Choosing S7 300 or 400

Plc_User

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As we have now powerfull S7 300 plc's as the 319 PN, what still considers you to choose S7 400?
In my experence I know that towards communication S7 400 is more powerfull. In the 300 all the rack IO is connected to the bus mpi. Also when you have an ethernet card, the bottleneck will be the backplane. And in bigger plants where you have lots of IO's you might want to split up your profibus network into different networks using profibus master cards on the backplane, however also here the backplane will be the bottleneck. For a new installation I am considering for these reasons using S7 400.
One drawback will be the price I guess. What could be the difference in price between 319 PN and an 400 with comparabele memory and onboard pn?
Why do you choose S7 300 or 400 in your projects?
 
Well I think the 300 will handle about anything you need. I did a job where I controlled an entire production line with one 317 Profinet CPU and still had 40% free memory. Then there is the 319 which is a 400 processor in a 300 chassis so jeez, how much cpu do you need? I wasn't doing motion but it was an entire production facility that could have been split into three CPU's if you broke up the segments.

Recognize the 300 has ethernet onboard the cpu so your backplane concerns are alleviated.

Things you don't get with 300 are all the OB's for time slicing and the ability to set how the I-O is polled. There are other things but I've never needed to manage systems with these tools in ten years of using Siemens.

Difference in price between 300 and 400 is like difference in price between PLC-5 and CompactLogix. :)
 
S7 300-400

Well I think the 300 will handle about anything you need. I did a job where I controlled an entire production line with one 317 Profinet CPU and still had 40% free memory. Then there is the 319 which is a 400 processor in a 300 chassis so jeez, how much cpu do you need? I wasn't doing motion but it was an entire production facility that could have been split into three CPU's if you broke up the segments.

Recognize the 300 has ethernet onboard the cpu so your backplane concerns are alleviated.

Things you don't get with 300 are all the OB's for time slicing and the ability to set how the I-O is polled. There are other things but I've never needed to manage systems with these tools in ten years of using Siemens.

Difference in price between 300 and 400 is like difference in price between PLC-5 and CompactLogix. :)

My big concern is the profibus IO I should want to split up unto two or more masters to keep the profibus cycle time short. But as I said, adding a profibus master card has the drawback of the backplane bottleneck.
In your big project, how many profibus slaves do you have (on the onboard profibus master) and what is the profibus cycle time?
 
The first few projects with S7 that we made was with S7-400, today we use S7-300 exclusively.

Regarding the backplane on the S7-300, then it is split into the K-bus (= MPI) and the P-bus (i/o). The K-bus is the slow 185.5 kbps. I dont know the speed of the P-bus, but someone has measured it with a scope and found that it operates at about 5 Mbps.
Thus, i/o via Profibus on a CP342-5 operates at a reasonable speed, whereas HMI connections, programming, datatransfers etc. via the same CP342-5 operates at the slow speed.
I am sure that an S7-400 is faster than an S7-300 for i/o via the backplane and CPs, but not so much as you might think.

There are more differences between the S7-300 and S7-400.
But I find that S7-300 has made quite big steps forward in the last few years, that I think it will be used more and more, and the S7-400 less and less.
If I look into my crystal ball it says:
S7-400 is on the decline.
S7-300 is rising.
PC-based (427C, 477, S7-mEC etc) is rising fast.
Profibus i/o is steady or dropping slightly.
Profibus HMI, datatransfer etc. is dropping fast.
Profinet i/o is rising.
Ethernet HMI, datatransfer etc, is taking over completely.

Therefore: Unless you have an extremely large project, or you need such things as hardware redundancy or a lot of communication power, I recommend that you stay with S7-300.
And I would also start to think about switching to Profinet in stead of Profibus.
If you must stay with Profibus, and the two DP ports on the 319 are not enough, you may consider the IE/DP link module (6GK1 411-5AB00), or you can add a few CP342-5 modules to the rack.
 
In an earlier project we used a 319, but used the mpi/dp port as mpi to put some TP177A panels on it, because we are not greedy to put panels on profibus. What do you think about panels on profibus? I thougt they introduced an extra master on the bus, so you go into multimaster, where the mastertoken has to be passed, so slowing down communication. Also some people talk about 'problems with panels on profibus'???
Is the MPI port set to DP-mode completly comparable to the native DP-port?
On that project I had a dp-cycle time of 30 ms (compared to plc cycle time of 2 ms). Thats one of the reasons I should choose now for two profibus busses. In a project from another machine constructor I saw once a profibus cycle time of 1 second????
What are your experiences with profibus cycle time on your projects?
Would you prefer a IE/DP coupler to a CP342-5?
Many questions, but I think worth asking them to colleages because for me they ar fundamental.
Thanks
 
With a S7-400 you are able to replace analog or digital signal-modules in RUN-mode of the CPU (if alarm OBs are programmed properly).
The S7-300 has these bus connectors as backplane, so if you remove one module you loose the connection to all following modules.

With a S7-300 you have the limitation that you can connect maximum 3 racks over the IM modules to the CPU. So you can plug max. 8 * 4 (3 racks + 1 CPU rack) = 32 SM modules.

In our bigger wastewater plants we use mostly S7-400 CPUs. I have a project with one single CPU an about 1500 I/Os. I think this would be too much for a 300.
 
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1. An MPI/DP port can be much faster than a plain MPI port.

2. It definitely slows down the Profibus if you put just a single panel on it.
We have a TTR of 30 ms on one of our projects because we have an OP on the Profibus.
For this reason we are going to change the OP to ethernet.
Today, there are even small panels with ethernet, so there is no reason to use Profibus.

3. 187.5k / 1.5M / 12M also makes a huge difference. The 1ms bus cycle must have been at 12M or it was a very small project.

4. I dont have experience with the IE/DP coupler, but I expect it to be at least as fast as a CP342-5.
 
In our bigger wastewater plants we use mostly S7-400 CPUs. I have a project with one single CPU an about 1500 I/Os. I think this would be too much for a 300.
1500 i/o is definitely not a problem for an S7-319.
But in a WWT plant, I am guessing you are using hardware redundancy, or .. ?
That would of course be impossible with S7-300.

I also think that the capacity of modules in racks is becoming irrelevant.
We dont have any i/o in racks any more !
Everything is via Profibus, even if it is the same cabinet as the CPU.
We are now in the beginning of the transition to Profinet.
 
1. An MPI/DP port can be much faster than a plain MPI port.

2. It definitely slows down the Profibus if you put just a single panel on it.
We have a TTR of 30 ms on one of our projects because we have an OP on the Profibus.
For this reason we are going to change the OP to ethernet.
Today, there are even small panels with ethernet, so there is no reason to use Profibus.

3. 187.5k / 1.5M / 12M also makes a huge difference. The 1ms bus cycle must have been at 12M or it was a very small project.

4. I dont have experience with the IE/DP coupler, but I expect it to be at least as fast as a CP342-5.

1. I meant the difference between the native DP-port and the Mpi-Dp port in DP-mode. I suppose the performance is comparable, but I have no experience in that.
2. You talk about a TTR of 30 ms, do you know what your TTR is whithout panels on the bus?
3. The TTR in that project was 1 second, I think because the number of drives large communication data.
 
2. Without the panel TTR drops to around 12 ms.
I suggest you experiment with various configurations in STEP7. It is free, and is a great way to learn.

3. OK, I misunderstood. 1 second is extreme. I guess that it may have worked acceptably because TTRtyp is much lower. However, it is not recommendable to rely on TTRtyp.
 
If I have some time left, I will try some configuration examples and find out the TTR with and without panels. Although I doubt the number of tags are included in the theoretical calculation. However in busses without panels when I compared TTR with the bus time measured with a diagnostic repeater, the calculation was pretty close to the measurement.
 
I work for Siemens and when I design a system it comes down to this.

When to use a 400:
1. Redundancy
2. Floating point safety (CFC)
3. CIR (Configuration in Run) - Adding I/O on the fly
4. Allot of SCADA resources (Think real big)

Why to tend to use 300:
1. Much more scalable
2. PN Ports have gobbs of bandwidth
3. I usually put no I/O on the main CPU rack (only on smaller systems)
4. I tend to use PN I/O and or IE/PB Links giving me a Profibus Master in the field right where I want it.

Lately many customers are using Microbox. Stellar performance if you can live with lengthy startup (Windows) but it has huge memory on tap. The next generation of CPU's will not have separate families. I could tell you but they would kill me. We just released the new generation of 315/317. Half the size (Now 40mm wide) and doubled the mem and speed. I suspect we were a little conservative. Customers who have run them side by side report even shorter scan times than half. I guess it depends on what you are doing and ho much port traffic you have. The future will be like the S7-1200. Much more dynamic and you will simply define OB time slices as you like.

If you don't need redundancy or floating point safety I would go with a 319. It will soon double up performance as well.
 
Jesper,
Failsafe (Safety) programming is done normally in LAD or FBD. On the Safety side you use standard functions that are approved by TUV. They are normally only INT format for math. Floating point is available in CFC but this can only be run on a 400F CPU. I suspect (but not sure) the library I makes use of the 4 accumulator only available on the 400's. The 300's being a 32bit CPU with two ACC's I think they use the top half of the 32bits as the safety CRC and time slot.
 
Nick

Your mixing up your systems. The S7400HF uses CFC and safety (the yellow safety CFC blocks) The 300F and 400F program the same way
 

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