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Using a Logic Probe Using a Logic Probe Hot

One of the simplest and cheapest tools you can include in your test equipment arsenal is a logic probe.  Although a lot of people seem overwhelmed by logic probes, they are actually very easy to use.  In regards to their purpose, consider a logic probe as a bridge between a meter and a scope.

While a meter is great for reading constant voltages (see Image 1) they fall short when a signal is pulsed (see Image 2, which is a 12 volt pulsed signal from the switch matrix).  What the meter will try to do with this signal is average it and give you a single voltage reading, which is not very helpful.  Of course a scope works great on pulsed signals, but is much more expensive, and complicated.

Specific to pinball, both the lamp and switch matrices are pulsed, plus circuits on the cpu, display and sound boards.  While you can infer readings from the switch matrix, for example, using a meter it is much easier to just use a logic probe.

Buying a Logic Probe

In Image 3 you can see my recommended logic probe, the Elenco LP-560, available at Amazon for $17.  You can spend more, but this is really all you need.   In addition to all of the standard features (which we'll discuss more in a little bit) it also provides an audible tone in addition to the led's.  While this will not benefit you much initially, as you become more proficient there are times when the audible tone will be a better indication of the pulsed circuit than the lights.

The only function that it does not have is a pulser, which allows you to apply a signal to a circuit.  This is a fairly advanced technique and most hobbyists will never have need for it.

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Using a Logic Probe
Using a Logic Probe
Using a Logic Probe
Using a Logic Probe
Using a Logic Probe
Using a Logic Probe
Using a Logic Probe
Using a Logic Probe

Logic Families

Note: The information in this section has  been simplified in order to align with the goal of a beginner level guide.  For example, both CMOS and TTL gates have different input and output logic levels, although we will consider them as being the same for our purposes.  If you want to fully understand the differences between TTL and CMOS logic levels see the following article at All About Circuits: Logic Signals Voltage Levels.

There are different logic families, or generations, of integrated circuits.  Each logic family has different behavior and within each logic family there can be subsets with different characteristics.  The only two we need to be concerned with in regards to our discussion are TTL and CMOS.

TTL chips use a nominal Vcc (Vcc is the fancy term for the supply voltage) of 5 volts and the inputs and outputs are always binary (low, high or pulsed).  TTL chips typically, but not always, use a standard naming convention of 54XX or 74XX.  On the other hand, CMOS chips can use a Vcc ranging from 3 - 15 volts and depending on the chip can have either binary (low, high or pulsed) or analog inputs and outputs.  CMOS chips typically, but not always, use a numbering convention of 40XX or 45XX.  If in doubt, you can always check the datasheet for any given IC.

One example of CMOS in a pinball machine is the LM339 voltage comparator used in Williams/Bally switch matrix circuits.  We'll discuss this in more detail as we get into the switch matrix examples, but for now the important part is to be able to recognize whether an IC is TTL or CMOS.

Based on the logic family of the chip there are different voltage ranges that are considered to be low or high in a digital circuit.  In the case of TTL the low range is 0 - .8 volts and the high range is 2 - 5 volts.  So any reading between 0 and .8 volts is considered a logic 0 and any reading between 2 and 5 volts is considered a logic 1.  

The specification for CMOS circuitry in a 5 volt circuit is a low range of 0 - 1.5 and a high range of 3.5 - 5.  For a 10 volt Vcc the low range would be 0 - 3 volts and the high range 7 - 10 volts.  The high and low voltage ranges scale linearly across the possible supply voltages of 3 -15 volts.

Thankfully you don't need to remember all that though, since there is a TTL/CMOS switch on most logic probes (the ones that don't have a switch are auto-sensing).  Put the switch in the correct position and it will correctly read low and high signals for that logic family.

Logic Probe Features

The first thing you will notice is that the logic probe has two wires (red and black) with alligator clips at the end.  This is where the probe gets it's power and they must be connected to ground and supply voltage.  If you're testing a 5 volt circuit, the red lead goes to 5 volts and the black lead to ground.  If you're testing a 12 volt circuit (parts of the switch matrix, for example) the red lead goes on 12 volts and the black lead on ground.

The pointy thing at the other end from the two wires is the probe.  Unlike a meter this single probe is all you need to take your readings.

There are two switches, TTL/CMOS and MEM/PULSE,  that will need to bet set properly.  If you're analyzing a TTL chip, put the TTL/CMOS switch in TTL and when checking a CMOS chip, put the switch in CMOS.  The MEM position on the MEM/PULSE switch will capture a pulse and retain the reading, which is advantageous in some rare situations, but for our purposes here you want it set to PULSE.

The last, and most important part, of the logic probe are the HI/LO and PULSE led's.  The red (HI), green (LO) and yellow (PULSE) led's are used to indicate the state of the measurement point.  Note: Some logic probes use different combinations of lights to indicate the status, so just a reminder, I'm specifically talking about the Elenco logic probe here.

In Image 4 you can see the various signals that can be indicated by the led's.  In most cases you can narrow these down to three issues: is the line high, is the line low or is the line pulsed.  Image 5 provides another representation, comparing the led's to what you would see on an oscilloscope.

Switch Matrix Example

Now let's look at a real world example to see how the logic probe works when testing the switch matrix.  Note: It is beyond the scope of this article to cover how the switch matrix works.  See the references for additional information.  Image 6 provides a generic WPC switch matrix circuit and we'll walk through what each test point should look like, starting with the column, or send, signals.

Note: There is an error in the WPC manual in regards to the switch matrix circuit in Image 6 (it shows a connection that does not exist), which has been noted in the image.

The ULN2803 is a TTL chip that uses 5 volt logic on the input (point B in Image 6) and controls a 12 volt signal on the output (point A in Image 6).  So the logic probe should be set to TTL and the red lead connected to 5 volts when testing inputs and 12 volts when testing outputs.  

Tip: If you look at Image 7 you will see three red circles with pull-up resistors and a supply voltage within them.  If the pull-up resistor is connected to a 5 volt supply you know you are working on a 5 volt circuit and if it's connected to a 12 volt source you know you're working on a 12 volt circuit.

With our logic probe connected to 5 volts and the probe on point B we will get a green light and the yellow light will be pulsing.  This indicates a low signal with high pulses.  This signal is a constant timing pulse and will not change based on the status of the switch.

The circle shown at point A tells us that the output signal from the ULNL2803 is inverted.  So a high input provides a low output, and a low input provides a high output.  Therefore, with our logic probe connected to 12 volts and the probe on point A we will get a red light and the yellow light will be pulsing.  This indicates a high signal with low pulses. 

The row side gets slightly more complex and the readings will change based on the status of the switch.  The first part of the circuit we are concerned with is the LM339.  It is a CMOS chip that takes a 12 volt signal on the + input (point C in Image 6) and provides 5 volt logic on the output (point D in Image 6).  So the logic probe should be set to CMOS and the red lead connected to 12 volts when testing inputs and 5 volts when testing outputs.  

With our logic probe connected to 12 volts and the probe on point C we will get a red light with the switch open, which indicates a high reading.  With the switch closed we will get a red light and the yellow light will be pulsing.  This indicates a high signal with low pulses.

With our logic probe connected to 5 volts and the probe on point D we will get a red light with the switch open, which indicates a high reading.  With the switch closed we will get a red light and the yellow light will be pulsing.  This indicates a high signal with low pulses.

The 74LS240 is a TTL chip and since there is a circle on the output we know that the signal is inverted.  So with our logic probe set to TTL, connected to 5 volts and the probe on test point E we will get a green light with the switch open and a green light with the yellow light pulsing with the switch closed.  The former indicates a low reading and the latter a low reading with high pulses. 

Image 7 provides a graphical representation of the logic probe led status for each test point.