Electronic Troubleshooting

Digital Systems
•
• Analyzing large systems based upon info in system diagrams
•
• Understanding Digital Systems
• Bus-oriented Systems
• Problems in digital systems
• Troubleshooting Digital Systems
• Testing and Troubleshooting Microprocessor systems

Understanding Digital Systems
•
• Most digital and computer (microprocessor or micro-controller) controlled systems can be represented by a block diagram similar to the one on the right

Understanding Digital Systems
•
• Simple Interval Counter
• Control subsystem:
• Control Flip-Flop
• Clock
• NAND Gate –regulates control signals
• Input Subsystem
• Reset, Start, & Stop switches
• Arithmetic Subsystem
• Tree 7490’s arranged in a multistage counter

Understanding Digital Systems
•
• Simple Interval Counter
• Output
• 7-Segement displays , drivers , and input signals
• Memory
• Lacks this subsystem
•
• If the subsystem inputs and outputs are known
• Subsystems operation testable using I/O values

Bus-oriented Systems
•
• Bus Symbols on diagrams
• Arrow with a diagonal line and the number of connected lines
• Replaced 7 lines between the 7447 IC and the 7-Seg display
• Busses structures require multiple devices to use circuits (lines) in a bus as Inputs & Outputs
• Can cause significant problems
• Example: Gate 1 tries to pull point X to a logic 1 and Gate 2 tries to put it at a logic 0 – Indeterminate result

Bus-oriented Systems
•
• Replace the gates driving lines of a Bus with gates that use external pull-up resistors
• Example 7401 NAND Gates w/external pull-up resistors
• The previous using these types of gates resolves to the gate putting out a logic 0 controls the line

Bus-oriented Systems
•
• Both “X” and
“W” inputs drive the lines of th bus
• 4-bit data bus
• Lines b
0
• Drives
– b f multiple gated outputs
• “Y” & “Z”

Bus-oriented Systems
•
• Better solution
• Has the faster rise time of the Totem Pole outputs
• Has 3 states : High, Low,
High Impedance
• Operation
• The added diode D1 will ground out the collector of
Q2 and the base of Q3
• Q2 and Q3 are off
• No current through R3 and
Q4 is off
• Equivalent Circuit – lower left

Bus-oriented Systems
•
• Example Circuit

Bus-oriented Systems
•
• Used to select ICs , memory locations, multi-line
Tristate inverters or buffers (same as inverters but no inversion of levels), etc
• Three binary address lines select one of eight outputs
• A way to select a bank of tristes to activate

Bus-oriented Systems
•
• Three lines on a 8-bit address bus is used to activate one of upto eight banks of tristates
• Controlling the writing data to another Bus
• Typical circuit in Bus based systems
• Microcomputers, micro-controllors, etc

Problems in digital systems
•
• Ringing and Reflections
• Power Supply Glitches
• Changes in Layout, Components, and
Temperature
•

Problems in digital systems
•
• Caused by long interconnecting lines
• As the interconnection lines length becomes significant to the wavelength or the signal frequency
• Load, source and transmission line impedance mismatch can lead to signal reflections and distortions
• Reflected waves interfere with new signals on the same line
» May be in or out of phase with the new signal
» End Result: the refection combines with the signal forming a new third signal.

Problems in digital systems
•
• Distortion call also occur on long interconnecting lines due to the different impedances seen by different components of the square wave placed on the lines
• All interconnecting lines have distributed capacitance and inductances
• The longer the lines the more the significant the distributed components
• Square waves have been analyzed as consisting of a large collection of signals with a large range of frequencies with differing amplitudes
• They react to long transmission line according to that analysis

Problems in digital systems
•
• Square waves have been analyzed - continued
• Higher frequency component waveforms suffer more attenuation than lower frequency waveforms
• Thus more distortion
• Noise pick-up and crosstalk
• Longer lines form better antennas to pick-up external signals/noise
• Longer lines form better antennas to pick-up internal signals from nearby lines caring other signals - aka
Crosstalk

Problems in digital systems
•
• Sudden changes in current draw by one of several components in a parallel connection to a regulated power supply can cause a voltage glitch
• Caused by the voltage developed across the distributed inductance of the line supplying the power
• Very short duration – only as long as the current draw is changing
• Voltage spike per the following:
See Example v
L v
L
L


L di
' dt

VoltageDev elopedAccr ossInduc
Induc tan ce

Henries tan ce
Problem 11-1 on page 320 di dt

Rate
 of

Change
 of

Current
 Amps
Second

Problems in digital systems
•
•

Problems in digital systems
•
• A large shared ground plane (as shown in the previous examples)
• Large current draws can lead to ground level fluctuations and related problems
• Best cure is at design time
• Provide each part of the circuit it’s own path to ground
» Thus minimizing the sharing of problems
• See Figure 11-14 on page 321

Troubleshooting Digital Systems
•
• Understand the circuit operation
• Apply typical inputs
• Successively split the system into smaller and smaller sections.
• Look for circuits that have good inputs and abnormal outputs
• Start at the approximate middle between inputs and outputs
• Sample circuit is analyzed
• A frequency Counter

Troubleshooting Digital Systems
•
• A frequency Counter
Pages 324 and 325

Troubleshooting Digital Systems
•
• Sample circuit is analyzed
• How it works (see page 325)
• Signal to be measure is feed into the Squaring Block
» TTL compatible square wave comes out
• Before the start of a measurement – the control circuit resets all the counters
• Then the squared input signal goes through the gate for 1 second COUNT-NOT pulses
» At the freq of the input
• COUNT-NOT pulses are feed to the counters for 1 second
» Gate is disabled to stop the counting after one second
» Counters hold the count of the number of input pulses that occurred during the 1-second measurement period

Troubleshooting Digital Systems
•
• Sample circuit is analyzed
• How it works (see page 325)
• Then a store pulse is feed the Latches – enabling the storage of the count that was on the output pins of the counters
• The latches feed the BCD to 7-Segement drivers which drive the displays
• The clock is a 555 chip and the output from pin 3 is a rectangular waveform that has a pulse width of 1 second
» Calibrated by adjusting pot R1
• The falling edge of the Clock (555 chip -pin 3 ) triggers the one-shot output from pin 13 of chip 74221
» 100µsec pulse – set by R3-C2
» Enables the latches to read the output pins of the counters

Troubleshooting Digital Systems
•
• Sample circuit is analyzed
• How it works (see page 325)
• The falling edge of the Latch enable one-shot pulse enables another one-shot output from 74221 – pin 5
» Pulse resets the counters
• The count of pulses stored in the latches represents the frequency of the input signal – since they only count during the one second measurement period
• Timing diagram on page 327 of the textbook
• Troubleshooting the sample circuit
• Inject a testing signal of a few hundred Hz
• Watch the displays
• Normal operation is obvious

Troubleshooting Digital Systems
•
• Troubleshooting the sample circuit
• Watch the displays
• Indications of the source of abnormal operation can also be discerned from the displays.
» If the least significant digit is operating correctly and the second and third aren’t
» All the Input, timing & control circuitry is working, also IC’s 7,
10, and 13 must also be functional - check the IC supporting the other displays
• If none of the displays are functioning normally
• Go to the middle of the circuit. Check the outputs of IC 7
• If good split the remaining part of the circuit and test again. etc
• Troubleshooting flow chart on page 328
• Large scale Integrated IC version on page 331

Testing and Troubleshooting
Microprocessor systems
•
• Designs that can be varied to meet a situation by changing the program in the device
• i.e., first circuit that input buttons on a WMS Bluebird Slot
Machine is a PIC microcontroller
• Programmed to de-bounce input button activations
• Some items can be checked even without a complete understanding of such a system.

Testing and Troubleshooting
Microprocessor systems
•
• Sample system - MC6800 Microprocessor single board system - See Figure 11-25 on page 333

Testing and Troubleshooting
Microprocessor systems
•
• Sample System - MC6800 – continued
• Has all the subsystems shown in Figure 11-1 on page 309
• MC6800 chip contains the control and arithmetic functions
• 74LS244 chip contains the Input circuits
• Tristate octal input buffer
• Connects external inputs to the data bus when enabled
» Pins 1G and 2G are active Low
• Output data leaves through the octal buffer 74LS373
• 2716 is EPROM that holds the system program(s)
• 8 data bits

Testing and Troubleshooting
Microprocessor systems
•
• Sample System - MC6800 – continued
• RAM - two 2112 ICs
• 4 data bits
• Need two for a byte of data
• Chip Select Circuit
• 74LS155 – 4 to 1 decoder
• 2- address lines are used to select one of four ICs (A
15
A
12
) and
» A
12
= 0 and A
15
» A
12
= 0 and A
15
= 1 selects output 2Y
= 0 selects output 2Y
0
2

Testing and Troubleshooting
Microprocessor systems
•
• Usually the major components are in sockets and can be removed
• Remove them an test surface mounted circuits that are isolated
• On the sample circuit the MPU, RAM and EPROM can be removed
• Obvious circuit to test is the clock
• On Sample: Pins 01 and 02 of the MC6875 and MC6800 chips
• Next check the Chip select circuits
• set MC6800 address pins A
12 pins on the other chips
& A
15 and check CE-NOT

Testing and Troubleshooting
Microprocessor systems
•
–continued
• Major components – removed - continued
• Check the input buffers by using external data settings and the appropriate address settings for A
12
& A
15
• Read the inputted data at the data lines for any of the removed ICs
• Continue with all the supporting circuits that can be directly tested.
• Any inverters and gates can be tested using a logic probe and digital pulser
• Retest with some or all the major components in place