frequency of data wires in a vehicle?
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Forum Name: General Discussion
Forum Discription: General Mobile Electronics Questions and Answers
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=107534
Printed Date: October 31, 2024 at 6:20 PM
Topic: frequency of data wires in a vehicle?
Posted By: trobins5
Subject: frequency of data wires in a vehicle?
Date Posted: September 18, 2008 at 2:05 PM
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!
Replies:
Posted By: n2ixk
Date Posted: September 18, 2008 at 8:07 PM
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.
Posted By: trobins5
Date Posted: September 19, 2008 at 7:12 AM
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!
Posted By: KPierson
Date Posted: September 19, 2008 at 7:59 AM
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
Posted By: trobins5
Date Posted: September 19, 2008 at 8:40 AM
KPierson, I thought you couldn't see frequency on a multimeter?
Posted By: megaman
Date Posted: September 19, 2008 at 9:07 AM
you can't SEE the frequency on a DMM, but you can test the frequency.
Posted By: trobins5
Date Posted: September 19, 2008 at 9:12 AM
How will I know with a multimeter if there is frequency present on a wire?
Posted By: KPierson
Date Posted: September 19, 2008 at 3:51 PM
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
Posted By: trobins5
Date Posted: September 19, 2008 at 5:59 PM
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.
Posted By: n2ixk
Date Posted: September 19, 2008 at 6:27 PM
Sounds like what you really need is a wiring diagram, not an oscilloscope...
Posted By: trobins5
Date Posted: September 19, 2008 at 7:34 PM
There is no wiring diagrams when you receive the vehicle before the dealers do.
Posted By: KPierson
Date Posted: September 19, 2008 at 9:13 PM
So what kind of signals are you looking for???? Data wires are typically smaller then power wires. I don't know exactly what you are trying to do, but there are "other" easier ways to tell if you have the correct wire. The only time I've ever need an o'scope on a car is to actually decode the data being sent. ------------- Kevin Pierson
Posted By: trobins5
Date Posted: September 20, 2008 at 9:52 AM
I am not looking for a data wire at all. I am looking for a constant and switched wire that do not have any data traveling on them. I have to be 100% sure there is no data on the wires I instruct my techs to use.
Posted By: megaman
Date Posted: September 20, 2008 at 1:37 PM
If your wire shows +12v DC constant or +12v DC switched, there's no data. period. They do not multiplex data on 12v DC wires. Computer data systems on cars are based on 5v format. Shove 12v DC onto a data wire and goodnight Irene. So, if you test the wire and it' shows 12v DC, you are fine. When the wire is multiplexed to control other features in the cars, such as Mopar lights and door locks, you'll see three different DC votages depending on what you are trying to control. Bottom line is that you will not see +12V DC on a data wire.
Posted By: KPierson
Date Posted: September 20, 2008 at 2:42 PM
Yeah, like I said if a wire has battery voltage on it constantly it can't have data on it. Throw a volt meter on it and watch it for 30 or even 60 seconds - if it stays constant then you are good to go. Another good rule of thumb is to never use any wire smaller then 18awg wire for power - that right there should eliminate your issue as most data wires are 20awg or smaller. Also, if you just need switched and ignition I would always look for the steering column wires or the ECU power wires - in both cases the wires will be larger then most other wires in the area. ------------- Kevin Pierson
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