CAN Bus, Star wiring and termination.

[Matt G]

I need to use CAN bus to send information to 12 remote displays from a
central control box. Each of the remote displays can be up to 50m away
from the control box and are connected back to the box with cat 5
cable. I'm looking for a data rate of around 125 kbps if this is
possible.

If we have this arangement the CAN bus wiring will effectivly be a
star centred at the control box. Is this acceptable if we terminate
the control box and each of the 12 displays with a 780R resistor to
give an overall impedance of 60R? OR as some other documentation says
just terminate the bus at the "hub" of the star with 60R?

Thanks

Matt

 

[Tim Wescott]

Rich Grise wrote:

On Thu, 02 Dec 2004 11:54:48 -0800, Matt G wrote:


I need to use CAN bus to send information to 12 remote displays from a
central control box. Each of the remote displays can be up to 50m away
from the control box and are connected back to the box with cat 5 cable.
I'm looking for a data rate of around 125 kbps if this is possible.

If we have this arangement the CAN bus wiring will effectivly be a star
centred at the control box. Is this acceptable if we terminate the control
box and each of the 12 displays with a 780R resistor to give an overall
impedance of 60R? OR as some other documentation says just terminate the
bus at the "hub" of the star with 60R?



Are you stuck with "CAN" bus? Isn't that for cars? Is this schoolwork?
Because what you've just described is a LAN. The last one of these I
worked on used ethernet. This is very common these days.

If you really are stuck with "CAN," and have no say in the matter, then
I'd say you'd probably have better luck looking up the spec than I would.
If you have the spec sheet and want to post a link, I can read it for you
and see if it answers your questions. Or any of the actual experts, of
course.

Good Luck!
Rich

Designed for cars, but not just for cars -- and when you find an 8-bit

micro with Ethernet for less than $3 let me know.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Tim Wescott]

Christoph Loew wrote:

Matt G wrote:

I need to use CAN bus to send information to 12 remote displays from a
central control box. Each of the remote displays can be up to 50m away
from the control box and are connected back to the box with cat 5
cable. I'm looking for a data rate of around 125 kbps if this is
possible.

If we have this arangement the CAN bus wiring will effectivly be a
star centred at the control box. Is this acceptable if we terminate
the control box and each of the 12 displays with a 780R resistor to
give an overall impedance of 60R? OR as some other documentation says
just terminate the bus at the "hub" of the star with 60R?

Thanks

Matt


The CAN bus is basically an line terminated with 120 Ohm on both ends,
stubs are limited to 50 cm IIRC. In your case the proposed star topology
is not applicable to the CAN bus; I'd recommend changing the topology to
a line (if you *have* to wire every display to the controller you'll
have to bridge some CAN lines so that the mechanical star becomes an
electrical line - a rather awkward 'solution') or change to a different
bus.

You might get away with disregarding the CAN bus standard for short
distances and low data rates but I'd advise against it.

If you *really* need a star topology as well stay within the CAN bus
specifications you'll probably need a seperate CAN interface for each
display...

I'm sure you've read it already but someone might be interested :
http://www.can.bosch.com/docu/can2spec.pdf

This AN covers some termination issues :
http://www.semiconductors.philips.com/acrobat_download/applicationnotes/AN10211_1.pdf

For the CAN bus's collision detection/priority arbitration scheme to
work a '0' bit transmitted by any one device on the bus has to be
detected before the bit interval is over, which means that any
transients on your cable need to have died down within about 1/2 to 2/3
of a bit time. 50m equates to 170ns in air, probably about 200ns on
cat-5 (what _is_ the velocity factor?).

I have no idea what your termination scheme is going to do to your
signal, but you could probably work it out mathematically or prototype
it fairly quickly -- I think you'll find that you're more or less OK for
the case where you're transmitting from the hub, but that any display on
the end of things is going to see a nasty transient, which may or may
not die down in time to make your data rate requirements.

If you can you may want to consider either 12 point-to-point links (in
which case why bother with CAN?), or having each wire run out to a
device and back, in which case (a) a single-point failure will take out
your whole bus and (b) you'll need two twisted pairs to each display.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Oliver Betz]

Rich Grise <rich@example.net> wrote:

[...]

Designed for cars, but not just for cars -- and when you find an 8-bit
micro with Ethernet for less than $3 let me know.

OK, fair enough. Do you know of an 8-bit micro that has a CAN bus?

Several Motorl^W Freescale 68HC(S)08 derivatives. BTW, CAN bus drivers
are also cheaper than Ethernet. And a powerful CANopen protocol stack
can be done with much less footprint than Ethernet.

Oliver
--
Oliver Betz, Muenchen (oliverbetz.de)

 

[Rich Grise]

On Sat, 04 Dec 2004 10:57:25 -0800, Tim Wescott wrote:

Microchip, Freescale, Cygnal, TI -- actually I think anyone who makes a
microprocessor makes one with CAN in it these days.

But they all assume that you have a copy of the spec from Bosche --
http://www.can.bosch.com/ -- so that's what you want to read. It's very
detailed, and quite lucid once you turn your brain upside down.

Thanks!
Rich

 

[Rich Grise]

On Sat, 04 Dec 2004 11:58:17 +0100, Oliver Betz wrote:

Several Motorl^W Freescale 68HC(S)08 derivatives. BTW, CAN bus drivers
are also cheaper than Ethernet. And a powerful CANopen protocol stack
can be done with much less footprint than Ethernet.

Thanks!
Rich

 

[Rich Grise]

On Sat, 04 Dec 2004 05:42:45 +0000, Barbarian wrote:

The Microchip PIC18F248 and related devices have CANbus hardware included.

Thanks!

Rich

 

[Oliver Betz]

I wrote:

[...]

are also cheaper than Ethernet. And a powerful CANopen protocol stack
can be done with much less footprint than Ethernet.

....a rather stupid comparison. I wanted to say that CANopen is a
powerful/versatile high level protocol with data abstraction etc. and
even simpler/smaller to implement than the plain data transfer part of
TCP(/IP) without any high level stuff.

Oliver
--
Oliver Betz, Muenchen (oliverbetz.de)