THE LOGIC PROBE
 

The logic probe we will be using is the Hewlett Packard model 545A. It is compatible with TTL, DTL, RTL, HTL, MOS, and CMOS integrated circuits. A logic probe detects the logic level of the test circuit by means of an indicator lamp on the probe located near the tip. The probe will indicate four digital states which are shown in table 1.
 
 

The floating state means that the voltage is in between the upper threshold level and lower threshold level. It could also mean that there is an open circuit. Sometimes the dim light is called a "bad" level indication. The probe is normally in the dim lamp state.

The flashing state indicates there is a frequency going through that part of the circuit. Any frequency will trigger the flashing lamp. The lamp will flash at around 10Hz. Single pulses as small as 10ns up to 40MHz CMOS and 80MHz TTL can be detected.

Another feature of the probe is the memory lamp indicator. The memory lamp will come on when

• The probe is turned on
• The tip is touched to a circuit
• The logic level goes from high to low or from low to high

The memory lamp is useful when the probe is used in an area of a circuit that is hard to see or if the user happens to look away for a moment while the probe indicated something. To clear the memory you must push the MEM-CLR button on the probe otherwise the light will remain on.

The last feature of the probe is the mode switch. This switch allows the probe to either operate in TTL mode or CMOS mode.  While in TTL mode the probe may be powered by a voltage supply in the range of 4.5 - 15 V dc.  In CMOS mode the voltage supply can be 3 - 18 V dc.  If the probe is powered by a source other than from the circuit under test, the grounds should be connected together.  For convenience, there is another ground connector on the probe, located just above the lamp.

Figure 1 shows a probe.

A logic pulser injects pulses into a circuit. The pulser we will be using is the HP 546A and it is compatible with TTL, DTL, and CMOS. The pulses injected by the logic pulser will override the current state of the circuit. For example, if the pin being tested is at a high, the pulser will input a low pulse to create an edge. The pulser automatically outputs the required logic polarity, amplitude, current, and pulse width to drive high nodes low and low nodes high.

A programmable push-slide switch determines the frequency and duration of the pulse injected. For example, one push followed by a hold will produce a 100Hz continuous output.  The pulser can also produce a burst output.  A burst frequency outputs the programmed frequency for a small length of time and then pauses for a small length of time.  A push is indicated by a dot and a hold is indicated by a line. Consecutive pushes must occur within 1 second of each other to be effective. Table 2 shows the sequence of pushes and holds that yield different output pulses. The information in the table is also labeled on the pulser.
 

By sliding the push-slide switch forward, the current program in the pulser is locked in so you don’t have to hold the button down. The lamp indicator near the tip of the pulser is slowed down for visibility.

The pulser operates in a tri-state manner - high, low, and off.  When the pulser is in the off-state it can be touched to a circuit and not have an effect on the circuit until the pulser button is pressed.  This is because of the high impedance of the pulser.
 

Figure 2 shows a logic pulser.

                                                               Figure 2: Logic Pulser
 
 

The current tracer detects current activity on logic nodes. It can be used to identify current paths and relative magnitudes. The electromagnetic field generated by a change in current is detected by the inductive pick-up on the tip of the tracer. The tracer responds only to a change in current, not DC current. A lamp indicator shows the relative magnitude of current going through the node. The brighter the light gets, the more current is going through that node.

The tracer has a sensitivity adjustment that can be used to control the lamp intensity over a 1mA to 1A range. The tracer may be powered from any source between 4.5 and 18V and requires less than 75mA to operate.

It is important to note that you cannot place the tracer on any grounded area unless the area is at the same potential as the negative lead of the tracer. The negative lead of the tracer is connected to the aluminum case so any voltage on the negative lead is also present on the case.

Figure 3 shows a current tracer.

Figure 3: Current Tracer
 

The current tracer must be held in the right way when in use to be effective. It should be used perpendicular to the board and the holes located on the tip must align with the current path as demonstrated in figure 4.
 
 

In most cases its not one handtool that is used in identifying faults.  Most often it is some combination of the three.  Below is a list of some common faults and how the handtools are used to identify them.  Two other useful tools used in troubleshooting digital circuits are the logic clip and logic comparator.  The logic clip is a device that clips onto an IC and displays the state of every pin by use of LEDs.  This is very useful when the output of many pins is needed at the same time, such as when troubleshooting counters and shift registers.  A logic comparator compares the analog voltage level at its two inputs.  Depending on which input is at a greater magnitude the comparator will output a high or a low.

LOGIC GATE FAULTS

Logic gate testing involves the probe and the pulser.  The pulser is used on the input lines of the gate while the probe monitors the output.  The pulser is usually used to drive an input line to the opposite state it is currently at so a change in the output of the gate can be observed.

Example:   AND gate testing
        Tie all input leads high except for one.  Pulse into the remaining one and monitor the output of the gate with the probe.  Because all inputs are high except for the one being pulsed, the output will follow the pulser.  Use the same technique to test the rest of the input leads.

Example:  2 input OR gate testing
        Tie one input low.  The output of the OR gate now depends on the other input. Pulse into this input.  The output should follow the pulser.  Now tie the other input low and repeat the procedure.

GATE TO GATE FAULTS

The current tracer and logic pulser are useful in finding low-impedance faults (shorts) in between two gates.  The majority of the current will pass through the low-impedance fault so the current tracer can be used to trace this path.  Once the current tracer moves past the fault, the lamp indicator on the tracer will dim because the current intensity went down.  This indicates where the majority of the current is going which is most likely the low-impedance fault.  The line may be tied to ground.  The pulser must be used to pulse the line in between the gates so there is current for the tracer to follow.

The probe and pulser can be used to find opens on the lines between the gates.  While pulsing the line the probe can be used to slowly move along the line to identify where the pulser signal stops.  This indicates the open.

SOLDER BRIDGE

Start with the current tracer following the path of current from the source.  Continue to follow the expected path of the current.  When the tracer comes to the bridge the indicator lamp will probably dim since the current has found some other path to follow.  Visually inspect the board for a bridge.
 
 

PROBE

• Turn on the uLAB
• Using the power connections at the top of the m LAB hook up the probe. Be sure to connect the lead with the red lines to +5 and the black to Ñ .
• Place the probe on the +5 source and verify that the lamp is brightly lit. Move the probe to Ñ and verify that the lamp goes off.
• Probe various areas of the m LAB and observe the state.

• Hook up the pulser in the same way the probe was. Leave the probe connected.
• Using table 2 or the information on the pulser, create all possible outputs. Try moving the switch to lock in the program.
• While pulsing, touch the probe and pulser tips together and verify that the probe flashes at the same time the pulser does.

TRACER
 

• Hook up the tracer
• Turn the sensitivity dial so that it is at approximately half of the maximum brightness.
• Try finding current on different lines on the uLAB. Remember to keep the tracer perpendicular with the board and keep the holes aligned with the line.
• Reset the microtrainer so that uLAB is in the display
• Set the current tracer sensitivity to show half brightness
• Place the tracer on a data line. Notice the light is bright. While holding the tracer there, press FETCH PC and then HARDWARE STEP. Notice that the light dimmed, indicating that the current level decreased.
• Look for current on address line A0 between the probe hole and the LED.
• While on A0 repeatedly press HARDWARE STEP and verify that the tracer lamp pulses each time HARDWARE STEP is pushed.
• Move to A1 and repeat the above step. Then A2. The lamp tracer flashes each time the address LED turn on or off. This indicates that the tracer is responding to a change in current value.

Now that we are familiar with the three tools we will begin to troubleshoot.
 
 
 
 

TROUBLESHOOTING ONE

1.  Reset the uLAB

2.  Press FETCH PC and then HARDWARE STEP

3.  Program the pulser for 10 Hz and pulse into the address bus, line 11. Make sure to pulse to  the left of the pad indicated in figure 5.

Figure 5: Address bus

5.  Now remove W1, located just below the address bus, and repeat step 4. What has happened? Removing W1 has opened A11 somewhere.

6.  Starting from the right, move the probe closer and closer to the pulser until you find the open. You should find that the line in open between the two pads on A11.

7.  Now move W1 to the rightmost holes. See if you can determine what happened to A11. HINT: pulse A10, again to the left of the pad shown in figure 5, and observe the other lines with the probe. Answer: A11 is shorted to A10 and A10 is open.
8.  Replace W1 to its original position (1^st two holes)
 

NOTE: when using the lines with blue paint on top of them you may have to press a little harder.
 
 

TROUBLESHOOTING TWO
 

Locate U14 of the schematic. Signals from the data bus are passed through this 3 state buffer and then onto U15, U16, and U17. The buffered data bus includes all 8 lines but only shows it as one. Looking at U14 on the schematic we can see that D6 input is pin 16 and the D6 output is pin 15. The D6 output then travels to U17 – pin 17, U16 – pin 17, and U15 – pin 17. Let’s verify this and also see what W9 does.
 

1.  Make sure the uLAB is still in HARDWARE STEP mode.

2.  Pulse into the D6 input of U14 and using the probe verify that the pulse in getting through to the D6 inputs of U15, U16, and U17.

3.  Once you have verified the above step, remove W9 and repeat the step 2. You should find that the signal is no longer appearing at U16 or U15. A probable cause for this is that removing W9 has opened the D6 line between U16 and U17.

4.  The lines located just below W10 are the lines between U16 and U17. Determine where the fault is using the pulser and probe. Use a method similar to the one used in troubleshooting one.
Answer is on figure 6.

Figure 6: Answer to troubleshooting two.
 
 
 

TROUBLESHOOTING THREE

1.  Have the uLAB in HARDWARE STEP mode

2.  Pulse D0 test hole at 100 Hz

3.  Place the current tracer on the pulser as shown in figure 7 and set the sensitivity to produce a dim light when touched to the pulser. Do not adjust the tracer after you’ve set it.

 


Figure 7: Current tracer on pulser

4.  With the tracer follow the current on the D0 line to the left and to the right of the pulser. You should see nothing. The current must be going somewhere so lets look at the back of the board.

5.  Pull up the board with the black knob on the right of the board. While still pulsing D0, follow the current on the back of the board. You should find that the current path is upwards and into pin 14 of U5.

6.  Using the same method determine where lines D3, D5, and D7 go. Answers: D3 = U5 pin 11, D5 = U6 pin 13, D7 = U6 pin 11

7.  Now reset the uLAB and press FETCH ADRS, 0000, and HARDWARE STEP.
Determine where the following lines now go. D0, D3, D5, D7
HINT: some paths may be half on the front and half on the back.

Answers: D0 = U4 pin 9
                D3 = U4 pin 13
                D5 = right most hole of W2
                D7 = U4 pin 17
 
 

1.   The 4 conditions that the logic probe identifies are:________ , ________ , ________ , _______ .

2.   The current tracer sensitivity control ranges from

       a) 1uA to 1A
       b) 1uA to 1mA
       c) 1mA to 1A
       d) 1A to 10A

3.  Briefly describe how the pulser is programmed.

4.  How is the pulser program locked in?

5.  T/F The logic probe can detect single pulses of less than 10ns in duration.

6.  The "bad" level indicator of the probe is shown as a ______. "Bad" level is a ______.

a) bright light, float condition
b) dim light, open circuit
c) dim light, short circuit
d) no light, float condition

a) electron flow
b) radiation
c) radio waves
d) electromagnetic field

a) ½ sec
b) 1 sec
c) 2 sec
d) 100msec

a) perpendicular to the board, holes aligned with the path
b) perpendicular to the board, holes perpendicular to the path
c) at a 45 degree angle with the board, holes aligned with the path
d) doesn’t matter

        a) device that indicates current
        b) device that indicates relative voltages by LEDs
        c) device that holds ICs together
        d) device clips off an AC waveform

Answers
1. high, low, floating/open, frequency
2. c
3. by a series of pushes and holds.  Each sequence of pushes and holds yields a different output.
4. by sliding the push-slide switch button forward
5. false
6. b
7. d
8. b
9. a
10. b
 

Related Material
Section 8-22 Tocci "Digital Systems"
Section 3-6 Kleitz "Digital Electronics"
Section 4.3.5.1 Perozzo's "Systematic Electronic Troubleshooting"