PLC memory Calculation

Hi guys,

I would like to ask, How to calculate the memory consumption of a given PLC? Specifically Simatic CPU-315-2 PN/DP? I can't find it in the help file. I just want to check if this cpu can handle 49 analog inputs and some digital input and output.

Your help is highly appreciated.
Thank you very much.

This should not be a problem for a recent version of the 315 CPU. Depending on which cards you use, u will probably have to install an expansion rack to.
The data sheet for the CPU shows you how big the address area is for the IO, but I would not go till the limit. The amount of memory you need is a bit based on experience. I always check the used memory and cycle time for every PLC I install. So after a while you get a feel for it. It is also very dependent on the complexity of the software.

Hello ry_fern;
And the mentionned data sheet can be foud here:
https://eb.automation.siemens.com/g...vetab=product&regionUrl=/uk#activetab=product&

You will find that the CPu 315-2DP (of model 6ES7 315-2AH14-0AB0) can handle up to 1024 analog inputs and 1024 analog outputs, so you should not have any roblems with your 49 signals.

Look for manuals on S7-300 CPUs here:
https://support.automation.siemens....tandard&viewreg=GB&objid=10805161&treeLang=en
Hope this helps,
Daniel Chartier

@ Jobbe

Yes, I have setup a remote IO network using the bus interface module IM 153-1. All my remote IO will be communicating through profibus or MPI network. Thanks for the "tip" about the checking of memory consumption and cycle times. I will include that routine in my next projects, including this one.

@ Dchartier,

Thanks for the links you have provided. They were very helpful.

I would like to add, is there a formula on how to calculate the actual consumption? I saw someone do that but I can't figure out how he'd done that.

Again, thank you very much.

ry_fern said:

I would like to add, is there a formula on how to calculate the actual consumption? I saw someone do that but I can't figure out how he'd done that.

Click to expand...

Memory consumption depends on how complex your code is.
For example you can use a start and a stop button to start a pump in 3 or 4 lines of code. You could control the same pump with extra control of a operator panel and a SCADA system on top, with interlocks, manual and automatic control etc. The second example will require more code and more memory. The same goes for analog signals. Do you use an analog input just to measure and visualize something or is it integrated in a complex cascading PID control?
Some companies develop their own spreadsheets that calculates the (estimated) required memory based on digital IO, analog IO, number of PID controls etc. This can be done if you use the same standard blocks every time. But it is still based on experience with previous projects.
Maybe Siemens has a tool on their site to help you with this, but I haven’t found it yet…

@ jobbe

Analog signals are for monitoring purposes only, so I think this will not consume much of the memory...

Thank you for your quick responses, I really appreciate them all.

Thank you.