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Siemens S7 - Allen Bradley PLC5 comparison
High speed machine control with 2-3 simultaneous processes.

I am told by my control manager that we cannot use an Allen Bradley/Profibus combination to control 2 simultaneous processes since the PLC scan time is too slow. He informs me that the Siemens S7 is the only solution and even this cannot cope with a 3rd simultaneous process. Is he right?

By Steve Myres, PE on 11 September, 2002 - 6:11 am

How many things a PLC can do "at once" depends upon the complexity and speed of the tasks. Can you give us a little more detail?

It strikes me that another relevant issue with this type of design is the consequences of a PLC failure. It sounds like the processes you describe are completely independent and each is of value without regard to the status of the others, rather than merely subtasks in one overall process.

If this is the case, it may be worthwhile to put a separate, perhaps smaller and less expensive, PLC, on each process. Then, if one PLC (or its IO network) has problems, it only takes down a single line.

By Donald Pittendrigh on 12 September, 2002 - 3:17 pm

Hi All

This certainly sounds like a strange claim, what kind of process are you referring to?

DP

We are concurrently cold forming metal in 2 or 3 dies simultaneously. There are approx.10 control loops for each part.

The PLC is also controlling the main machine parameters at the same time.

I am not an expert on the referenced product lines, but I think you are comparing apples to oranges, the S7 and PLC5 are different-generation controllers. The AB PLC5 should be compared to the Siemens S5. The Siemens S7 should compared to the AB SLC500 or ControlLogix.

I have no idea how this narrow contest came to be,
but the S7 - AB5 comparison doesn't pass the smell test.

I suspect there is something that you are leaving
out of this question or your control manager is the owner's nephew. Just about any PLC can control a handful of external processes in a quasi-concurrent manner. Or did you mean process in a multi-tasking OS sense ?

Scan-time is a function of processor speed, memory
access speed, and bus ( I/O communication ) speed. It would take one sharp cookie to read the specs, even at face value, for a couple of PLCs, and quantify which one of the two is really faster.

Then there is the problem of PLC design, compiled or interpreted, Ethernet/IP or OurBus-OurProtocol
communications, RLL or FBD, and so on.

I know, not a satifying answer, but that's all I
can say.

Jay Kirsch

By Ranjan Acharya on 16 September, 2002 - 1:31 pm

Sounds like salesman claptrap to me. There is a case study to prove anything.

Why are you using PLC5? I would be more inclined to a newer offering from A-B such as CLogix. S7 and PLC5 are generations apart.

The S7-400 is a super PLC with lots of process capabilities, especially when coupled to PCS-7. However, are you already a Siemens shop? If not then you have to teach all your technicians about OBs, WinCC, Step7, PCS-7 and so on. Plus, PCS-7 has a PCS-7 way of doing things that you are best to follow.

CLogix has an equally capable array of offerings that when coupled to RSBatch, RSView and so on would give you just as good a junior DCS as PCS-7.
If you are an A-B shop, then the leap to CLogix from PLC5 is quite manageable.

I see more of the costs and headaches with intangibles such as support (what time is it in Germany right now anyway?), licensing and so on rather than raw issues such as PLC scan time or the number of PLCs you should be using (independent processes should have their own PLC, don't be cheap).

RA

By Maguire, Kevin on 18 September, 2002 - 11:21 am

Regarding <<I see more of the costs and headaches with intangibles such as support (what time is it in Germany right now anyway?),>>...

Siemens Customers in the USA receive tech support through our US hotline - based in Johnson City, TN. The time there is the same as New York City and an hour ahead of the time in Chicago. :-)

By Ranjan Acharya on 19 September, 2002 - 3:51 pm

<clip>
Regarding <<I see more of the costs and headaches with intangibles such as support (what time is it in Germany right now anyway?),>>...

Siemens Customers in the USA receive tech support through our US hotline - based in Johnson City, TN. The time there is the same as New York
City and an hour ahead of the time in Chicago. :-) </clip>

Yes, I have had excellent service from Johnson City (shame about the lack of APT!) and Chicago. However that being said, I have had to call both the UK and Germany for support issues with Siemens products. Time zones do apply (OK, a Global Village and all that stuff, but it can still limit support options).

Also, many times I have been on Siemens web site (which is not useable by any stretch of the imagination, but I can sympathise there, it must be hard to put all that data in order) - I always seem to end up (virtually) in Germany and once in a while I still end up either with a German page or a document that is only available in German.

It happened just a few months ago with some MicroMaster VFD stuff. I eventually found the English version of a manual by going through my local rep., but the web site only had it in German.

This is not just me. Other Siemens users I talk to at trade shows and my customers all have the same observations. You only have to be stymied once.

":("

RA

The biggest difference in the Siemens layout and
the AB PLC5 layout is the data table, and the
general organization of the PLC program.

AB thinks that a PLC program is a series of
consecutive rungs, each one solving a boolean
equasion of some sort, in order to manipulate some field device.

Siemens, OTOH, believes in manipulation of data first, and actually turning on or off something later.

If you like manipulating data, use Siemens.
If you can think in objects, use Siemens.
If you think you could do a lot more with a PLC
if you could just pass _9_ variables into a
subroutine, by all means, use Siemens.

The only hard part of Siemens is figuring out
how the software guy and the hardware guy ever
had time to write their 'figuring out Siemens for an Allen Bradley user' manual. (hint: It does not exist..)

Definitely it's a no sense comparison, since their are from a different generation (more than a decade!)
I'm sure that you will success if try to use A-B ControLogix platform:
-CPU scantimes several faster than PLC5.
-Multiprocessor system.
-Backplane controlbus communications.
-Etc...

Good luck