Has the Profibus FDL been derived from FMS? What is difference between them?

 

Hello, Ali;

Both FDL and FMS services were developped for fdifferent purposes, around the same time.
FDL was meant to be a secure and easy link (ISO Layer 2, Data Link) between S5 and S7 CPUs, with configured connections and unformated data.
FMS was meant to be a peer to peer connection between Profibus masters, exchanging structured data (ISO Layer 7: Application). All data transfered over FMS is converted from the CPUs natural format to a neutral format (FMS variables) to allow data transfer between masters of different families (S7 and TI505 for example).
Have a look at the manual @Communicating with [email protected] that you can download from Siemens' support website, at:

http://www4.ad.siemens.de/WW/llisapi.dll?func=cslib.csinfo&lang=en&objID=4000024

Hope this helps,
Daniel Chartier

 

> Both FDL and FMS services were developped for fdifferent purposes, around
> the same time. FDL was meant to be a secure and easy link (ISO Layer 2,
> Data Link) between S5 and S7 CPUs,
<clip>

As a slight correction to this, I believe that FDL long predates the Siemens S7 series. I believe that it was originally intended to allow peer to peer communications between S5 PLCs (and PCs with an appropriate CP card) at a lower cost than the complete FMS protocol. In this sense, both FDL and FMS both address the same application area. However, FMS has features which FDL does not. One form of FDL (Send/Receive) is supported by S7 hardware to allow communication between existing S5 and newer S7 systems.

Although I do not know if the following is strictly correct from a technical standpoint, you can more or less consider FDL to be a lower cost subset of FMS. While Profibus DP appears to be very well established and still growing, both Profibus FMS and FDL appear to be fading away and seem to be mainly useful for support of older hardware.

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Michael Griffin
London, Ont. Canada
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Hi All

FMS was/is available on the S5, it was available before Profibus DP, and seemed for a while as if it was going to be "the Profibus" standard, however as we have seen Profibus DP has pretty much turned out to be "the Profibus". It is pretty much possible on DP to do everything that FMS had to offer, however creating point to point links for moving data blocks between PLC's over a DP network is missing on DP links, enter FDL, a point to point connection which has all the characteristics of old RS232 comms standard.

FDL was available at the same time Profibus DP started becoming available, that means it was there during S5 times, as Profibus was available for a long time before S7 was released.

I have to admit I have never really gone into the details on either spec but I would expect that what goes up and down the purple cable doesn't change much from one to the other. The real difference is in the setting up, Profibus FMS required that each variable or VMD be defined manually and in detail, whereas DP with its clever use of GSD files has eliminated this.

Regards
Donald Pittendrigh.

 

> Although I do not know if the following is strictly correct from a
> technical standpoint, you can more or less consider FDL to be a lower
> cost subset of FMS. <

This is not really an accurate way to look at it. FDL is a data link protocol. In other words a way to send data from one node to another.

FMS is an application protocol. FMS specifies a format for the data that is sent using FDL.

FDL alone is not sufficient. If there were Siemens controllers that supported FDL and did not support DP (Siemens had another name for it was at some point I think) or FMS, then you can rest assured that there was some kind of protocol, perhaps unspecified, in use that organized the data that was sent over FDL.

> While Profibus DP appears to be very well established and still
> growing, both Profibus FMS and FDL appear to be fading away and
> seem to be mainly useful for support of older hardware. <

Although I can't personally dispute this, I think the Fieldbus Foundation guys might argue that FMS is not diminishing. FMS is used as one of the application protocols supported by the Fieldbus Foundation.

Regards,
Ralph Mackiewicz
SISCO, Inc.

 

FDL on the S5-95U actually has 4 different operating forms or modes. I believe the most commonly used was "cyclic I/O", which more or less acts as a form of shared memory between groups of CPUs for small amounts of data. It requires minimal programming; mainly a bit of configuration in DB1. It resembles DP in the sense that you have one "master" which controls the automatic scanning of the passive CPUs.

There are also modes called the "Standard Connection", "PLC-PLC", and "Send-Receive". The "Standard Connection" is used for sending larger amounts of data than cyclic I/O, but requires more work (your program controls when messages are sent). "PLC-PLC" is used for communicating between active stations. "Send-Receive" uses lower level calls ("level 2 services") and requires more work. However this is the only mode (of FDL) which will interoperate with the newer S7-300/400.

FMS is available on the full size S5 models, but the S5-95U only provides FDL. The intended role for FMS/FDL was coordination between independent PLCs (and other complex devices), but it now seems as if Profinet over ethernet is intended to fulfil this function. I don't know if Profinet has generated much customer interest yet.

--

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Michael Griffin
London, Ont. Canada
************************

 

> This is not really an accurate way to look at it. FDL is a data link
> protocol. In other words a way to send data from one node to another.
>
> FMS is an application protocol. FMS specifies a format for the data
> that is sent using FDL.
<clip>

Siemens is a bit vague about what is really in certain versions of the S5-95U. It gets referred to generically as "FDL" because they will tell you that it isn't "FMS". The S5-95U actually supports what appears to be a collection of proprietary application protocols layered on the data link protocol (Cyclic I/O, PLC-PLC, and Standard).

> FDL alone is not sufficient. If there were Siemens controllers that
> supported FDL and did not support DP (Siemens had another name for it
> was at some point I think) or FMS, then you can rest assured that
> there was some kind of protocol, perhaps unspecified, in use that
> organized the data that was sent over FDL. <

Siemens used to call Profibus "L2". The S5-95U came in versions with built in ports for "L2" or "L2 DP". This PLC did *not* have an FMS option (you had to use a higher end PLC for this to be available). The one referred to as just "L2" was the one which was commonly called "FDL", to distinguish it from L2 DP and proper L2 FMS. When Siemens introduced the S7-300/400 series, they made it clear that they had a proprietary protocol called "MPI", so there wasn't the same confusion about that situation.

<clip>
> I think the Fieldbus
> Foundation guys might argue that FMS is not diminishing. FMS is used
> as one of the application protocols supported by the Fieldbus
> Foundation.
<clip>

There seems to be very little hardware outside of Siemens' PLCs which supports Profibus FMS. There is very extensive support from many companies for DP. If Profinet on Ethernet is successful, it is difficult to imagine what future purpose Profibus FMS will serve. The Fieldbus Foundation may recognise FMS, but I would be rather curious as to how much it actually gets used.

--

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Michael Griffin
London, Ont. Canada
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FOUNDATION Fieldbus uses FMS at both the field-level (H1) and the host-level (HSE). For HSE we called it FDA but it really does the same thing. The FMS application layer used by FOUNDATION Fieldbus is not identical to the FMS application layer in Profibus-FMS.

I agree that "Profibus-FMS" is on the way out, even the Profibus technical description says so. I think Profinet v2 will take its place.

Fortunately you don't need to understand FMS to use Fieldbus. But if you are a propeller head that want to know more about it, take a look at the last chapter in the book "Fieldbuses for Process Control: Engineering, Operation, and Maintenance" (buy online in hardcopy or download immediately in softcopy). The focus of the book is the more down to earth issues like installation, configuration and commissioning, suitable for system integrators that need to do projects based on FIeldbus. Only the last chapter covers what goes on down in the engine room. If your email does not support this hyperlink feature correctly, please copy the URL and paste it into your Internet browser. Mind the line wrap, make sure to get the complete path all the way to the 3036: http://www.isa.org/Template.cfm?Section=Shop_ISA&Template=/Ecommerce/Product Display.cfm&ProductID=3036

Jonas Berge SMAR
==================
[email protected]
www.smar.com

 

Dear Mr. Daniel, Mr. Griffin, Mr. Pittendrigh, Mr. Mackiewicz, and Mr. Berge:

Thanks for your helpful comments.

Best,
ARFereidunian,
TAM Co.