logic question the common sense kind

I have an encoder connected to a cnc board. there is an intermittent problem with either a cord to an encoder or the board. If the problem arises, all you have to do is push lightly on the connection and it will go again for hours, days, weeks ... who knows. I took the plug on the cord apart and checked the connections ... I don't think it's the cord but I don't know for certain.

I rigged up a short cord that connects between the board and the encoder with LEDs each drawing a little less than a milliamp, connected between each of the pulse and quadrature pins and ground, and 5vdc and ground.

When I plugged it into the board before plugging the cord into it, all the LEDS lit up. when the encoder turns all the LEDs except the 5vdc flicker.

I thought this was going to show which was bad ...the board or the cord, but now, except for the 5vdc, I don't know exactly what I'm looking at.

Apparently pins on the board float high, and the encoder pulls the signals down.

Finally the question... When this thing quits, what will I see that will differentiate between the cord and the board?

Just some thoughts -

Can you press on the board itself without touching the connector? Possibly there's a cold solder joint on the board - could be right where the connector attaches to the board.

Can you rig up a little breadboard circuit to check the encoder independent of the board? You'd need a power supply and probably a couple of resistors and LEDs.

Do you have spares for any of these parts?

#1 wire. #2 connector pins - surface corrosion. #3 board.
Give a light tug to each wire. If wire is broken, insulation will expand. If crimp is bad, wire will pull.
TTL or differential? If differential, look up RS422 communications. Uses same chips.
Put a 'scope on it. You'll see spikes that PLC will ignore.

thanks for replying... I found the problem today ... a broken wire ...the strain relief and the shrink tubing held it solidly enough that when I checked it before, it didn't seem to be broken... but it was

today it finally quit altogether and I found it


thank you again

... a broken wire ...

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Keith knew that already.

#1 wire.

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hey thanks for replying, and keith was right... BUT ... I was looking for some logical thoughts about how the LEDs on the little cord I rigged up would look that would prove whether it was the cord or the board.

I had an LED between each pin on the plug and ground... if the 5v pin was not lit up, the problem was in the board ( or I suppose it might be possible that the wire was grounded somewhere in the cord or encoder)

When I plugged the LEDS into the board all of them lit up... when the encoder turned , they flickered ... except for the 5v ...I would think that was normal.

At the time when the system would go into an error condition, if one of the LED's never went dark or flickered, I suppose that would indicate that the problem was in the cord, and not the board, would it not?

I thought perhaps someone might be interested in considering the logical possibilities that might indicate definitively if the fault was in the cord or the board, because I expected everything but the 5v to be dark until I plugged in the encoder ... when they all lit up , it threw me for a curve

... if one of the LED's never went dark or flickered, I suppose that would indicate that the problem was in the cord, and not the board, would it not?

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I don't know. As Keith asked, is your encoder feeding a TTL or a Differential input on the CNC board? In order to answer, I would need a complete circuit diagram, with the Encoder, CNC, and LED part numbers and specifications. LEDs are tricky devils, and your circuit may be working opposite of what you think. Apparently your LED circuit could not diagnose or indicate the broken wire.

When i plugged the leds into the board they all lit up with no encoder or anything else plugged in. When the encoder was running and the system was functioning normally the leds flickered except for the 5 vdc one

Oh that complete diagrams, or even a reasonable knowledge of their own product by the manufacurer themselves existed! Oh that you could call someone and not get voicemail! Oh that stuff wasnt obsolete by the time you buy it ....what a wonderful world it would be!

That seldom is the case . The reason all this nonsense is going on in the first place is because the maunfacturer cant supply us with reasonably functioning cnc boards.. We get a replacement, it might very well be worse than the one we have.

What if it WAS TTL ? What if it was something else? I dont see what difference it makes... The info available is the info available.

I wish i had the complete diagrams etc available even once in a while If i had all that stuff available i could probably diganose things while i slept

. The wire wasnt "broken" when i had my jigger connected. It was making connection. It was a very intermittent thing

I thought maybe someone might be interested in forming some deductions from the info at hand.

but i guess not.... Sorry i asked

I thought maybe someone might be interested in forming some deductions from the info at hand.

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I am interested, but do not have all "the info at hand". TTL circuits are usually 5 VDC. Draw your circuit as you built it, and post that, along with the LED specifications. Did you use 2 Volt LEDs, or something else? Is the current rating of the LEDs exactly 1 mA? What is supplying the power to light the LEDs (CNC or encoder)? Are you sure which way that power is flowing? Where does that power go when a LED is OFF? Everything in a circuit makes a difference about how it operates. Leave out one thing, and the operating characteristics can change 180 degrees. (I guess you found that out the hard way).

From where do you derive your conclusions?
Your posts contain no useful information and serve only to irritate

Sorry that I irritated you, but I have made no conclusions, nor can I until you provide information about your circuit. I am in the dark about what you wired up, or why you expected it to operate differently.

Lancie1 said:

Sorry that I irritated you, but I have made no conclusions, nor can I until you provide information about your circuit. I am in the dark about what you wired up, or why you expected it to operate differently.

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Apparently your LED circuit could not diagnose or indicate the broken wire.

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(I guess you found that out the hard way).

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I would call that a conclusion... what is it exactly that I found out the hard way?

If that conclusion was not intended for the sole reason to be condescending and to try to make me look stupid, perhaps you would be so kind as to explain for what it was intended ...

And there was nothing preceding it in your post that provided any basis for it .
Just a bunch of your irrelevant comments... If you can't dazzle em with brilliance, baffle em with b. s. ...

I told you all the info I had available to me. I told you that very seldom, if ever, do I enjoy the luxury of having all the information you wanted to know.

I told you that each LED was connected from one of the pins on the plug to ground... I would assume you would be able to deduce that any "power flowing" in the LED's went from anode to cathode. I told you they all lit up without having the encoder plugged it. And I told you that except for the 5v, they all flickered when the encoder operated normally.

This is about the third time I told you all that stuff.

I've seen you make similar comments to other posters in other threads .. probably about as often as not. And then to come round saying you were sorry if you irritated me... I'd say that was more than just a little disingenuous.... If I didn't know better, I'd say you were just a troll

I'd prefer that you did not respond to any more of my posts... after, of course, clobbering me on this one

I told you they all lit up without having the encoder plugged it. And I told you that except for the 5v, they all flickered when the encoder operated normally.

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LEDs are famous for being able to turn on merely from the leakage current through non-conducting transistors. In other words, the transistors do not have to be in the on or in a conducting state, but only connected to their normal power source, in order to pass enough leakage current to turn on a LED. For that reason, I would be surprised if you could take only some random LEDS and make an effective trouble-shooting device for some transistor circuits. Most likely the pin with the broken wire was getting fed from the opposite end through another pin and then back through a transistor circuit.

If you do a site search for "leakage and LEDs" you will find many cases of that same problem.
http://www.plctalk.net/qanda/search.php?searchid=3729868

realolman said:

When i plugged the leds into the board they all lit up with no encoder or anything else plugged in. When the encoder was running and the system was functioning normally the leds flickered except for the 5 vdc one

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The pullup resistors inside the input module were driving the LED's. Won't affect your LED test setup.

realolman said:

What if it WAS TTL ? What if it was something else?

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The LED setup will work as you expected with TTL (AKA Quadrature). I assume that's what you have. The other type is differential. Differential (AKA Line Driver) probably wouldn't drive your LED's correctly, because it references one wire to the other wire, not ground.

Encoder manufacturers are competitive. Although internal electronics may differ, their outputs will be compatible. Therefore, you can use the schematic from another encoder to troubleshoot yours.
Typical TTL outputs use SN7414. See the second schematic on this page.

Thank you.