Multiplexing / CAN

ACTIVE8 · Post by **ACTIVE8** » 23 May 2006, 13:34

I agree entirely, twisted data cables are used, because it's the cheapest way to do it, but it's not the best way to do it.

Cars are now built it seems more by the accountants, than by the engineers that should be more in control of the project.

miked · Post by **miked** » 23 May 2006, 15:40

Because the CAN signal is a single differential signal on a pair of wires (not seperate data and clock wires as someone suggested above) in my opinion a twisted pair cable is more suited that a coax cable. A coax cable is more suitable in cases where the two conducters are a single data conductor and a ground, in which case the ground conductor would act as a shield around the data wire to prevent noise. If CAN were to use coax, the outer conductor could not be tied to ground and would pick up noise.

Kowalski · Post by **Kowalski** » 23 May 2006, 16:44

If you think twisted pair is used for cheapness rather than other reasons, you clearly haven't heard of STP, that is shielded twisted pair.

http://en.wikipedia.org/wiki/Shielded_Twisted_Pair

Twisted pair uses something called "differential signaling" in essence in means you send a positive signal down one wire and a negative signal down the other. The "differential" part means you are only ever interested in the difference between the two signals. If your twisted pair receives some interference, it will either have a voltage spike or a voltage trough induced in it, with twisted pair both pairs receive the same interference, so when you compare the two at the end, the difference is unaffected.

If you have screening on a cable (e.g. coax) it serves the purpose of stopping intereference from getting onto your data lines inside, twisted pair and differential signalling allows you to cope with interference. For screening to be effective you need a good earth, cars tend to be electrically noisy and the electrically noisy systems share the same earth as your sensitive data systems so coping with electrical noise is important, trying to keep electrical noise out of your systems is probably close to impossible.

wheeler · Post by **wheeler** » 23 May 2006, 18:19

Kowalski wrote:
Twisted pair uses something called "differential signaling" in essence in means you send a positive signal down one wire and a negative signal down the other. The "differential" part means you are only ever interested in the difference between the two signals. If your twisted pair receives some interference, it will either have a voltage spike or a voltage trough induced in it, with twisted pair both pairs receive the same interference, so when you compare the two at the end, the difference is unaffected.

the CAN high & low signals are mirror images of each other for that reason that you said

AndersDK · Post by **AndersDK** » 23 May 2006, 18:28

Been working with electronics since late 60'es - there is no gain in "quality" using screened cabling for twisted pair signalling.
Twisted pair signalling means you have 2 leads each carrying the exact same signal - execpt the one is in anti-phase with the other.

This means that the signal data is the differential between the 2 leads - not the difference between ground and each of the leads.
The huge advantage of such methods signalling data is that any noise will be common to both of 2 leads - related to ground. I.e. you will always find exactly the same noise signal on both leads related to ground. This is called common mode signals.
As the data mode differential - the nosie is cancelled out.

In fact there are large riscs that a screening, earthed shield will induce even more common mode noise levels - which can in fact make the receiving circuits go beyond their common mode noise levels - in which case you get a "blackout" of data - not directly a data error.

sub42 · Post by **sub42** » 23 May 2006, 20:45

pugxpert wrote:data wires are useally twisted together to stop interference

I have always twisted wires together in an in-car auxilliary harness. The man from Clifford taught me that in 1996. He just took all the cable pairs, shoved them into the end of his new-fangled 6 Volt cordless, and twisted them up. I have copied that every since. EMF reduction, and a reduction in falsies he claimed......