breadboards. Distribute these to all other vertical rails to create +5V and GND buses. Use wire of the same colour for all +5V connections, e.g. red, orange or yellow. Use another colour for GND, e.g. black, brown, green, blue.

  

 

  • Connect the Vcc and GND pins of the IC to the power bus using short jumpers. Be consistent with use of colour.


 

 

  • Make use of coloured wires to differentiate different connections.
  

 

  • Try to keep all wiring short and tidy and avoid laying wires over the IC, just in case you have to remove the IC. Make use of colours to differentiate the signal functions.


 

 

  • Do not trim the leads of resistors and capacitors. Arrange them neatly on the solderless breadboard and connect using short wires.


  

 

Working with Integrated Circuits and other components.

 

  • All semiconductor components are sensitive to electrostatic discharge (ESD). While total damage from ESD is more easily detectable, it is the unseen and undetected weakening of devices that are more costly in the long run. In electronics assembly plants, the cost to replace a defective 25¢ component increases exponentially as the product travels down the assembly line. Eventually, the cost to replace the same component when the equipment is out in the field can run into thousands of dollars. Time and money spent in tracing "bugs" and intermittent and unexpected failures is surprisingly high. Companies concerned with the reliablility of their products ensure that proper handling procedures are in place throughout the assembly process.

To reduce the potential for damage from ESD, the following care is required.

Where provided, make use of anti-static controls and material handling techniques, i.e., wrist-band grounding straps, anti-static foam and anti-static bags, grounded workbenches and workstations.

 

Avoid excessive handling of components, cards, cables, computers, etc. If you are not wearing a wrist-band grounding strap, discharge yourself by touching the grounded metal benches in the laboratory before handling ICs and equipment. This is especially important in the winter after taking off or putting on any garments, for example, sweaters and coats.

  • Before using an IC, make sure that you identify the proper part number. In the picture shown, the proper part number is SN74LS139N. The LS stands for Low power Schottky and the suffix N is the package style, i.e. dual-in-line plastic (DIP). The letters XAB is the batch code and the numbers 9611 is the date code (11 week of 1996 in this example).


  • Check that all the IC pins are straight. If the pins are not straight, use the needle-nose pliers to straighten them.

  • If the IC is new and never used, straighten all leads on each side by placing the IC on its side on the bench and press firmly on it in order to bend the pins to a 90-degree angle.

  • Do not use your fingers to remove the ICs from the breadboard. Use the flat screwdriver to gently pry away the IC by alternating at both ends of the IC. Do not crowbar the IC from one end.

  • Similarly, check the pins on interface cables. Make sure all pins are straight. If there are any pins missing please notify the lab supervisor immediately so that a new cable can be provided.
  • Do not pull on interface cables when removing from the breadboard. Pull on the pull tab instead.
  • Connections to the solderless breadboards are made by stripping about 6 mm (¼ inch) of insulation off the end of a length of #22 AWG solid hookup wire and inserting the bare end into the tiny holes in the breadboard. Do not use ends that are twisted or bent. Prepare a fresh end.

Computer System Design

As with software design, the design of computer hardware systems requires a well planned systematic approach. A common pitfall of the inexperienced designer is to hastily put together a complex system only to see it fail on first trial. Trouble shooting the design then becomes a frustrating experience in chaos and the designer has to go back to the drawing board to start a new design. Proper planning and testing would have prevented this scenario.

A systematic approach to computer system design involves three stages.

  1. Design
  2. Implementation
  3. Testing and documentation

In the first stage the problem must be clearly identified and specifications defined. Examine all possible solutions to the problem. Both top-down and bottom-up approaches are required. With top-down approach you will develop conceptual plans of how the solution will be put together. At this stage the design will be void of details of actual implementation, as with top-down programming.

Applying bottom-up design means that you must be familiar with the specifications and operation of all the devices to be used in the design.


In the implementation stage, you convert your conceptual design into hardware components. To accomplish this you will need to examine truth-tables, timing diagrams and device specifications. Rough hand-drawn schematics are approriate at this stage and must not be overlooked. However, it is important that these drawings be complete logic schematics with part numbers, pin numbers and signal names. An IC package layout or a physical wiring diagram is NOT appropriate.

As parts of the implementation are put together, they should be tested in isolation. This is the modular approach to hardware design and testing. Using this technique increases the chances of a working system when all the parts come together.

Finally, "The job's not over until the paperwork is done!" Your rough schematics must be converted into proper drawings using either drafting templates or a computer aided design (CAD) program. Complete documentation will also include a discription of the circuit's operation and its performance, along with truth-tables, timing diagrams, etc.

Schematic Drawings

There are established standards for drawing circuit diagrams. The sample schematic is used to demonstrate the following guidelines:

  1. Draw signal flowing from left to right.
  2. Junctions are indicated using a dot. U1,7.
  3. Logic symbols are used and not the IC package layout. U10.
  4. Pin numbers are placed outside the symbol. U10,11.
  5. Pin functions are placed inside the symbol. U10,11.
  6. Label all ICs, e.g. U10.
  7. Label IC type, e.g. 74177.
  8. Do not show +Vcc and GND pins except for larger or uncommon ICs.
  9. All unused inputs should be connected to +Vcc or GND as required. The symbols 1 and 0 may be used as an alternative. U9,2.
  10. Negated signals, e.g. RESET, should reflect the active state condition, i.e. active LO. U6,6.
  11. Gates should reflect the logic function (i.e. AND or OR) and not the IC type used. Use DeMorgan's theorem. U5,8.
  12. Negated outputs, represented by the small circle, should match up with a negated input where ever possible. U4,11.
  13. Differentiate between rising edge triggered clocks (U9,3) and falling edge triggered clocks, U10,8.
  14. Where it is useful show which edge is being used as the trigger source.


A schematic drawing shows the logic function of devices. It is not a wiring diagram.


Here is an example of a badly drawn schematic.

DeMorgan's Theorem

DeMorgan's Theorem provides us with the following two boolean identities:

Because of these identities, it is possible to draw the same logic functions in different ways:

The rule for drawing logic symbols is as follows:

Draw the logic function using POSITIVE logic. That is, if the function is the AND function then show the AND symbol. If the function is the OR function, show the OR symbol. The bubbles at the inputs and outputs are then added if the signal is NEGATIVE logic, i.e., ACTIVE-LO.


For example, do not consider this as a NAND gate, but an AND gate with an ACTIVE-LO output.

Similarly, this is not just an AND gate, but an OR gate with ACTIVE-LO inputs and an ACTIVE-LO output.

Do not draw the circuit only on the basis of the IC package. For example, just because the 7408 package is an AND gate does not mean that you draw the AND gate.


In this example, the 7474 flip-flop is cleared by either ABORT or CLEAR. Therefore an OR gate is shown even though a 7408 AND gate is used in the circuit. Note that the clear (reset) function on the flip-flop is ACTIVE-LO.


When analyzing circuit diagrams, it is useful to consider the bubbles not as inverters but as beads on a string. You can slide the beads along the string without altering the outcome of the logic. When two such beads bump into each other, they cancel each other.


Introduction to Electronics

This section will review some basic electronic components which you will be using in the laboratory.

Resistors

The resistance value and tolerance are identified by coloured bands on the body of the resistor. Resistors in the lab are generally 5%, ¼-watt resistors and will have four coloured bands.

Bands A and B indicate the first and second digits while band C is the multiplier. Thus, the resistance value is given as AB × 10 C ohms. Coloured band D indicates the tolerance of the resistance value, i.e., the maximum expected deviation or error from the nominal value.

Resistor Colour Codes

The numeric value assigned to each colour is shown. Note that the digits from 2 to 7 are the six colours of the rainbow in order. For example, the resistor shown in the photograph with colour bands of yellow-purple-red-gold indicates a 4700 or 4.7k, 5% resistor. In international notation, this value is shown in schematics as 4k7 where the multiplier (k) replaces the decimal point. This notation avoids error from an obscure decimal point.


Standard Resistance Values

1.0 1.2 1.5 1.8
2.2 2.7 3.3 3.9
4.7 5.6 6.8 7.5
8.2 9.1    

Multiply these values by any multiple of 10 to arrive at standard values for 5% resistors. These values are approximately 20% apart.


An interesting note on tolerances

It can be beneficial for electronic companies to periodically check on the quality control of component manufacturers. Suppose that the resistance values of 1000 resistors, with a nominal value of 100 ohms, taken from the same batch of resistors were measured and tabulated. One can expect a distribution of resistance values centered about the nominal value. Here are some typical histograms showing different distributions.


In this distribution, the values are well within the ±5% tolerance limits.


Here the manufacturer has good process control and manages to keep the tolerance to ±2%.


While these values still meet the specified tolerance, this shows that the manufacturer is experiencing some process control problems.


Here the manufacturer has poor process control and relies on measurement to reject resistors falling outside of the 5% tolerance limits.


Worst of all, this manufacturer sorts and removes the 1% resistors which can then be sold at a higher price.


Variable Resistors

Variable resistors are also commonly known as potentiometers or pots for short. These are used to provide output voltage Vout as a variable fraction of an input voltage in a circuit called a potential divider.

They can also be used to provide a variable resistance which will allow fine tuning of frequencies, time delays, RC constants, etc. Note that every pot comes with three terminals. The centre terminal is usually the slider or wiper arm which provides the variable contact along the body of the resistor.

Capacitors

The standard unit of capacitance is the Farad. However, 1F is a huge value. Non-polarized disc capacitors are commonly available in the range 10pF to 1µF. These are used in timing and frequency dependent applications such as RC circuits and filters. The capacitance value may be printed on the capacitor in a number of ways:

  1. value in pF, e.g. 4700 = 4.7nF
  2. value in µF, e.g. .022 = .022µF = 22nF
  3. value as per resistor code, e.g 104 = 100nF = 0.1µF
  4. value as per international code, e.g. n15 = 0.15nF = 150pF

(micro = × 10-6, nano = × 10-9, pico = × 10-12)


With familiarity, you will learn to identify the proper value judging from the physical size of the capacitor.


Electrolytics are polarized and must be inserted into the circuit with the negative lead towards the more negative potential. The negative lead is identified by the -ve sign or the arrow. These capacitors are commonly used in power supplies, large RC timing and DC blocking and are available in much higher values than discs, ranging from 1µF to 10,000µF.


"Super-caps" with values such as 1F are now available. Capacitors of these values are more likely to be used as batteries!

Oscilloscopes

Just as a programming language debugger allows you to follow the inner workings of a program, an oscilloscope is your window into the inner workings of the hardware. It is therefore important that you become comfortable as well as competent in the use of the oscilloscope.

Sometimes a simple light indicator or light emitting diode (LED) might be used to monitor output levels. The oscilloscope, however, is far superior, whereas an LED can be misleading at times. In particular, an LED will not indicate the following conditions:

  1. A high frequency oscillation.
  2. A floating input or output.
  3. A glitch or very small spike.
  4. An output with insufficient drive (a low HI).
  5. An output with insufficient current sinking ( a high LO).
  6. A noisy signal.
  7. A signal with ringing on the rising and falling transitions.
  8. A signal with instability at the transitions.
  9. A delay between signals.

Furthermore, the oscilloscope allows you to make precise time and voltage measurements. For these reasons and many others it is important to use the oscilloscope at all times when testing and debugging circuits and associated systems.

Getting Started

First, you must become familiar with the usage and settings of all the controls of the oscilloscope. The controls are identified by the numbers shown and described below. Go over all the controls and make sure that you understand their functions.

Picture of Tektronix Model 2213, 2213A and 2225 oscilloscope.

BEAM CONTROL  
1 INTENSITY Controls trace brightness
2 BEAM FIND Locates the beam if off the screen
3 FOCUS Focuses the beam
5 POWER ON/OFF Power switch
6 Power Indicator Light comes on when POWER is ON (2225 only)
   
VERTICAL CONTROL  
7 CH1 POSITION Channel 1 vertical position control
9 CH2 POSITION Channel 2 vertical position control
10 CH1-BOTH-CH2 Selects CH1, CH2 or BOTH
11 NORM-INVERT Inverts Channel 2 when depressed
12 ADD Sum of CH1 and CH2 is displayed
ALT Beam alternates between CH1 and CH2
CHOP Beam is rapidly switched between CH1 and CH2
13 CH1 & CH2 VOLTS/DIV Vertical gain or sensitivity
14 CAL Rotary knob must be in indented position for calibrated vertical scales
15 AC Only the AC component displayed
GND Shows zero volt reference line on the screen
DC DC and AC components are displayed
16 CH1 & CH2 INPUT BNC input jacks for CH1 and CH2 signals
   
HORIZONTAL CONTROL  
17 COURSE POSITION  Course horizontal adjustment
18 FINE POSITION  Fine horizontal adjustment
19 X1-ALT-MAG  
X1 Normal (unmagnified) waveforms
ALT Alternating display of normal and magnified waveforms
MAG Magnified waveforms
20 SWEEP (SEC/DIV)  Horizontal sweep rate, i.e. horizontal scale
21 CAL  Must be in CAL position for calibrated horizontal scale
21 x10  When pulled out, the sweep rate is magnified by 10 (2213 only)
22 x5-x10-x50 MAG  Horizontal magnification (2225 only)
   
TRIGGER CONTROL  
25 SLOPE  Selects rising or falling trigger source
26 LEVEL  Selects the trigger threshold level
27 TRIG'D  Light shows when the horizontal sweep is triggered
28 MODE  
AUTO  Triggers automatically at preset levels
NORM  Triggers when the threshold level is exceeded
TV FIELD  Triggers from television field signals
SGL SWP  Triggers once only at threshold level
29 RESET  Used to reset and arm trigger for SGL SWP
30 HOLDOFF  Varies the time delay before the next horizontal sweep
31 SOURCE  Selects CH1, CH2, LINE frequency or EXTernal trigger source
32 COUPLING  Selects AC or AC & DC trigger signal
33 EXT INPUT  BNC connector for external trigger signal



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