RE: different cables in the same network

Tamas,
That was the same issue my client was discussing.  He had the bus cable and
most taps off were quite short and he had at least 14" between taps to
follow the tests Steve Corrigan at TI did a few years ago.  But then there
was that one RM1001 display module hanging as the last tapped item on the
bus with a 2m long cable into the cab of the vehicle.  Termination was after
that tap on the main cable.  Ideally the right way to do this is to extend
the bus cable into the cab of the vehicle and then shorten the RM1001 cable
to be under 30cm.  But apparently that wasn't possible due to some sort of
large panel connector issue.  Or so I was told.  I'll probably find that it
could have been done correctly but would have been too much work.

It's like dealing with computer programmers (I'm one of them).  First
reaction from them is "It can't be done" or "It will take a really long
time".  Which is code for "I've not figured out how to do this and seems
like a lot of work so I'd rather not do this".

A friend of mine (also a programmer) once said that to me after I wanted a
particular solution to a problem.  I went away and came back with a one page
function that did exactly what I wanted to show him that it could be done
and wasn't that hard.   His reaction to that was to do a quick look at my
one page of code function and then turn it into a 1/2 page of code function.
Far more efficient and elegant.  After that I never believed his "Can't be
done statements".  

John Dammeyer

> -----Original Message-----
> From: [email protected] [mailto:canlist-
> [email protected]] On Behalf Of Hadarits Kft.
> Sent: November-23-14 12:54 PM
> To: [email protected]
> Subject: RE: [CANLIST] different cables in the same network
> 
> Dear John,
> 
> Thanks for your detailed answer. As I know at the J1939 network the max.
> allowed bus lenght is 40m, so I hope our newly installed plus 4m stub
lenght
> will remain within its tolerance. But I'm afraid this newly installed stub
> (without termination resistor) will be longer than the existing FMS CAN
> network (with the two 120ohm resistors). As you adviced I will check it
with
> scope.
> 
> Regards. Tamas
> 
> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]] On Behalf Of John
> Dammeyer
> Sent: Sunday, November 23, 2014 6:44 PM
> To: [email protected]
> Subject: RE: [CANLIST] different cables in the same network
> 
> The short answer is yes.  You can mix it.  I just has a similar discussion
> with a client yesterday.
> There's nothing magic about CAN bus wiring. Although CAN isn't a truly
> differential signal, since it's referenced to ground, the principles are
> exactly the same.  A signal change somewhere on the bus propagates in
> both
> directions.  When it reaches the end of the bus if there isn't any
> termination or in other words if it hits a really high impedance it
> reflects.  Like throwing something at a wall.  It bounces.   A change in
> cable impedance by changing cable types can also do the same thing but to
a
> lesser degree.
> The reflection which, is lowered in signal level goes back down the bus
> until it hits the other end and then bounces back. Each time with less
> energy.  On the way back it runs into the other reflection and either sum
or
> subtract creating a larger or small signal.  As long as that point isn't
at
> a tap for a CAN device it doesn't matter.
> Only if there's a CAN device at that point does it become a problem.  If
> this signal happens to coincide with a new bit in the same message or the
> next message and it's strong enough to change the signal then you get a
bit
> error in the message.  CAN handles this by retransmitting the message so
> it's transient.    And the next message may have a different value in the
> data or CRC part and the reflections do no harm.  So the errors are very
> transient and incredibly hard to track down.
> That's why we have termination resistors at each end.  These are matched
to
> the impedance of the cable.  You can use 45 Ohm cable if you want and have
> 22.5Ohm resistors but that pushes the drive capability of the CAN
interface
> so people stay with 100 to 120 ohm and live with minor reflections.
> The faster the bit rate and the longer the cable the more likely breaking
> the rules will result in peculiar errors.
> So the advice is adhere as best as possible to standard engineering
> practices but if you can't at least scope the bus and look at the signal.
> CAN is pretty robust.  Odds are switching cable types won't make a
> difference if the network is short.
> John Dammeyer
> 
> > -----Original Message-----
> > From: [email protected] [mailto:canlist-
> > [email protected]] On Behalf Of Hadarits Kft.
> > Sent: November-23-14 9:05 AM
> > To: [email protected]
> > Subject: [CANLIST] different cables in the same network
> >
> > Hi all,
> >
> > we deal with installation of fleet management devices into trucks. One
> > of our latest work was to make connection between the fleet
> management
> > device and the digital tachograph to reach the possibility to download
> > the data from the tachograph via CAN network. In this case the fleet
> > management device was already connected to the truck's FMS CAN
> network
> > and we had to connect to this network the tachograph too: FMS
> > gateway+fleet management
> > device+ tachograph. I hope it is understandable till now.
> > In the trucks I found only unshielded twisted pair wires for CAN
purposes.
> > Now we got from the fleet management device's producer shielded
> > twisted pair wires to make the connection between the fleet
> management
> > device and the tachograph (the required wire lenght is appr. 4-5m).
> >
> > My question: is it possible to use shielded and unshielded wires
> > together
> in
> > one network?
> >
> > Thanks in advance: Tamas Hadarits
> >
> > www.tachografszerviz.hu
> >
> > --
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> > problems to <[email protected]>
> 
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