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frequency of data wires in a vehicle?


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trobins5 
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Posted: September 18, 2008 at 2:05 PM / IP Logged  
I am wanting to know the frequency of the data on the data wires in a vehicle. I am buying a portable scope and need to know if I need a 20, 40, or 60MHz scope. Thanks in advance!
n2ixk 
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Posted: September 18, 2008 at 8:07 PM / IP Logged  
As far as data communications go, an automotive databus is VERY low speed, a few MHz at most.
What will you be trying to measure? If all you want to see is if data is present on the bus, just about any scope will do. If you want to make critical measurements of pulse risetime, clock jitter, etc. You are going to need a good wideband scope.
trobins5 
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Posted: September 19, 2008 at 7:12 AM / IP Logged  
All I am wanting to see is if Data is on the wire. I do not need to measure the Data. If I see Data on the wire then I will move on until I find a wire without Data. At my place of employment, we install aftermarket electronic systems into vehicles and I do not want to tap into a Data wire. So, I would be safe by just buying the 20MHz scope? Thanks for the input!
KPierson 
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Posted: September 19, 2008 at 7:59 AM / IP Logged  

The 20mhz scope should work fine, but a multimeter will do the trick for much less.

With a decent meter you can measure either voltage or frequency of the signal you are attached to.  A data bus won't stay at 12vdc consistently and a data wire will show some sort of frequency while its communicating.

Kevin Pierson
trobins5 
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Posted: September 19, 2008 at 8:40 AM / IP Logged  
KPierson, I thought you couldn't see frequency on a multimeter?
megaman 
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Posted: September 19, 2008 at 9:07 AM / IP Logged  
you can't SEE the frequency on a DMM, but you can test the frequency. 
trobins5 
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Posted: September 19, 2008 at 9:12 AM / IP Logged  
How will I know with a multimeter if there is frequency present on a wire?
KPierson 
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Posted: September 19, 2008 at 3:51 PM / IP Logged  

Set the meter to frequency and watch it for a second.  Some Fluke meters tend to read 60hz even if there isn't a frequency present so watch out for that.

You won't be able to see the actual data, or read it in any way, and chances are the frequency won't ever stablize as the bit information will most likely be constantly changing resulting in different frequencies.

Like I said though, if you are looking for power wires in a vehicle just check to make sure you have battery power.  Data signals will usually read really low with a DC meter, around 2-3vdc (not 100% of the time, but mostly).  It should be impossible to accidently tag a 12vdc wire that is a data wire, IF you meter the wires first.

What exactly are you trying to do?  What signals do you need to find?  Why are you so paranoid about tapping in to a data wire?

Kevin Pierson
trobins5 
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Posted: September 19, 2008 at 5:59 PM / IP Logged  
I do not want to tap a Data wire and cause any sort of check engine light, TPMS light, or any malfunction to the vehicle. I am in the fleet environment and if I instruct my techs to tap a Data wire, we could screw up thousands of vehicles, not just one.
n2ixk 
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Posted: September 19, 2008 at 6:27 PM / IP Logged  
Sounds like what you really need is a wiring diagram, not an oscilloscope...
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