Hello all,

I have a question of how s7400 is better? I have worked all siemens PLC like s71200, s7300 and s7400 series. And people follow certain pattern in selection of this products. Like if it is machine or small process take s71200, For medium level machine, assembly lies or processes there is s7300 and high level process or assembly lines require s7400. 

I know s7400 is superior in terms of memory, and reliability through my experience but is its performance really higher in terms of capability of processing instructions. I mean is it true that analog signal processing and PID control will be better than s7300 in s7400. I know for a fact that s71200 is not suitable for handling critical PID loops. Why is there such difference in performance?

In other words if I have to control four very critical PID loops, and not much memory is required then what is better solution? s71200, 300 or 400? If s7400 is preferred than what makes it preferable?

Thanks.

Kbv.

Nowadays you should take into consideration S7-1200 or S7-1500 for new projects. Unless you need redundant solution then the only option is S7-400 AFAIK.

Or you don't have TIA portal, then yeah...

Anyway, few PID loops doesn't sound serious at first glance. And what do you mean by critical? Is it sampling speed? Or rather when cotroller fails you will blow whole district out of ground?

Hello,

Thanks for you interest in question.

So, I guess, we cannot point out obvious advantages of S7400?

Actually example of critical application is glass furnaces. There are few loops like gas flow, oil flow, furnace pressure control. But if any of the loop doesnt give good performance then it could affect production a lot in irreversible way or worse it may blow up three floor large furnace.

In this case obviously one can think that I have to control just 4-5 things, I can take s71200. But no, we always use s7400 or now s71500 in such applications.

Is there any parallel processing in s7400. As I think PID are much more stable when run in S7 400.

Thanks.

Kbv.

Hi Kbv,

PLC is micro-controller device. S7-400 is 32 bit micro-controller with complex architecture which offers dedicated functions like redundancy, diagnostics etc. It also has more internal memory which results in lesser scan time. Hope this helps!!!!!!!!!

Hello,

S7400 is having faster scan cycle? Well I work with CPU 410 and it has scan cycle of 200ms. I dont think that has effect over processing of CPU. IF any, i think PIDs are more stable in this CPU. I agree that 400 is obviously powerful in terms of communication capabilities and features like redundancy. But what if I dont want use those features. Then 300 and 400 are the same?

Some one once told me that In PLC PID are implemented by approximation of continuous control by digital control. The more loops you add, this approximation is worse and this will affect the quantities which are controlled. In case of s7400 CPU has ability to process analog inputs parallelly so this approximation will be as good as single PID for 100 PID loops. Is it true?

Another question that comes to my mind that siemens is promoting CPU 410 as DCS solution. The only difference I found is that it has fixed scan cycle and I can only download charts CFC. It cannot download block. Is there any hardware related difference that makes CPU410 from different than others and Suitable for DCS application.

Thanks.

 Read this. I hope this will give you answer how DCS and PLC are different.

Hello jacek d,

I have seen this document numerous time before. It differentiates between DCS and PLC application areas. This is clear to me. I am asking what makes CPU 410 smart classified as DCS? Is it slow scan time of 200ms, is it integrated memory or is it the fact that block cannot be downloaded to it only charts can? In my opinion this differences are limitations not features.

Well I have not worked in any other DCS system but as per my knowledge in DCS, control should be distributed so that failure of one area does not affect the other area. There is centralized engineering facility. and user doesnt have to put much effort or face limitations in inter-controller communications participating in DCS. System is capable of maintenance during running plant. I know this is possible in siemens but no tailor made solution is available. With great engineering effort and proper planning, this can be implemented. So when siemens is saying now CPU 410 smart is specially designed for mid level DCS market then there must be some hidden features?

I am not asking these questions just for knowledge. It is obvious that client wants to know what is superiority of S7400 whe he is investing significantly more. and saying that it is having more memory is not enough as I said memory requirement in my application is satisfied with S7300 also.

Thanks.

Kbv.

I think you ask wrong questions. S7-410 alone is not a DCS. Nor S7-400 is.

When you say: DCS, in Siemens world it sounds (let's put aside legacy systems): PCS 7.