Hi,
On Thu, 9 Jan 1997 9:12:31 -0500 (EST) [email protected] wrote:
> We have been looking at fieldbus's to replace the BITBUS for
> new projects at Bates. Both PROFIBUS and CAN look promising.
> Can someone comment on the advantages or disadvantages of PROFIBUS
> verses CAN?
>
> Thanks,
>
> Coles Sibley
We looked at various field bus options for use with EPICS on the Gemini project
a couple of years ago. Profibus and CAN were two of the options we considered
most seriously. We decided on CAN because it suited our application better.
CAN is ideal for fast, reliable, exchange of control data. CAN is probably the
most rugged of the field busses. It incorporates prioritisation of messages
using a CSMA Collision detection/Collision avoidance scheme. It can be used in a
deterministic way if required. A CAN message can contain up to 8 data bytes.
2032 message identifiers are allowed on a CAN bus. The maximum bit rate is 1
Mb/s. Most of the data layer (OSI layer 2) is provided in the hardware of a CAN
control chip, making the software interface relatively simple. The CAN standard
doesn't include an application layer (layer 7), however there are now several
standards that provide this, such as Device Net (Allen Bradley), SDS (Honeywell)
and CAL (CiA). Having said this, we have written our own, simple, application
layer since we did not have the resources to implement one of the standards.
CAN controller chips are available as separate chips or integrated into
microcontrollers such as the 8051 (Phillips, Siemens), 6805 (Mot.) and a host of
others. We use a 16 bit microcontroller with an integrated CAN controller, the
Siemens C167. Although about $20 a time more expensive than an 8051 derivative,
we figure its cheaper in the long run because the programmer doesn?t have to
spend time optimising and shoehorning code to get it to run and fit in an 8 bit
device.
Many CAN products are now available at board level, these generally have one of
the layer 7 interfaces.
I don't know so much about Profibus because we didn't use it, so the following
may be a little hazy.
Profibus seems better suited to applications where larger amounts of data are
being transferred (~250? bytes per message). It can be used to transfer smaller
amounts of data of course, but the overhead per message is much greater than
CAN. The maximum bit rate is 500kb/s?. All the Profibus protocol is handled in
software. You can purchase Profibus libraries from equipment vendors (this is
also true for CAN). I believe there is a runtime licence required for each
Profibus node, none is required for CAN.
There are at least two EPICS/CAN implementations available. Ours is a
'lightweight' interface that uses 'dumb' (i.e. cheap), industry pack carriers
and CAN industry packs, giving up to 4 CAN buses from a 6U VME card. It should
also run on a MVME162 + IP's.
I believe DESY also have a EPICS/CAN interface working, running through their
Common Driver Interface.
DESY were also planning a Profibus interface which I guess would be working by
now.
Our experience with CAN has been very good. The only sticky bit is getting the
bit rates, bit sampling times, bus termination etc. right. Once the bus is
working it tends to work with no problems. We do have a bus analyser which
allows us to examine network traffic and has been invaluable.
I hope this doesn't sound too much like a CAN advertisement!
Cheers,
John.
_____________________________________________________________________
John Maclean
Royal Greenwich Observatory,
Madingley Road, Cambridge, CB3 0EZ, UK.
Phone: +44 (0)1223 374814 Fax: +44 (0)1223 374700
email: [email protected]
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