About: Logic Probes

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whiteshadow
Posts: 142
Joined: Mon Jun 01, 2015 1:10 am

About: Logic Probes

Postby whiteshadow » Fri Jun 19, 2015 9:19 pm

The following is information about Logic Probes & beginners' instructions.

    A Logic Probe is low cost & easy to learn tool you can use for electronics repair on systems like pinball machines.
    They are very easy to learn & to use. A logic probe is a bridge between a meter & a scope.

    A Meter is for reading constant voltages (see image 1 below), but not great when a signal is pulsed (see image 2 below, which is a 12 volt pulsed signal from a switch matrix). A meter averages the signal to provide a single voltage reading, which is not very helpful.
    A scope is good for pulsed signals, but is much more expensive, and more complicated.

    Image 1 : 12 Volt Signal:
    logicprobe-1.gif
    logicprobe-1.gif (36.18 KiB) Viewed 1568 times


    Image 2 : Switch Matrix Pulse:
    logicprobe-2.gif
    logicprobe-2.gif (38.46 KiB) Viewed 1568 times


    For pinball machine testing & repair, both the lamp & switch matrices are Pulsed, plus there are circuits on the cpu, display and sound boards. You can infer readings from the switch matrix using a meter, but it's easier to use a logic probe instead.

When buying a Logic Probe :

    Image 3 below is 1 possible recommended logic probe. It is an Elenco LP-560 and can be purchased on Amazon for about $17.
    There are more expensive logic probes, but this one will do well.
    This particular logic probe provides both sound & visual LED's as reading indications, plus more features described below.
    While this will not benefit you much initially, as you become more proficient there are times when the audible tone provides a better indication of a pulsed circuit than the LED's.

    Image 3 : Logic Probe (Elenco LP-560)
    logicprobe-3.jpg
    logicprobe-3.jpg (27.81 KiB) Viewed 1568 times


    This specific product does not include a Pulser.
    A Pulser allows you to apply a signal to a circuit. That is an advanced technique & most hobbyists may not need or use it.

About "Logic Families" :

    Both CMOS & TTL gates have different input & output logic levels, but consider them as being the same for these purposes.
    To learn more about the differences between CMOS & TTL logic levels see the following information (not needed for beginners):
    http://www.allaboutcircuits.com/textboo ... ge-levels/

    There are different logic families, or generations, of integrated circuits. Each logic family has different behavior & each logic family can have subsets with different characteristics. The only 2 we need to learn right now are CMOS & TTL.
    TTL chips use a nominal Vcc (Vcc is a term for supply voltage) of 5 volts & the inputs & outputs are always binary (low, high or pulsed). TTL chips typically, but not always, use a standard naming convention of 54XX or 74XX.
    CMOS chips can use a Vcc range of 3 - 15 volts & depending on the chip can have either binary (low, high or pulsed) or analog inputs & outputs. CMOS chips typically, but not always, use a naming convention of 40XX or 45XX.

    An example of CMOS in a pinball machine is the LM339 voltage comparator used in Williams/Bally switch matrix circuits. More info below in the switch matrix examples. Right now it is important to recognize whether an IC is TTL or CMOS. If in doubt, always check the datasheet for that IC.

    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.
    TTL has a low range of 0.0 - 0.8 volts & a high range of 2 - 5 volts. So any reading between 0.0 and 0.8 volts is considered a logic 0, & any reading between 2 and 5 volts is considered a logic 1.
    CMOS circuitry in a 5 volt circuit is a low range of 0.0 - 1.5 and a high range of 3.5 - 5.0. 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.

    You don't need to remember all that, since there is a TTL/CMOS switch on the Elenco (and on all logic probes except those that are auto-sensing). Put the Logic Probe switch in the correct position (based on the previous information about CMOS & TTL) and it will correctly read low & high signals for that logic family.

About Logic Probe Functionality :

    The logic probe has 2 wires (red & black) with alligator clips at the ends. 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 be 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 the first image below 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. The next image provides another representation, comparing the led's to what you would see on an oscilloscope.
Last edited by whiteshadow on Sat Jun 20, 2015 12:04 am, edited 8 times in total.

whiteshadow
Posts: 142
Joined: Mon Jun 01, 2015 1:10 am

Re: About: Logic Probes

Postby whiteshadow » Fri Jun 19, 2015 9:21 pm

logicprobe-4-indicators.jpg
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logicprobe-5-hi-lo.png
logicprobe-5-hi-lo.png (79.27 KiB) Viewed 1567 times
Last edited by whiteshadow on Fri Jun 19, 2015 10:04 pm, edited 1 time in total.

whiteshadow
Posts: 142
Joined: Mon Jun 01, 2015 1:10 am

Re: About: Logic Probes

Postby whiteshadow » Fri Jun 19, 2015 9:49 pm

Switch Matrix Example :
    Here is areal world example (Williams WPC in this case, but the theory is the same on other games) 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 following link for more information on the switch matrix.
    http://pinballrehab.com/1-articles/soli ... leshooting
    The first image below 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.

    The ULN2803 is a TTL chip that uses 5 volt logic on the input (point B) and controls a 12 volt signal on the output (point A). 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 the second image below 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.
    Note: Thanks to zaza for the corrected switch matrix diagrams (these are incorrect in the WPC manual and show an additional connection on the 10K resistor that does not exist).

    logicprobe-8-pullUpResistors.png
    logicprobe-8-pullUpResistors.png (50.37 KiB) Viewed 1567 times

    logicprobe-6-switchMatrixCircuit.png
    logicprobe-6-switchMatrixCircuit.png (32.24 KiB) Viewed 1567 times


    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) and provides 5 volt logic on the output (point D). So the logic probe should be set to TTL 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.
    The image below provides a graphical representation of the logic probe led status for each test point.
    I've really tried hard to keep this at a beginner level while still providing the necessary information and would appreciate any feedback on stuff that needs clarification or is confusing.

    logicprobe-9-switchmatrixreadings.png
    logicprobe-9-switchmatrixreadings.png (61.99 KiB) Viewed 1567 times


    Question: How does the rate of the blink on the Pulse LED relate to the actual frequency. In the one diagram it says "the rate of which indicates the frequency of pulses". It couldn't be a one for one relationship or you wouldn't see it blinking (in the case of the switch matrix for example)?

    Answer: The signal is manipulated before it goes to the led, for example there is a pulse-stretching circuit so fast pulses can be seen. This is why the audio can often provide more information once you get a good ear for it. At a certain point though, it will just look like the light is on solid.
Last edited by whiteshadow on Fri Jun 19, 2015 11:28 pm, edited 2 times in total.

whiteshadow
Posts: 142
Joined: Mon Jun 01, 2015 1:10 am

Re: About: Logic Probes

Postby whiteshadow » Fri Jun 19, 2015 9:54 pm

Tips for the Pro's
    People often comment they are concerned about sticking a probe or lead into a pinball machine backbox while the game is on.
    Here are a few products that most professional technicians carry that will help make your life easier, and safer. In both cases you can setup all of your test points with the game off, and then turn it on to take your readings.
    I should clarify by safer that I meant for the game, as in not shorting a couple of pins/components together. As long as you keep your fingers away from AC and the DMD high voltage section you've got nothing to worry about.
    The first is a minigrabber test clip, which has a small J shaped, spring loaded clip for attaching onto resistor, capacitor and transistor leads (see image below--the bottom test clips are the minigrabber type). Due to their design they have little chance of shorting to another component once installed. Buy ones that are long enough you can place the other end on the playfield glass.
    The next item is a DIP clip for testing IC's (see image below). The clip is placed over the IC and provides extended test points where you can use a minigrabber or your test probe to take readings. They come in a variety of sizes, but if you don't want to buy a full selection, you can use a 14 pin test clip which will fit on IC's with more pins, although not all pins will be available for testing at one time.
    In some cases you can also use a larger DIP clip on a smaller IC if there is no physical obstruction on the board (since the clip will extend beyond the IC).
    Great Plains Electronics sells the 3M product at the price of $18 for a 14 pin.
    https://www.greatplainselectronics.com/ ... .asp?cat=6
Attachments
logicprobe-12-DIPclip-and-J-clips.jpg
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logicprobe-11-testclips.jpg
logicprobe-11-testclips.jpg (23.86 KiB) Viewed 1567 times


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