Re: control area network (CAN) bus speeds > 1Mbps possible?

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On Fri, 08 Aug 2003 14:31:41 +0100, Dave Hudson

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In fact the drop would be much more drastic than that due to the fixed
propagation times in the CAN controller and the CAN transceivers.

At lower speeds the distance drops to one half when the speed is
doubled. However the maximum distance for 500 kbit/s is 100 m, for 800
kbit/s  50 m and for 1 Mbit/s only 25 m (without optocouplers, with
optocouplers only 9 m).

The problem is that the total propagation time has to be counted twice
when processing the ACK bit. Thus, even if the CAN controller is
overclocked, the internal propagation delays remain the same. With
typical CAN controller delays of 50 to 62 ns, transceiver delays 120
to 250 ns and cable propagation delay 5 ns/m, you could not reach very
far even at 2 Mbit/s.

With 50 ns controller and 120 ns transceiver delay it would take at
least 340 ms back-to-back propagation delay even if the devices are
wired directly to each other and no optocouplers are used. This might
be just enough for a 500 ns bit time (2 Mbit/s) but I really don't see
how anything more than that could be used.

Paul
  

Re: control area network (CAN) bus speeds > 1Mbps possible?
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I'm unaware of any CAN controllers you can 'overclock' to do that kind
of thing.

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I think you'd be better trying Ethernet or USB now, or waiting for
FlexRay, which looks to be the 'real' answer... ;)

pete
--
snipped-for-privacy@fenelon.com "there's no room for enigmas in built-up areas" HMHB


Re: control area network (CAN) bus speeds > 1Mbps possible?

: > Even though we would deviate from the standard a bit, is there any way
: > to run a CAN network faster than 1Mbps?  like, 5Mbps?
: >
:
: I'm unaware of any CAN controllers you can 'overclock' to do that kind
: of thing.

If the BRP is adjusted it should be easy to overclock fcan, the frequency
of the ÁC is always far higher than at the CAN-controller. The only problem
with high baudrates is maximum propagation delay. By adjustig the Sample-Point
up of 87% maximum prop-seg is granted. The system should work although it
is not specified in the CAN-spec.

: > I really like CAN and its feature set, but our data rate requirements
: > are pushing us to other options.
:
: I think you'd be better trying Ethernet or USB now, or waiting for
: FlexRay, which looks to be the 'real' answer... ;)

That's not the answer. If you're looking for an arbitrating protocol there
will always the prop-delay-issue limiting it to 1 MBaud. I am in doubt if
FlexRay is an answer, TTP/C is the far better protocol with proven mathematical
models of it's security and message latency predictibility. The possibility
of using a higher baudrate only is not the main selling feature of this
protocol. If you're looking for a cheap bus for sensor applications CAN
will still be your choice.

Regards,
Taso




Re: control area network (CAN) bus speeds > 1Mbps possible?

: > > > I really like CAN and its feature set, but our data rate requirements
: > > > are pushing us to other options.
: > >
: > > I think you'd be better trying Ethernet or USB now, or waiting for
: > > FlexRay, which looks to be the 'real' answer... ;)
: >
: >
: >
: See http://www.flexray.com It does appear to be the next step from CAN,
: providing most of the same elements and adding a statically allocated
: message portion and using a different signalling layer so that can get to
: much higher message rates.  Unfortunately it doesn't exist as a useable
: product yet.

FlexRay is not a CAN-migration, it is a protocol evolving from Byteflight
and TTP/C. The reason why FlexRay is evolved are political feuds between
OEM. TTP/C is a project which started 20 years ago. It is the protocol
with a big research effort, it's aim is to provide a time-triggered data
protocol. FlexRay is a product developed in short time, they still have
problems with the protocol specification because they would violate patents
given to TTP/C. Both protocols will be introduced for time-triggered automotive
by-wire-applications, they are not senseful for cheap data busses as used
in various applications. Btw, TTP/C products are available for a long time
know! Only the physical layer is missing.


Re: control area network (CAN) bus speeds > 1Mbps possible?

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take a look to http://www.byteflight.com This is a CAN protocol
working with 12 bytes instead 8 using 10 Mbit speed. Look for Chips at
Motorola

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