seimens processor

What is the different between S7-300 and S7-400 and which one is better .

Depends on the system, what you need it to do, etc.
They are two different levels.

http://www.automation.siemens.com/m...matic-s7-controller/s7-200/Pages/Default.aspx
http://www.automation.siemens.com/m...matic-s7-controller/s7-300/Pages/Default.aspx
http://www.automation.siemens.com/m...matic-s7-controller/s7-400/Pages/Default.aspx

Thank you

Thank you Mr. tharon for your replay actually we are using S7-400 in our steel plant for main plant and for the auxiliary machine we are using S7-300

Makes sense. S7-400 is for large machinery, process automation, communications, etc. Probably similar to Allen Bradley's ControlLogix platform. I have never used S7-400 though.

I'd wager that S7-300 is similar to A-B's SLC-500 line. Handy for machines with a larger number of I/O.

S7-200 probably similar to A-B's MicroLogix. Limited I/O and communications, but just fine for small, simple machinery.

I really have only used S7-300 and the ET200S modules. And never developed for them from scratch, just maintained and modified existing machinery.

Here's how I think of it. To control an entire plant and use PCS7 software would require the 400. The 400 is on par with DCS. To control a machine or a group of machines can be done with the 300. The 200 is a great platform but is limited on I-O and the resolution of the analog inputs is low. The 200 is for smaller machines.

The top end 300's are on a per with the 400, tyhe bottom end far exceeds SLC's.