In your first post you said

So whay are you both trying to design one ? Its a fundamental flaw in approach if you really ask me. Let me tell you something since I also do these kind of full machine controls with MCUs.

* First and foremost they are not to be called PLCs. One of the basic qualification for PLCs, as the industry understands, is the ability to program in ladder logic or one of the modern GUI based tools like what Siemens/Allen Bradley etc supply. ( Look at the snapshot posted by Steve). These are standardized and the customer can be trained to handle these as they are regular software products.

* I call the control systems that are built around MCUs by me as "Embedded 8B" and do not forget to add the terms "(C) Winstar Hydraulics Ltd" which happens to be my company's name.

* My client is fully aware that code modifications are possible only by my company. He is willing to do this based on the past experience ( very reliable) and cost advantage for the facilities he gets - VFDs for user interface and configuration, 12 bit analog channels and 8 bit DACs that are almost free as compared to even the low cost PLCs...

* In India you can still get by with an offering that is not CE certified. Assuming your client insists, what Certification will you offer for your "control system". Quite frankly I cannot for mine.

* How about warranty and spares management ? I do about 6 to 8 machines in a year with such MCU controls alone and it is a big investment to maintain spare cards and details of changes made in each machine. It is an expense but HAS to be done. You cannot let down your client by sloppy documentation and poor spares management.

* All in all it needs plenty of homework and real exposure to machine control to be able to do it with MCUs. Make no mistake : In the industry SAFETY of men machine comes topmost and it here that one has to be very very careful. I have been doing this for donkey years and still, recently had a mishap of 415V meeting 24V. Fortunate I am alive to tell it.

My sincere suggestion would be :

Looking at the I/O summary you had given, better open a dialog with Messung PLC ( they are in Mumbai ) and with thier MicroLogix PLC you will get upto 64 DIO, and 12 BIT AIO as per need. User interface will be a HMI_Sr that has something like 28 user configurable keys and 4 linex20char. VFD display. And programmed with ladder logic. I said Messung PLC purely on cost - you have others like Omron / Mitsubhishi as alternates in the low I/O count modular stuff. And if cost is not an issue you can try Siemens / Allen Bradley for thier rack versions.

Reserve the temptation to develop your own PLC for a later date when you have been adequately exposed to industrial controls. Your customer will be happy you did that.

Play it safe and right.

Good luck

Raghu