Data Exchange

[Colin Carpenter 10]

A mate of mine has got himself into a BIG problem. He's working on a project that demands a lot more knowledge, mathematics and equation solving than he thought it did. I did warn him, but there you go ..... The job is just at the commissioning phase, and I think that the brown murky stuff is just about enter to the air space of the revolving air handling mechanism, so he has to sort out a solution pretty quickly. The customer demands that the PLC on this piece of plant is an Allen Bradley, so a CompactLogix PLC has been installed. I know how to do all the maths, the equations, the data look up and the problem solving, but I do it using an FX2N or an FX3U. I've never used the CompactLogix PLCs, so to effect a reasonably quick solution, I'm thinking of proposing a "Black Box" solution, namely a FX3U that I can programme and test using the experience and routines that I've built up over the years. The theory is that the CompactLogix would handle all the analogue ins, anlogue outs and the PID control, but that the the FX3U would do all the number crunching to inform the CompactLogix of the setpoints that it should be controlling to. So, what I need is a reliable method of exchanging integer values between the two PLCs. The CompactLogix reads the analogue ins, sends them to the FX3U, which crunches the numbers and sends back PID setpoints for the CompactLogix to control to. Speed is not really an issue ... the system response time could happily be in the order of tens of seconds. So, can anyone recommend (have used) a comms method that will reliably allow an FX3U and a CompactLogix PLC to transfer data between each other, with update times in the order of a few seconds? Thanks.

[Tech Fred 3]

I maybe mistaken but I do not believe there is a communicaitons path between the 2 controllser so you are going to have to use a server to read data from each device and then use a bridging software to move the data back and forth. As luck would have it Kepware has driver for the compact logix and a driver for the FX PLC's The last piece you will need is the LinkMaster product which is an OPC bridging software. You can download full functioning demos of the server with all of the drivers and the LinkMaster from the Kepware Website (http://www.kepware.com). Technical Support is free via e-mail at TEchnical.Support@kepware.com. Fred Loveless Application Engineer Kepware Technologies http://www.kepware.com

[Crossbow 295]

CompactLogix to FX3U is not really an easy mate. Can the CompactLogix do Modbus? The FX3U has modules for Modbus communications. Otherwise an HMI with data transfer capabilities or a PC with OPC will be your best bet.

[WKla 0]

Why not using a user dedicate protocol? Just send over serial port the analog values and the FX3U would answer with the setpoints. The word to be send should have a header, a terminator and sum check if necessary.

[burnerman 4]

I would have thought that the learning curve for establishing communications between the two platforms will be much steeper and longer than learning how to do the number crunching in the CompactLogix. I can't imagine that there is anything that an FX3U can do that a CompactLogix can't. If you get stuck on anything I'm sure that there are people here that will be willing to help.

[andybr 5]

I would have thought that using DeviceNet would probably be the easiest solution but I cannot think of any type of calculation which would be easier to perform in an FX3U than a compact logix. It would seem much simpler to me to persevere with the CLX than to add the extra complication of another PLC to the mix. Almost any calculation you would need to perform can be done simply using the CLX compute instruction. Andybr

[Veganic 8]

I'd stick with the CLX as well. I be a pretty angry client if I'd asked for AB and received a Mitsu. Do you have a HMI on this project? One solution would be to use an E series HMI. Connect one PLC to the RS232, the other to the RS422 and use Controller Data Exchange. (edit - just noticed Crossbow has already suggested this.) But stick with the CLX. Edited 8 May 2008 by Veganic

[Colin Carpenter 10]

I think the HMI is the solution as well, I just wondered if there was another way of connecting the two together. Hopefully the new range of MAC screens will have a CompactLogix driver available. The question regarding who specifies the PLC crops up a lot these days, and causes loads of problems. For "run of the mill" control then I can see that the customer has a case for demanding a certain "site standard PLC", so that he can add things to it, his maintenance guys can alter the code etc., etc. However for a highly specialised application, where the customer expects the programmer to guarantee the results in a measurable way, and to install it in a small downtime window, and to get it right first time or face the consequences, then there is no substitute for working with what you know and trust. For a small one or two man company, imagine the cost to have "up to date" working versions of all the software for all the major PLCs, and to keep up with all the changes and patches and updates that come so quickly now, just so that you can respond to every customer demand for a certain type of PLC. I did one job where the customer demanded a ControlLogix PLC to perform a task that a basic SLC500 or A series PLC would have "eaten for breakfast", but he insisted because it was the "site standard" ... just the CPU and the programming software license cost more than my car!! I always think it's a bit like me deciding to splash out on a new BMW M5, but then telling BMW that I didn't like the brand of controller that they used for all the car's engine management and other systems, and insisting that they change it all to another brand and that they guarantee the performance of the brand that I insisted on. On a highly specialised software package, that the customer will never touch, isn't it better sometimes to accept that the supplier knows best?

[Veganic 8]

All heartfelt and agreed but it's not me you have to convice. But on the other hand if you client is set up for AB you'll always be the idiot that supplied the wrong unit and whatever goes wrong will be because of that, right or wrong. Your friend must have the software in this case though? Mitsu can do devicenet on FX2s I think. Let us know how it goes. Edit : The quickest dirtiest and cheapest solution that would at least buy some time would be as follows? You want to send integer values, say 16 bit? use a 16 point output and 16 bit input on each PLC. Add additional points as required to inform the other PLC of what the "data register" is. Send the info as a 16 bit binary, convert to integer, calculate and send the result back. Depending on the total number of values to be sent you might be able to think of some 'protocol' that gets the number of bits right down. And Easily 'do-able' in your time frame. What do you think? Edited 9 May 2008 by Veganic