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Difference between MODBUS and PROFIBUS
Communications systems and equipment. topic
Posted by Anonymous on 12 January, 2009 - 5:52 am
Hi,

Can anyone explain, what is the difference between MODBUS and PROFIBUS. I think they are communication protocols but what is the basic difference.

Regards


Posted by M Griffin on 12 January, 2009 - 10:59 pm
They are two different communications protocols. If you want a rough analogy, it's like two different languages. Profibus is speaking German and Modbus is speaking English. Both languages work, but the two speakers can't communicate.

Profibus has certain protocol features that let certain versions of it operate in multi-master mode on RS-485, while Modbus could be only single master. However, Modbus can operate on Ethernet (including multiple masters) while Profibus can't (Profinet can, but Profinet is not the same as Profibus).

In short, Profibus was good in its day, but its specialised features tied it closely to RS-485, which has resulted in it becoming obsolete (although there's lots of it still around).

Modbus is still going strong because its simplicity let it evolve to adapt to Ethernet without significant change. It's now one of the major Ethernet protocols used in automation. The older serial (RS-232 and RS-485) versions are still around as well.


Posted by Anonymous on 13 January, 2009 - 11:58 pm
Thanks Griffin

Nice comparison.

Can you also explain what is Field bus protocols.



Posted by M Griffin on 14 January, 2009 - 12:30 pm
There are several different things which are sometimes called "Fieldbus". In one sense it is a generic name used to describe any industrial network that connects field devices. As a generic description it was fashionable at one point, but seems to have fallen out of use lately.

It is also sometimes used as the short form of a particular protocol called "Foundation Fieldbus". This is just another protocol like Profibus or Modbus, but it has features which are intended specifically for process industries.

It was also sometimes used as the short form for a failed IEC effort at standardising industrial networks. That standard is pretty much forgotten now, as all it said was that any of the proprietary protocols could call itself "standard".


Posted by Steinhoff on 14 January, 2009 - 3:03 pm
Yes, there is a bunch of fieldbus systems standardizised by the IEC 61158. Each of them are standardizised, open specified and are not proprietary protocols.

The process to get part of the IEC 61158 standard is a real hard ...
there is no simple "call itself standard".

BTW ... here is a short description what a fieldbus is:

Industrial data communication by serial bus systems is the core of every distributed control and IO system. *Serial bus systems designed for real-time data communication are called fieldbuses*. Fieldbuses are industrial networks for deterministic real-time data communication between bus masters, distributed actors and sensors.They are designed for low latency and low transmission volumes. The packet sizes of fieldbuses are in general in the range of 1 to 200-300 bytes.

The transmission speed can be choosen in a wide range from e.g. 12Mbit/s down to 1200bit/s. This flexibility of transmission speed is a big advantage against Ethernet based networks, e.g. The useable segment length depends on the transmission speed, transmission media, and the topology of the individual fieldbus.

In general, high transmission speed decreases the segment length of bus oriented fieldbuses to a length of 50 to 100 meters; low transmission speed allows a segment length in the range of a few kilometers. Fieldbuses with a *ring topology* like INTERBUS or EtherCAT are working often with a physical extension in the range of kilometers because of the signal refreshing which is done by the fieldbus I/O modules. Fiber optical transmission medias allow in general extended segment lengths and increased noise immunity.

Best Regards
Armin Steinhoff


Posted by Curt Wuollet on 15 January, 2009 - 12:09 am
Yes, that's an example of trying to standardize without giving up anything. So now they have complete control of nothing much.

Regards

cww


Posted by Steinhoff on 14 January, 2009 - 2:44 pm
<M Griffin: They are two different communications protocols. If you want a rough analogy, it's like two different languages. Profibus is speaking German and Modbus is speaking English. Both languages work, but the two speakers can't communicate.>

Yes, that's a real rough analogy.

<M Griffin: <Profibus has certain protocol features that let certain versions of it operate in multi-master mode on RS-485, while Modbus could be only single master. However, Modbus can operate on Ethernet (including multiple masters) while Profibus can't>

Profibus is a real sensor/actor fieldbus. It can operate at different transmission speed from 1.2 kb/s to 12Mbs und is highly optimized for speed. BTW, a fieldbus is a technical term for serial buses used for industrial communication .... like USB(universal serial bus), but it interconnects industrial devices und not multimedia devices.

<M Griffin: (Profinet can, but Profinet is not the same as Profibus).
In short, Profibus was good in its day but its specialised features tied it closely to RS-485, which has resulted in it becoming obsolete (although there's lots of it still around)>

The use of Profibus is still increasing and it will have a lot of
additional good days :)

Specialised sensor/actor fieldbuses like Profibus have their place in
the industrial automation today and in the future.

<M Griffin: Modbus is still going strong because its simplicity let it evolve to adapt to Ethernet without significant change. It's now one of the major Ethernet protocols used in automation. The older serial (RS-232 and RS-485) versions are still around as well.>

The concept of Modbus is simply outdated. Look to the newer ones:
EthernetPowerlink, EtherCAT, EthernetIP, SyncNet, ProfiNet a.s.o

RS-232 doesn't provide a physical media for serial BUS systems .....

Best Regards

Armin Steinhoff
http://www.steinhoff-automation.com



Posted by M Griffin on 15 January, 2009 - 12:11 am
In reply to Armin Steinhoff: You said: "The concept of Modbus is simply outdated. Look to the newer ones: EthernetPowerlink, EtherCAT, EthernetIP, SyncNet, ProfiNet".

Would you care to elaborate on that? EthernetPowerlink and SyncNet appear to be specialised motion control networks. They're intended to replace things like Sercos, not Modbus. EtherCAT is another specialised network, aimed at replacing things like CANOpen.

EthernetIP and ProfiNet aren't so much protocols as they are brand names for collections of protocols intended to address a variety of niches. They're not exactly new either. EthernetIP has features to handle some of AB's legacy products, while ProfiNet handles drive synchronisation (which Modbus doesn't) as well as I/O.

What does seems to be happening to the market is that companies are coming out with Ethernet based products to replace their older systems which relied on RS-485 and other similar media.

As for Modbus, it's a basic general automation protocol. It doesn't do multi-axis motion control over a network, but that's not something that most people want to do with it anyway. What it offers is that its an open protocol that is easy to implement, which means that is has been widely adopted.

As for RS-232, yes it is point to point, but the question was about protocols, not media. Any protocol is point to point on RS-232. Protocols that can use RS-485 or Ethernet are "bus" (network is a better term) protocols. Of the protocols mentioned, Modbus and Profinet can operate on RS-485, and all of the ones mentioned except Profibus can operate on Ethernet.

As for whether Profibus is obsolete, Siemens was telling everyone a while ago (6 - 8 years ago if I recall) that Profinet was supposed to replace Profibus for field devices. I must admit though that I haven't seen much sign of that happening. I haven't seen anyone using Profinet, while Profibus is still being installed on new equipment today. If you look on the ODVA (EthernetIP) or Profinet web sites, either lists only a couple of compatible PLCs.


Posted by Steinhoff on 16 January, 2009 - 6:18 am
Hello,

This is a reply to the recent posting of M. Griffin.

Ethernet Powerlink (EPL) and EtherCAT are *general purpose* fieldbuses, which are also (like others) used for motion control. EPL and EtherCAT doesn't replace CANopen... they are using the CANopen protocol on top of their Ethernet media, that means CANopen has a broader use.

Yes, SyncNet has a strong relationship to motion control networks, but it offers also generic I/O modules. I don't see how Ethernet Powerlink can replace Sercos in general because of EPL isn't fast enough to do so.

ProfiNet isn't a brand name of a collection of protocols, but it offers different profiles like profiles for drives, intelligent pumps and the PROFIsafe profile. It doesn't support RS-485.

Don't care about the old marketing bubbles from Siemens... Profibus
will not be completely replaced by ProfiNet. Profibus PA can't be replaced by ProfiNet, e.g. I don't believe that Modbus is a general automation protocol. It deals, e.g. with registers and coils of PLCs... so it has a limited use.

Best Regards,

Armin Steinhoff
http://www.steinhoff-automation.com


Posted by M Griffin on 17 January, 2009 - 11:05 pm
In reply to Armin Steinhoff: When I look at Ethernet Powerlink, most all the devices which use it are drives or drive related. Perhaps it may not be a direct replacement for Sercos, but it does seem to be targeted at the drives market. There are very few general purpose I/O available for it.

For EtherCAT, I believe it is a high cycle rate protocol that operates directly on Ethernet (not through IP). My understanding of EtherCAT is that it is for small networks which need very fast reaction times.

As for whether Modbus is a general purpose automation protocol, what I meant was that it fits in the same market as typical PLCs running ladder logic programs do. The fact that it deals with coils and registers is exactly what makes it "general purpose". Someone writing ladder logic deals with coils and registers, so Modbus fits that need fairly closely. Solenoid valves, proximity switches, push buttons, pilot lights, beacons, analogue sensor readings, etc. are all "coil and register" applications.

I didn't mean to imply that Modbus can fill any and all automation needs. "General purpose" doesn't mean "any and all purpose". It just means that it fills a common non-specific need.

As an aside, I saw an announcement a couple of days ago that a driver for Ethernet PowerLink is now in the staging tree for the Linux kernel. If it gets cleaned up enough to meet the Linux quality standards then it will be a standard part of the kernel. This will make it easier to use in PC applications, which in turn may make it more popular.


Posted by Steinhoff on 18 January, 2009 - 1:55 pm
In reply to M Griffin:
General purpose I/Os for Ethernet Powerlink are available from B&R, Wago and others. Wago provides more or less every types of modular I/Os. Yes, at the first time there was much more offerings for drives ...

The sources of Ethernet Powerlink are available at: http://openpowerlink.sourceforge.net ( Linux, GPL and BSD license)
It's available since a long time ... and it was clean from the beginning :)

EtherCAT has a ring topology and its devices have a very little propagation delay, but the speed of the network is bound to the transmission speed of the 100Mb/s Ethernet. It takes still more than 120us until a full packet has passed a device. Adding N devices to a EtherCAT network does only add N x ~50ns to the bus cycle ... that means the bus cycle depends heavily on the size of the packets and the number packets which are necessary to transport the I/O data of the ring. The offering of general purpose I/Os are comparable with the offering of Wago, but there are also additional third party offerings.

It seems so that we have a different understanding the term 'general purpose' regarding Modbus. A general purpose fieldbus should at least support bus cycles in the range of 5ms to xxx ms, IMHO.

Best Regards
Armin Steinhoff
http://www.steinhoff-automation.com


Posted by M Griffin on 18 January, 2009 - 6:00 pm
In reply to Armin Steinhoff: So far as Modbus/TCP speeds are concerned, that depends on the particular implementation. There are vendors offering I/O modules with less than 5ms response time. For example, there is a certain party who posts on this list regularly who would be happy to sell you hardware which he claims meets this spec.

Some I/O are a lot slower but that is a factor of the vendor's hardware and software, not something that is inherent to the protocol. Some hardware is intended for certain process industry applications where they really don't care about speed. You will also see a lot of analogue I/O modules with 3Hz low pass filters on the front end that are intended for the same market.

There is also hardware where the bottleneck is not the communications, it's the time required to scan and update the actual I/O. The overall I/O cycle time then depends on how many I/O modules you have in the rack, not on the speed of the communications module.

The < 5ms operating range is more than adequate for almost all PLC type general I/O applications. Filter time constants on the input hardware are typically slower than that, and valves and relays are usually much slower than that (100 ms closing time is not unusual for even a small motor contactor).

By comparison, AB sold a lot of Remote I/O where the update time was > 100ms. Given their position in the market, I don't think we can say that their hardware wasn't "general purpose".

I would classify I/O (by today's standards) as:

a) Process industry > 100ms
b) General automation 5ms to 20ms
c) Special purpose high speed < 1ms.


Posted by Dick Caro on 19 January, 2009 - 3:31 pm
More on latency times:
A lot depends upon the sensor device. If the sensor is a contact limit switch or any relay with a mechanical contact, then you must filter that input for contact bounce. The filter may be an analog circuit, a digital timer, a software smoothing filter, or a software bounce monitor. Nominally, any reading of a mechanical device that is less than 50ms is suspect. If the sensor is a TTL switch, then faster times are appropriate, and filtering is not often used. Some intelligent I/O have built contact bounce filtering into the sensor, i.e. Honeywell SDS use of the CAN chip in their limit switches.

Dick Caro
===========================================
Richard H. Caro, Certified Automation Professional, CEO, CMC Associates,
Subscribe to the CMC Wireless Report at http://www.CMC.us
2 Beth Circle, Acton, MA 01720
Tel: +1.978.635.9449 Mobile: +.978.764.4728
Fax: +1.978.246.1270
E-mail: RCaro@CMC.us
Web: http://www.CMC.us


Posted by Steinhoff on 15 January, 2009 - 7:34 am
> They are two different communications protocols. If you want a rough
> analogy, it's like two different languages. Profibus is speaking
> German and Modbus is speaking English. Both languages work, but the
> two speakers can't communicate. <

Yes, that's a real rough analogy. Profibus is a real sensor/actor fieldbus. It can operate at different transmission speed from 1.2 kb/s to 12Mbs und is highly optimized for speed.

BTW, a fieldbus is a technical term for serial buses used for industrial communication... like USB (Universal Serial Bus), but it interconnects industrial devices and not multimedia devices.

The use of Profibus is still increasing and it will have a lot of
additional good days. :)

Specialised sensor/actor fieldbuses like Profibus have their place in
the industrial automation today and in the future.

The concept of Modbus is simply outdated. Look to the newer ones:
EthernetPowerlink, EtherCAT, EthernetIP, SyncNet, ProfiNet a.s.o.

Best Regards,

Armin Steinhoff
http://www.steinhoff-automation.com


Posted by BIBHUDATTA MAHAPATRA on 23 January, 2009 - 8:21 am
Profinet is advanced version of profi-bus only. But is that modbus is still a requirement for serial bus communication (for data acquisition) to plant DCS from individual controls where interfacing with local LAN is also required? We want a X-terminal to view plant DCS data from local LAN. DCS is used for data acquisition in plant. Boiler and Turbine control is separate. Please explain.


Posted by Steinhoff on 25 January, 2009 - 5:50 am
MAHAPATRA: "Profinet is advanced version of profi-bus only."

That's not correct. ProfiNet is completely different to Profibus:

- Profibus is based on RS485 with different transmission speeds ... from 9200 b/s up to 12Mb/s
- ProfiNet is based on Ethernet, specialized Ethernet switches and is
bound to 100Mb/s

The protocols of Profibus and ProfiNet are complete different and are
offering very different features.

MAHAPATRA: "But is that Modbus is still a requirement for serial bus communication (for data acquisition) to plant DCS from individual controls where interfacing with local LAN is also required? We want a X-terminal to view plant DCS data from local LAN. DCS is used for data acquisition in plant. Boiler and Turbine control is separate. Please explain."

A solution could be a Unix/Linux machine supporting Modbus TCP if the
DCS is able to communicate by Modbus TCP.

Best Regards
Armin Steinhoff


Posted by M Griffin on 25 January, 2009 - 10:55 pm
With regards to X terminals, as someone else has already mentioned, X is a standard feature in Linux (the popular GUIs all operate through X). You can also buy X terminal thin clients. X is used for a lot of software on the Mac, and there are third party X drivers for MS Windows.

Just search for "x terminal" or "x terminal thin client" in Google and you will get lots of options. X is the most widely used graphical windowing system there is (most operating systems use it). The cheapest solution though is usually to just install any popular Linux distro on a PC.

I'm not sure why you are drawing a connection between Modbus and X. Modbus is a communications protocol, while X is the standard way of operating a windowing system over a network. The computer your DCS system is running on would have an X server. The GUI on that computer would use X to display windows (and their contents) on the local monitor. If you logged in over a network, your remote PC would make a connection to the X server on the DCS computer and the remote screen keyboard, and mouse would operate just as if they were directly attached to the DCS computer.

If the above isn't clear, post a more detailed question.


Posted by redmund on 27 December, 2009 - 1:28 pm
Mobdus can never die,in fact every industrial equipment ranging from flow meter, motor manager, plc, temperature controller, pressure controller, protection relays and some with built in Modbus, either Modbus RTU or Modbus TCP. so dead simple and can easily network without specialty software.Besides its virtually hardware independent for communications cable.whew! no sweat.


Posted by Jonas Berge on 25 January, 2009 - 1:43 am
In my personal opinion, PROFIBUS has one good thing going for it: EDDL (www.eddl.org) IEC 61804-3 electronic device description language is an integral part of the protocol. By loading the EDDL file into a configuration software using EDDL, that software can configure variable speed drives, electric actuators, and other kinds of devices using PROFIBUS-DP.

No need to map registers to get access to full configuration and diagnostics etc. With the Modbus protocol it may be possible, but laborious and error prone to achieve the same result. The same single software can be used to configure all kinds of PROFIBUS, Fieldbus, HART, and WirelessHART devices.

Cheers,
Jonas

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