Buss Contention and Buss interference

Discussion in 'General Electronics Chat' started by SamEricson, May 1, 2015.

  1. SamEricson

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    Apr 25, 2015
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    When troubleshooting buss contention and buss interface problems. You put +VCC on Oscope ch1 and on Oscope ch2 you put it on the buss line you think is causes the buss contention or buss interference problem? If there is a problem the +VCC will fluctuate or start to fluctuate in time. How else do you solve these type of problems?
     
  2. #12

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  3. SamEricson

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    Apr 25, 2015
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    Buss contention is when more than one device tries to drive data line at the same time.If one device is trying to drive it high and other tries to drive low then, we call it bus contention.

    I'm talking about troubleshooting logic boards that is causing Bus Contention errors and problem from either a damaged logic chip that is shorted or how the traces or sockets are causing interface or coupling issues.

    When using either an Oscope or Logic Analyzer, you put +VCC 5 volts on Channel#1 and then monitor the rest of the logic signals that is causing the logic VCC +5 volts to that glitches, random drops, fluctuates or start to fluctuate in time from heat , etc.

    What type of troubleshooting is this called? i thought it was called buss contention troubleshooting
     
  4. #12

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    You are right.
     
  5. SamEricson

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    Apr 25, 2015
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    How do you guys troubleshoot buss contention or buss interference problems and errors?
     
  6. #12

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    We start with the schematic of what we are working on and examine the voltages on the outputs and power busses.
     
  7. WBahn

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    You'll only see that if the contention places a sufficient load on the devices to pull down the supply. It is far from guaranteed that that will happen.
     
  8. WBahn

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    You also have contention when two devices are trying to put control a wired-AND line by actively pulling it down to assert a LO and allowing it to be passively pulled up to assert a HI. But in this case monitoring Vcc won't tell you a damn thing. Some people call this a bus conflict or a bus collision instead of bus contention, but plenty of people call it bus contention, too.
     
  9. SamEricson

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    Apr 25, 2015
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    But the main focus is you're comparing the power busses to the output voltage or other busses to see if anything is pulling or interfering with the power busses right? this is called buss contention

    Also buss contention can be when logic signal look like stepping ramps or wave shaped with steps because of voltage deviations. This happens when logic chips are damaged or shorted that are "tied" to a buss causing the logic signals to be modified with deviations or steps.
     
  10. SamEricson

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    Apr 25, 2015
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    Yes, if you have circuit boards when these types of problems, how do you troubleshoot them? pull each IC chip one by one that are "tied" to that buss line until the buss line is back to normal?
     
  11. WBahn

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    That may or may not be a viable option. Are you really going to pull a multi-hundred pin BGA package off a board just to see if the bus line goes back to normal?
     
  12. SamEricson

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    Apr 25, 2015
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    I would pull everything else before pulling a BGA package. But mostly it's show the BGA was soldered or heated which is most of the problems now of days.

    I'm talking about TTL and CMOS circuit in the 70's and 80's that have buss contention problems.
     
  13. dl324

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    I'd use an HP547A Current Tracer, possibly in combination with an HP546A Logic Pulser. Google it before you ask a ton of questions.
     
  14. SamEricson

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    Apr 25, 2015
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    Yes, I have a BK logic pulser

    How would you use the logic pulser and current tracer? to troubleshooting circuit boards with this type of problems?

    You will print out the datasheets for each IC chip and then do what please?
     
  15. dl324

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    I asked you to Google HP547A before firing off a lot of questions that you could answer on your own with a little work.

    The HP547A allows you to trace current back to the gates that are in contention for the bus. Assuming you know which one should have control, the other one is the culprit.

    Search for HP547A and you'll find a link for an HP Journal from 1976 that describes how to use the probe and how it works. It also covers how to use it in conjunction with the HP546A logic pulser to troubleshoot stuck at issues. I don't know if your BK logic pulser has the power or intelligence to substitute for the HP pulser.
     
  16. SamEricson

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    Apr 25, 2015
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    I know, I was just asking how did you use these in your experience to know how to use them better
    examples of using them would help me
     
  17. SamEricson

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    Apr 25, 2015
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  18. #12

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    This is the first time the TS has named any piece of equipment, working with or working on, since he joined, 104 posts ago.
     
  19. SamEricson

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    Apr 25, 2015
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    The HP pulser does have a feature that you can either pulse or "force" the logic state. New pulsers doesn't have the forcing high or low state because of overloading the output stages of TTL and CMOS logic chips. I'm not sure if this is turn or not but all the newer pulsars don't have this feature anymore.
     
  20. dl324

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    If you have a bus contention fault, you use the current tracer to trace back to the drivers with high currents. Only one of them should be trying to drive the bus, so the others are at fault. Either the at-fault driver is stuck or something upstream is defective.
     
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