This is the beginning of a long message to describe what to be the perfect arduino for me !
First, I want to drop out a couple of ideas to help developing a new PLC board base on Sam3x.
I can't design it myself for 2 reason: one I don't own a full version of eagle so the board will not fit in the free version, second, I'm clearly not competent! I'm an electrician that learn a basic of programming for fun.
That said, I looking for a board that is versatile, easy to program and powerful in automation and control application. So I own an arduino DUE and use it for a while. It is a lot of i/o and programming is fun. But with Ethernet shield, communication is quite slow. and I thaw that it will be great to have all feature in the same board, without sheild.
Then I saw pic micro's and specially mikroe.com/picplc16
At first, it look so perfect; on-board ethernet, 485, 232, 16 relay, 16 isolated DI, and those ports! for what is missing, they have add-on !!! then back to reality... It was so badly design. heaven 485 and ethernet can't work together without conflict.
So, came back studying arduino, and, reading the data sheet, I notice the ethernet pins that was not used in arduino due and start searching for info and learn that it was once on project...
Finally, my thinking process lead me to this message to suggest the perfect arduino PLC that I can think of...
Now the goal is to be versatile, powerful, and cheap. So probably some feature I suggest could not be possible for that last reason.
Start with arduino due basic design. add it is builtin Ethernet and sd card. I know palliser, Graynomad and other have work on it... but it have been a while. Hope there is development.
next, I would keep only arduino UNO pin headers, for shield compatibility (explain my point later).
Ok, now begin the new thing. One thing that I have found buggy with arduino is fitting on design to another. Often, you have to redraw all the electronic because it interfere once with another because of the ''one on to to another shield''. So I that of a concept that, I think, could help it.
First, I will not use pin that is fitting with arduino uno, so any shield can be reusable on the board.
The problem of conflict is the stackable shield. If they use the same pin, you're f***! That's where, at first, the design of the mikore PLC16 shown above seems so great. They have side by side add-on !
But the design, again, was bad and monopolize to much pins because they use an independent SPI port each time !
Then I merge the two ideas; Shield switchable by pins and side by side add-on.
I think of a 6 pin 2 row header port, VCC5, VCC3.3, GND.
MOSI, MISO, CLK SPI pin.
Add it 5 controller pins for SPI control or 4 bit mux or demux, in case your missing pins...
So if portA have control pin 20 to 24, and portB 25 to 29, heaven if you use the same add on, they will never interfere one with each other.
So in resumé, digital pins could look like this:
-14,15,16 = RS485
-17,18,19 = RS232
-20,21 = 2wire sda slc port
-20 to 24 = mux digital opto isolated input
-25 to 29 = demux relay output
-30 to 34 = portA
-35 to 39 = portB
-40 to 44 = portC
-45 to 49 = portD
another aspect is the analog inputs... In the optic of PLC, standard input signal should be integrated.
I am currently working on an input that can read 0-5 0-10v 4-20mA or 10k inputs. Those are the most common analog signal for probe...
Finally, the only thing that differ from industrial device is power supply. Most are powered at 24Vac. Bu it can raze the device production cost, so I just mention it.
I hope it gave some taste. I will work on the part that I can and if anybody want to get on board...
and I'm willing to put some cash on a kick starter if enough people manifest some interest.