Sizing up a PLC for the Job

I was wondering how you would go about choosing the correct model of S7-300 PLC. My system is going to contain two profibus loops with various pieces of instrumentation on each. There will also be an ethernet CP card for HMi communications. As for the code..well it is mostly just alarming conditions and signals out to the HMi.
I am aware that the working memory increases as you move up the models (as ive had a look in HW catalogue) and it would be easy to pick a 315-2DP which i know would do the job no problems but what is the correct approach in choosing the right one?

Sounds like you already have selected a model, just listen to yourself describe what your requirements are.
Rule of thumb, 1.5 times what you think you need, if your afraid you missed something then,,,,
1.75 times what you think you need. NOBODY ever got fired for comming in under budget, those left overs get shifted into the spare parts inventory budget.

Hmmm..i'll be honest with you, i was expecting a flood of posts suggesting how to guage the memory size required for the job. I suppose i will just go for one that i know will do the job plus cover me for future expansion!
Thanks anyway!

LH:beer:

Look for a post or reply from JesperMP a while back.
I remember a bunch of info on Working memory and such for Siemens S7-300 series CPUs. Do a search and you may find it.
Hope this helps a bit.