Which PLC for 32 low voltage inputs and 32 240V AC

[Guest Thomas Björklund]

I have checked a few of the large manufacturers' web pages, but I have a hard time making up my mind about which model to choose. This is the reason why I'm posting this message, since I hope that you guys might be able to provide me with some valuable input. What I need is a PLC can be wall mounted (no bare circuit board), preferably on a DIN rail. I need approximately 32 low voltage inputs, 32 output relays that can handle 240V AC and a few amps each, and four PWM-capable low voltage outputs. What would be nice to have (not need to have) is the capability to have a distributed solution, so that all I/O does not necessarily need to be located at the same phycial location. Further, it would be nice if one could connect two PLCs of the same type, and let them share I/O. The Siemens Logo! seems nice, and there are plenty of them available on eBay. However I got the impression that it does not have any PWM-capable outputs, and the analogue modules are quite expensive. I would appreciate alternative suggestions of which PLCs/PICs to use, preferably not too expensive. Thanks!

[TConnolly 13]

The only PLCs of which I am aware that can share physical IO between PLCs is the Allen Bradley ControlLogix. And even in the CLX, only one processor may own an output point - though all others can see its status. There might be others. Otherwise, you cannot share the IO - however you can send information back and forth that allows one PLC to signal another PLC to turn on an output - or you can send a message from one PLC to antoher that a particular input is on. But that is not quite the same as sharing IO. Edited 15 Sep 2005 by Alaric

[GerryM 2]

What part of the world are you in? That has alot to do with your choice if service will be required. Also, are you doing simple logic, motion control, etc.? They will affect your choice as well. I like Automationdirect.com plcs. They do not share I/O but can communicate, and they have remote I/O, I'm not sure about PWM outputs. However, there are many others that i like and would work fine as well.

[gravitar 3]

Well this is probably too crude to be considered as "shared" I/O, but I would think that if you wanted two PLCs to see the same input, you could just wire them in parallel to the sensor. Depending on the current sourcing capabilities of the sensor and the input impedance of the PLC, you likely could wire one sensor to several PLCs. Many years ago when ControlLogix was first introduced, I recall shared outputs as being a hot-button issue. I remember it being said that there's no technological limitation in doing so, and A-B experimented with allowing control of outputs to be shared, but that capability was removed because of the safety implications. I don't think this dilemma has yet been solved. The original poster mentioned wanting the ability to "wall mount" the PLC. I visualize that as a free-standing component, without an external enclosure. Such PLCs might exist, but I have yet to see one. A-B is starting to push in the direction of "on-machine control", which emphasizes standalone I/O modules not requiring an enclosure.. I believe at this point though, they do not offer a controller in this form. Might just be a matter of time. Maybe he should look at an Opto22 Optomux type system? He could mix-n-match I/O as he pleases, and control it from basically whatever he wanted. If he used a PC, he could then share the I/O as he wished with other PCs, PLCs, etc. What is the application for this control system??

[aldarin 0]

Well, There are plenty of choices you can use in your project. I have an idea for I/O sharing: In fact I/O shared configurations are quite expensive. But you can use a network among processors and share I/O among them by using link registers. Link registers are registers that are used to communicate with other plcs on the an network. For example if one of the processor receives X0 you can send this Input but asssigning X0 to L0. On the other side if processor receives L0 you can understand that X0 is pressed on the other side. Mitsubishi is a quite good choice in Europe. Units are not as expensive as other brands like Siemens. You can use CClink for linkiing among processors. For more information you can look at Mitsubishi forum. On the other hand GE fanuc series is another choice. You can share I/O with Genius Bus. Best wishes.