May somebody knows the main applicational differnces between CAN and Profibus Fielbuses? especially from the view point of a process engineer who is deciding to select one of them for an industrial automation purpose.

 

This is not a completely straightforward question, as there are a couple of CAN based protocols in wide use (DeviceNet and CanOpen). However the following notes may help:

1) Profibus DP is essentially a master/slave protocol. It essentially uses memory exchanges between master and slaves to very efficiently transfer input and output values, and has a large packet size (around 240 bytes maximum I and O per slave, though 16 or 32 bytes is more typical). It's very easy to set up and maintain, largely since slaves tend to be based on a particular ASIC (SPC3) and tend to work in very similar ways. It's an excellent choice for an extended PLC backplane where there is a central PLC or controller and distributed devices, but the drawback is that the master part of the system tends to be a little expensive and there's no peer to peer comms between slaves (there are some extensions to DP, DPV3, which allow 'peer to peer', but in fact this can't be done without a master in the system. Although identified with Siemens, in fact there are masses of third party devices available for DP, which is a great plus. The raw speed 1.5M or 12M looks good too, though because the protocol isn't particularly smart, I don't think it has particular throughput advantages over the competing networks.

2) DeviceNet, though promoted as a peer to peer network on CAN is more often Master/Slave, since most slaves are what are known as group 3 only servers. Because of this, and because the packet size is small (8 bytes), it is a pretty limited and limiting network. It will work well in particular contexts (small machines), but for more complex set-ups requiring very high speed, bandwidth is limited. This is somewhat compensated by smarts in the protocol to optimise throughput. Because there are a number of implementations available, and people can and do roll their own, there are inconsistencies between the way even similar types of device work, and the network setup and management is therefore more fraught than Profibus. The reason for choosing it is really in circumstances where you need to connect third party equipment to AB networks (I think in proprietary AB systems, DH or DH+ is more often used; this is certainly the story I have heard though it would be interesting to confirm this). I don't think I'd choose to use it where I had DP available and wasn't using AB kit.

3) CANOpen uses the underlying CAN protocol properly, i.e. allowing true peer to peer comms. I don't think it's quite as well established as the first two protocols above, but support is growing, particularly in countries that feel the need for an anti Siemens backlash. However there are a good number of devices available, which are well defined and have a good interoperability strategy, and it's worth a close look.

Cheers

Tim Linnell (Eurotherm, though speaking for himself)

 

CAN is faster and used for short distances. Profibus is slower and used for longer distances.

 

Profibus is a complete specification for both the network and the user-level interfaces. It is widely supported by devices and software from many suppliers. Profibus allows master/slave communications using a data frame up to 246 bytes. For process industry use, Profibus-PA uses an intrinsically safe wiring plan common with Foundation Fieldbus that also supplies DC electrical power to process field instrumentation.

CAN is a specification for only the protocol of a simple communications chip. The physical network and the user interface are not defined. DeviceNet, SDS, CANopen, and CAN Kingdom are all different and non-interoperable specifications for CAN implementations. CAN allows a maximum data frame of 8 bytes.

Dick Caro
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CMC Associates
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One of the main differences is CAN is limited to 1Mbps and PROFIBUS can go up to 12Mbps Also the CAN frame is only max of 8 bytes (could be more with segmentation). Profibus frame is 244 bytes. The CAN transmitter broadcasts to all nodes. The Profibus Master talks only to its slaves when it needs to. CAN max distance is 1Km @ 50Kbps. Pofibus is 1Km @ 9.6Kbps with wire and expandable with repeaters to 10Km. With Mono Glass Fiber you can go 15Km on one segment.

Check this link for further comparisons http://www.synergetic.com/compare.htm

 

Check here. There is information about most fiedbusses and links to sites. http://ourworld.cs.com/rahulsebos/

 

In fact CAN is slower (1mbaud maximum, 500K typical) and used for relatively short distances. Profibus is faster (1.5 or 12 meg), with distance falling off with speed (I think 50m for 12mbaud). So Ivo's statement is more or less back to front.

 

Bad advice! CAN has a maximum speed of 1 Mbps bps for short bus lengths. Profibus-DP has a maximum speed of 12 Mbps for bus runs of about 1500m. Profibus-PA and Foundation Fieldbus H1 both operate at 31,250 bps for line lengths up to 1500m, and support intrinsic safety and DC power delivery. CAN available as DeviceNet supports power delivery using a 4-wire cable, but not intrinsic safety.

Dick Caro
============================================
Richard H. Caro, CEO
CMC Associates
2 Beth Circle, Acton, MA 01720
Tel: +1.978.635.9449 Mobile: +1.978.764.4728
Fax: +1.978.246.1270
E-mail: [email protected]
Web: http://www.CMC.us

 

Can someone explain to me how 1Mbps (CAN)is faster than 12Mbps (Profibus)?

 

You can not only compare by physical bus speed! Both are using very different communication principles. While Profibus is a Master/Slave bus, mostly used for cyclic data transfer, CAN is multi Master and used for event driven communication. Because of its simplicity and the flexibility higher layer protocols like CANopen provide, it is more used in machine automation.

Regards Heinz +==================================================================| Heinz-Jürgen Oertel port GmbH http://www.port.de | mailto:[email protected] | phone +49 345 77755-0 fax +49 345 77755-20 | Regensburger Str. 7b, D-06132 Halle/Saale, Germany | CAN Wiki http://www.port.de/cgi-bin/CAN | Newsletter: http://www.port.de/engl/company/content/abo_form.html +=================================================================>

 

Almost correct. Its 100m @ 12mbps

 

Just one minor correction. Profibus DP can go a max of 1000m @ 12mbps on copper with repeaters. Glass mono fiber you can achieve distances of 15KM.

 

More used where in machine automation? Certainly not in Germany? Yes CanOpen is more used in your automobiles but its doubtful that it has more nodes than Profibus in Germany or for that matter Europe in the machine automation world. If it does its only because it has been around longer.

 

On August 3, 2003, Heinz wrote:
>You can not only compare by physical bus speed! Both are using very
>different communication principles. While Profibus is a Master/Slave bus,
>mostly used for cyclic data transfer, CAN is multi Master and used for
>event driven communication.

CAN is not only used for multi master schemes... e.g. CANopen and DeviceNet are using a (polling) master/slave protocol. DPV1 and DPV2 of PROFIBUS are also event driven protocols...

>Because of its simplicity and the flexibility higher layer protocols like
>CANopen provide, it is more used in machine automation.

If you need speed and data volume... 8 bytes/ per frame at 1Mb/s offers in a lot of cases not the neccessary transmission throughput. PROFIBUS-DP has more performance with 244 bytes per packet at 12Mb/s... yes, CAN and PROFIBUS-DP are comparable

Best Regards

Armin Steinhoff

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Armin Steinhoff <[email protected]>
STEINHOFF Automation & Fieldbus-Systems
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TEL +49-6431-529366 http://www.dachs.info
FAX +49-6431- 57454 http://www.steinhoff-automation.com

 

Ron,

Of course is CANopen used in Germany, mostly in Germany and Europe and it starts gaining shares in the US as well. It is not often used in automobiles, and when used in cars, these are mostly special kinds of trucks or so. Please look at <http://www.can-cia.de/cleanopen/> for usage of CANopen in Municipal Vehicles. This market share is increasing too. And yes, may be it has not the same amount of installed nodes as Profibus, but this is because Profibus is longer in use than CAN based automation hardware (not CAN in cars). For Application fields I quote from CiA: : CANopen is used in a variety of applications today, : not just in industrial control systems. : CANopen networks are also used in off-road vehicles, : maritime electronics, medical equipment, and railways, etc. : The very flexible application layer : and many optional features are well suited for designing tailored networks. : <http://www.can-cia.de/canopen/application/> According communication speed:

> Can someone explain to me how 1Mbps (CAN)is faster than 12Mbps (Profibus)?

Even compared with 12 Mbit/s Profibus, CAN can be faster communication from one node to another, without using a masternode to do so, than Profibus. A high Priority message can be sent immediately, gains the bus and can received by all other nodes. This way CAN can communicate a certain type of information very fast to all other nodes.

with best regards / mit freundlichen Grüßen

Heinz-Jürgen Oertel +==================================================================| Heinz-Jürgen Oertel port GmbH http://www.port.de | mailto:[email protected] | phone +49 345 77755-0 fax +49 345 77755-20 | Regensburger Str. 7b, D-06132 Halle/Saale, Germany | CAN Wiki http://www.port.de/cgi-bin/CAN | Newsletter: http://www.port.de/engl/company/content/abo_form.html +=================================================================

 

> CAN is not only used for multi master schemes... e.g. CANopen and
> DeviceNet are using a (polling) master/slave protocol.
> DPV1 and DPV2 of PROFIBUS are also event driven protocols...

This is definitely not true for CANopen.
CANopen PDO (process data object) communication is not at all a master slave
protocoll.

--

with best regards / mit freundlichen Grüßen

Heinz-Jürgen Oertel

 

I'm impressed with the "my bus is bigger than your bus" arguments, guys. As with most automation technology, both have valid applications and published specs, which we can recite to each other all day.

Get Back to Work!