[s7] What processor?

Hi guys,
Could you share your knowledge on what factors you take into consideration when decision is being made on how much memory is needed to handle a system?

I've been asked a question whether Siemens ever published any guidance on how 'big' processor (memory wise) is needed to handle certain amount of I/O points...

From experience a lot depends on how complex your plant is, but maybe you will point some valid points that I haven't thought about.

Cheers!

I have seen Rockwell provide some kind of estimation sheet for their Controllogix processors, depending on i/o count, comms channels etc.

But to my honest opinion, the memory requirement is so totally depending on the programming style and functionality, that such a guide is at best a rough guesstimate, and possibly totally misleading.
The best method is to write a small program in your own programming style and with the required functionality, and see how big it becomes. Then extrapolate from that. Not perfect but much better than memory = f(i/o count).

JesperMP said:

I have seen Rockwell provide some kind of estimation sheet for their Controllogix processors, depending on i/o count, comms channels etc.

But to my honest opinion, the memory requirement is so totally depending on the programming style and functionality, that such a guide is at best a rough guesstimate, and possibly totally misleading.
The best method is to write a small program in your own programming style and with the required functionality, and see how big it becomes. Then extrapolate from that. Not perfect but much better than memory = f(i/o count).

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I feel his pain, though - your advice is good when you can write the software before buying the hardware, but usually you have to buy hardware well before that point.

The way I've tended to select S7 processors in the past is as follows:
1. Quick search though old projects done by my company to see what processors they used. This is the best and most reliable guide for selecting processor power as you're using representative hardware and software for a representative task. This is also one reason my company logs the scan time as part of the "finalization" tasks for a project, in order to facillitate this retrospective.
2. Limit myself to no less than 315-2DP (or equivalent) for larger machines or smaller systems. Typically, below this point the value and processing power drops below what I consider useful in the sort of industries I work in (machine scan time no longer than 50ms under any circumstances, and prefer 20ms and under)
3. At the higher end, I generally use an informal scale built up from information in point (1), then round up substantially as I would rather save money elsewhere in my hardware budget if need be.
4. If the customer has similar-scale plants elsewhere, I will ask what processor they used there.
5. For really big or complicated projects, I will consult with Siemens on their suggested processor requirements (although the Rockwells and Siemens of the world typically like to go nuts with the hardware spec when it's someone else's coin)

I know this is a bit heuristic, but as Jesper has already said - processor selection is a bit of a gentle art, especially in Siemensland.

Binaural said:

I know this is a bit heuristic, but as Jesper has already said - processor selection is a bit of a gentle art, especially in Siemensland.

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Thanks for your input!
You touched the heart of the problem when it comes to decision - sales department has to quote a certain CPU well before engineering department is even aware of the project. Their knowledge is unfortunately limited and together with Siemens complexity creates issues at the start.
Siemens support is ever worse - they advised hardware for 3 systems within the last year and none of them could physically work. Sales people get very frustrated.

I suggest that you post the requirement here, and we can give our best estimates. Since we are end users, we probably have a better idea than Siemens have themselves.

Post i/o count.
Type of application.
Anything out of the ordinary.

Siemens support is ever worse - they advised hardware for 3 systems within the last year and none of them could physically work.

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That sounds interesting. Could you give us some more details about that ?

JesperMP said:

I suggest that you post the requirement here, and we can give our best estimates. Since we are end users, we probably have a better idea than Siemens have themselves.

Post i/o count.
Type of application.
Anything out of the ordinary.

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Thanks for your offer, but unfortunately the only info I've got is the quote prepared by our ex-sales guy who did rubbish. I don't know the plant at all, have only got number of I/Os.

That sounds interesting. Could you give us some more details about that ?

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As an example - one advised to use PA couplers and ET200M modules on the same rack 'behind' one IM153-2.

I don't know the plant at all, have only got number of I/Os.

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But you know what "kind" of plant it is at least.

As an example - one advised to use PA couplers and ET200M modules on the same rack 'behind' one IM153-2.

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I see what you mean. I guess that is because PA is less well-known than DP, even within Siemens. But of course that is no excuse.