How to use a logic probe

ArcadeGreg

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This seems to be a frequent question and topic, but somehow never gets fully explained. I have mastered the multi-meter, and the next logical step (no pun intended) is a logic probe. I have a current project that I'd like to tackle. I would like to start slow and simple. Can someone help me to probe a 27512?

How do I probe a 27512 chip?
What information is need from the data sheet?
What do I set my probe to CMOS or TTL/LS?
Set it to normal or pulse?

Any information would be greatly appreciated.

Greg
 
a logic probe is generally used to see if there is traffic on a data / address line.

ie, checking the OE/CE pins on an eeprom to see if the processing adddress decoders are properly working, checking the address pins to see if there stuck low/high. etc.
 
Go check my repair logs for a sticky post on how ROMs and RAMs are read.
 
This will help you find the problem and not just shot gun the chips. It is usually used to find about were the problem is, floating pins, bad logic. The best method, in my opinion, is with a working board next to a bad board and an oscilloscope. If you have a probe with audio, you can hear the pcb audio through that probe.

I have fixed my Galaxian pcb with a logic probe and the help of Buffett and Orion333. I could easily see the address lines on the 74ls138's was messed up replaced them and no joy. The only other thing left was the 74ls245 replaced and boom, working game.

Just start getting used to using that thing and the more you use it the better you will get. It may take some time at first but soon you will have no problem finding things that don't look right. I never get tired of watching the z80's on Galaga suddenly spring to life after a few secs of being powered on.
 
Go check my repair logs for a sticky post on how ROMs and RAMs are read.

I read it, thoroughly. However, I think it is a little advanced for me as a beginner. I need some to use lame man's terms. For example, if you watch one of my "how to" videos I make it so anyone can do the things I am doing in the video. I brake it down step by step, so even an idiot can figure it out. I'm not asking anyone to go out of their way, though. Just take it one step at a time with me. I have been doing a shit ton of reading, but I learn best with an experienced teacher.

Once I figure out the logic probe, I will make a "how to" video explaining everything as simple as possible.

So, taking it one chip at a time, how do I check a 27512?
Is it a CMOS or TTY?
Pulse or normal setting?
Let's start with pin 1-14, what should I be reading, seeing, or hearing for each pin?
1-
2-
3-
4-
5-
6-
7-
8-
9-
10-
11-
12-
13-
14-


This is the probe I'll be using to troubleshoot.
6653d868572c8f13f424b8dfd7e64a0a.jpg


Thanks for all the help guys!
Greg
 
Funny this was brought up as this was on my list of things to do today. I've got an X-men board with a ram error and I've already replaced the ram and 2 other chips and still no go, so I guess this is my next step :(
 
I'm far from an expert, but let me share some stuff that's helping me. First off, you need to know what a chip does. I do that by using google, and looking at the data sheet and information about the chip.

Now, I don't know if this is the same chip as you are looking at, but going off the number you gave I'd first read up on this: http://www.futurlec.com/Memory/27512-250.shtml and see what it's function is. VCC is supply voltage, pin28. Meter that to see if it's got supply voltage, wont work w/o getting power. E-bar is chip enable, pin20. To read, that needs to go high. So hook the logic probe up to a 5v supply, set to TTL & Pulse, and probe that pin. It should pulse between high and low as it's being told to read. If it's not pulsing, the problem is before that chip (not excluding that chip, but it's too early to tell).

A pro can chime in if I'm wrong, but that's how I go about trying to figure this stuff out. For me, the real trick is trying to figure out if the INPUTS are correct. It's easy enough to see what a chip is supposed to do with a given input, but how do you know if the input is correct in the first place...not always so straight forward.
 
I too am very interested in a layman's explanation on how to use a logic probe. Seems like everything I find out there expects you to have more advanced knowledge than beginners.
 
I am by far an expert to, but I think the best way, is first get the probe hooked up, then probe some chips and get familiar with the different readings. Then read about the different chips you are probing. IMHO, this is not something you are going to pickup lickity split. It gets a little confusing which then leads to frustration. Be prepared to do a lot of reading and research. Patience is a virtue.
 
I too am very interested in a layman's explanation on how to use a logic probe. Seems like everything I find out there expects you to have more advanced knowledge than beginners.


A logic probe is not a beginner's tool. To use it effectively requires an understanding of logic and how logic is gated. People assume it is a beginners tool, my guess is because they are cheap and look very simple.

That being said, the method of using a known good board for comparison is pretty straight forward though it may take a long time since you'll be guessing on where to start and then guessing which component on that line is the culprit.


Here are a few basics to get you started:

1. The logic probe should have it's source power and ground from the board you are testing, not an external source. This ensures that the logic levels are consistent.

2. Before using the logic probe on the board determine if it is operating at TTL or CMOS levels. The easiest way of determining this is to look at the chips themselves. The first three colums of this chart show you the voltage differences. Also, pay attention to the letter codes on the bottom of each column. These codes will be on the ICs somewhere and are used to identify the components as TTL, CMOS, etc.

Chart-IC-Voltage-Switching-Levels-Grpah.png


3. As mentioned earlier you should understand each chip you are testing. Data sheets for most chips can be found online and sometimes the schematic you are following while troubleshooting will include them as well. If you don't understand And gates, Or gates, Inverters, etc. then you need to learn how each of these things operate before you can effectively troubleshoot.

4. Practice, practice, practice.
 
I read it, thoroughly. However, I think it is a little advanced for me as a beginner. I need some to use lame man's terms. For example, if you watch one of my "how to" videos I make it so anyone can do the things I am doing in the video. I brake it down step by step, so even an idiot can figure it out. I'm not asking anyone to go out of their way, though. Just take it one step at a time with me. I have been doing a shit ton of reading, but I learn best with an experienced teacher.

Once I figure out the logic probe, I will make a "how to" video explaining everything as simple as possible.

So, taking it one chip at a time, how do I check a 27512?
Is it a CMOS or TTY?
Pulse or normal setting?
Let's start with pin 1-14, what should I be reading, seeing, or hearing for each pin?
1-
2-
3-
4-
5-
6-
7-
8-
9-
10-
11-
12-
13-
14-


This is the probe I'll be using to troubleshoot.
6653d868572c8f13f424b8dfd7e64a0a.jpg


Thanks for all the help guys!
Greg

Set it for ttl and normal.

next probe the 1-10 pins and if that is working you should see rapid switching from low to high. this is the address lines. 11-13 are the data lines and will probably have activity on them. pin 14 should be LOW. pins 15-19 should have some activity on them as they are the rest of the data lines. Pin 20 is the standby line, if this is high the chip is in standby mode. Pin 21 is another address line (a10) and should be flashing like a rr xing sign with at train going by. pins 22,29 are kind of important and they are not ttl logic level. pins 23 to 27 are address lines and should be flashing like the RR xing. Pin 28 should be high as it is the power input.

Now it is your turn what are you seeing on them there pins. Here is a data sheet you need to learn but I wanted to just get you started. It is not much fun to have to do lots of reading on stuff you probably don't understand fully and can't play with your new toy. So I hope that helps you and after you look at the pins with the probe then look at this data sheet. It may then help you understand why your probe is acting the way it does.


http://www.futurlec.com/Memory/27512-250.shtml
 
So, taking it one chip at a time, how do I check a 27512?

Use the right tool for the job: try reading it with an EPROM programmer.

A logic probe is not a beginner's tool. To use it effectively requires an understanding of logic and how logic is gated. People assume it is a beginners tool, my guess is because they are cheap and look very simple.

That being said, the method of using a known good board for comparison is pretty straight forward though it may take a long time since you'll be guessing on where to start and then guessing which component on that line is the culprit.

+1 on all that. No offense to the OP, but... Physically USING (attaching, setting the switch, and probing with) a logic probe is fairly straightforward, and conducive to a step-by-step "for dummies" type explanation. However, the trick isn't phyisically using it, but knowing how to make sense of what you see; i.e. knowing what's "good" and what's "bad." Aside from the "known-good-board-comparison" technique, I'm afraid the usefulness of a logic probe is more or less directly proportional to the user's understanding of digital logic. So, don't worry about "how to use a logic probe." Learn about digital logic (start with google), as well as how simple microcomputer systems function, and then "how to use a logic probe" will become essentially obvious.

As several have pointed out, perhaps the easiest thing to understand in digital logic is the interter gate. Whatever you see on the input, you should see the opposite on the output. Anything else, and it's bad. In a similar vein are ICs such as flip-flops with complimentary outputs. That means that one output if a compliment of (i.e. exact opposite of, or inverted version of) the other.

Unfortunately I don't know of any "shortcuts" to using a logic probe. It kind of goes hand-in-hand with reading and understanding schematics (although schematics aren't strickly required if you understand common microcomputer design, and have a databook... or are using the board-comparison method).

If you want a tool that requires less thinking, consider a logic comparitor...
 
Set it for ttl and normal.

next probe the 1-10 pins and if that is working you should see rapid switching from low to high. this is the address lines. 11-13 are the data lines and will probably have activity on them. pin 14 should be LOW. pins 15-19 should have some activity on them as they are the rest of the data lines. Pin 20 is the standby line, if this is high the chip is in standby mode. Pin 21 is another address line (a10) and should be flashing like a rr xing sign with at train going by. pins 22,29 are kind of important and they are not ttl logic level. pins 23 to 27 are address lines and should be flashing like the RR xing. Pin 28 should be high as it is the power input.

Now it is your turn what are you seeing on them there pins. Here is a data sheet you need to learn but I wanted to just get you started. It is not much fun to have to do lots of reading on stuff you probably don't understand fully and can't play with your new toy. So I hope that helps you and after you look at the pins with the probe then look at this data sheet. It may then help you understand why your probe is acting the way it does.


http://www.futurlec.com/Memory/27512-250.shtml


A logic probe is not a beginner's tool. To use it effectively requires an understanding of logic and how logic is gated. People assume it is a beginners tool, my guess is because they are cheap and look very simple.

That being said, the method of using a known good board for comparison is pretty straight forward though it may take a long time since you'll be guessing on where to start and then guessing which component on that line is the culprit.


Here are a few basics to get you started:

1. The logic probe should have it's source power and ground from the board you are testing, not an external source. This ensures that the logic levels are consistent.

2. Before using the logic probe on the board determine if it is operating at TTL or CMOS levels. The easiest way of determining this is to look at the chips themselves. The first three colums of this chart show you the voltage differences. Also, pay attention to the letter codes on the bottom of each column. These codes will be on the ICs somewhere and are used to identify the components as TTL, CMOS, etc.

Chart-IC-Voltage-Switching-Levels-Grpah.png


3. As mentioned earlier you should understand each chip you are testing. Data sheets for most chips can be found online and sometimes the schematic you are following while troubleshooting will include them as well. If you don't understand And gates, Or gates, Inverters, etc. then you need to learn how each of these things operate before you can effectively troubleshoot.

4. Practice, practice, practice.


I'm far from an expert, but let me share some stuff that's helping me. First off, you need to know what a chip does. I do that by using google, and looking at the data sheet and information about the chip.

Now, I don't know if this is the same chip as you are looking at, but going off the number you gave I'd first read up on this: http://www.futurlec.com/Memory/27512-250.shtml and see what it's function is. VCC is supply voltage, pin28. Meter that to see if it's got supply voltage, wont work w/o getting power. E-bar is chip enable, pin20. To read, that needs to go high. So hook the logic probe up to a 5v supply, set to TTL & Pulse, and probe that pin. It should pulse between high and low as it's being told to read. If it's not pulsing, the problem is before that chip (not excluding that chip, but it's too early to tell).

A pro can chime in if I'm wrong, but that's how I go about trying to figure this stuff out. For me, the real trick is trying to figure out if the INPUTS are correct. It's easy enough to see what a chip is supposed to do with a given input, but how do you know if the input is correct in the first place...not always so straight forward.


Thanks for all the help and suggestions guys!

I will test the chip as recommended, and report back my findings.
 
I don't really agree with the whole understanding of what the chips should and should not be doing. If I was new and a bit timid about this new thing and had to do a bunch of reading about something I don't fully understand, I might get discouraged and not want to use this new tool. I don't fully understand the addressing magic but I do know what the logic probe and scope should be showing me. I do know that when I put my logic probe on a rom chip (working) it should be flashing rapidly and at various speeds, not rhythmic like when a z80 cpu is watch dogging. I have used one for some time and I can tell the game is working and if the inputs to the controls are working, just with the logic probe, power supply, speaker, and a wire to ground the controls. I am the kind that will jump into a car and try to drive it without doing much reading about it. That is how I learned to drive a motorcycle then car. I am terrible at taking test so I always take the test cold turkey and then study. This helps as you learn about the test taking process and about the questions that are being asked.

I just don't agree with the whole you gotta fully understand this before it will be of any use. Yes the proper tools are great but the programmer will tell you if the program is correct. It will not tell you if the hardware is accessing the program.

Sorry guys but I just don't agree with you and I will stand my ground here.
 
I don't really agree with the whole understanding of what the chips should and should not be doing. If I was new and a bit timid about this new thing and had to do a bunch of reading about something I don't fully understand, I might get discouraged and not want to use this new tool. I don't fully understand the addressing magic but I do know what the logic probe and scope should be showing me. I do know that when I put my logic probe on a rom chip (working) it should be flashing rapidly and at various speeds, not rhythmic like when a z80 cpu is watch dogging. I have used one for some time and I can tell the game is working and if the inputs to the controls are working, just with the logic probe, power supply, speaker, and a wire to ground the controls. I am the kind that will jump into a car and try to drive it without doing much reading about it. That is how I learned to drive a motorcycle then car. I am terrible at taking test so I always take the test cold turkey and then study. This helps as you learn about the test taking process and about the questions that are being asked.

I just don't agree with the whole you gotta fully understand this before it will be of any use. Yes the proper tools are great but the programmer will tell you if the program is correct. It will not tell you if the hardware is accessing the program.

Sorry guys but I just don't agree with you and I will stand my ground here.


I never said you need to fully understand but you do need a basic understanding. As I said before, you can grasp at straws and as long as you keep on grasping you will eventually find the short one. However, that doesn't demonstrate understanding, it demonstrates using a process of elimination. The key to understanding is it gives you a better focus allowing you to repair boards more efficiently. Understanding also allows for you to learn from your repair creating greater focus and speed in the future.
 
I never said you need to fully understand but you do need a basic understanding. As I said before, you can grasp at straws and as long as you keep on grasping you will eventually find the short one. However, that doesn't demonstrate understanding, it demonstrates using a process of elimination. The key to understanding is it gives you a better focus allowing you to repair boards more efficiently. Understanding also allows for you to learn from your repair creating greater focus and speed in the future.

I guess I am just a hands on kind of guy, probe and probe and probe some more. If you get a 'short straw' probe some more till something doesn't quite look right. Kind of like Sesame Street, Which one doesn't look like the others. If you are not sure ask somebody to probe theirs or probe a good board. This is about how I learned to use the thing. It started with the Galaga boards and I didn't have a probe at first, I was using a multi meter. It worked but was kind of a pain. Then I built my own from these plans.

http://www.swansontec.com/sprobe.html

and here is the thing I built from them plans.

http://s1224.photobucket.com/albums/ee365/kb0jjn/?action=view&current=Logicprobe.jpg

I do agree a basic understanding is needed but this is not rocket science. The probe is flashing in a good way or a bad way, or it is just stuck. The first thing is to just use it, in my opinion, then ask questions as to why you got the results you have. After you get use to seeing the different results with the logic probe then you will kind of know what should be seen when the program is correct and running proper. It doesn't matter what the game is, if I probe a rom chip with the logic probe, it should act about the same if it is working proper. The main thing here is to just use it and the more you use it the more you will understand it.

My 5th grade step son can tell you if a board is working or not with a logic probe and he doesn't know what a data sheet is or how to read it. He just started probing things with it and asked me what does that mean and what is wrong. I told him to just probe different things and watch the results but just be careful not to touch more that one chip leg at a time. He concentrated on the roms for some reason and after a few questions, more probing, he knew the working boards from the bad in about 20 mins.

Here is some useful info for you guys that are getting tired of my ramblings.....

http://wiki.arcadeotaku.com/w/Logic_Probe_Guide
 
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First off, I appreciate this thread and all of the information you guys are putting in it. Let me ask this......... Are all logic probes basically the same, or are there better ones to get? In other words, if someone is looking to buy their first probe, are there specific ones to buy/avoid? I do not want to derail your guys' conversation, but I want a probe to learn from as I am figuring out logic and do not want to get a dud. Thanks in advance.
 
First off, I appreciate this thread and all of the information you guys are putting in it. Let me ask this......... Are all logic probes basically the same, or are there better ones to get? In other words, if someone is looking to buy their first probe, are there specific ones to buy/avoid? I do not want to derail your guys' conversation, but I want a probe to learn from as I am figuring out logic and do not want to get a dud. Thanks in advance.


I have never seen a bad new probe, and since new probes are as cheap as $15 I would say buy new. I have seen some older probes that were damaged being sold as "untested" so I really wouldn't recommend grabbing one of them.

There are probes that have special features - some probes will connect directly to your o-scope and others have a small digital screen that can display a rough wave form. The special features can often be helpful once you understand what you are looking for but aren't really needed.

Here is a probe I use in teaching classes. It is certainly not the best probe on the market but it is more than adequate for getting started. The biggest problem I have with this probe is the leads are short (a common problem amont logic probes), but I just use jumper wires to extend them if testing becomes difficult. http://www.mcmelectronics.com/product/72-190
 
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