RE: different cables in the same network

CAN is pretty robust, and the great majority of FMS networks as implemented
by the OEMs are nowhere near the limits of CAN. I wouldn't worry too much
about the type of cabling, let alone the stub length one particular node as
long as it's not measured in the tens of meters. I'm pretty sure that you
could connect most FMS nodes using a pair of metal coat hangers, and still
have it work.

And having seen a lot of installations of telematics equipment in HGVs, I
would even think that the metal coat hangers wouldn't be that bad a choice
in comparison with what's already out there ;-)

Cheers,
Bram
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Hadarits
Kft.
Sent: zondag 23 november 2014 21:54
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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