Modicon to Micrologix

[PhilipD 0]

Hi all, I work as a maintenance mechanic in a factory that produces corrugated (cardboard) boxes. We have a machine that uses three Modicon 110 CPU 311-01 brick type PCLs. One acts as a "parent" and the other two (each in a separate cabinet) are "children." My boss want me to convert these to a AB system and cost is a big factor. I would like to use three Micrologix 1000 units so I won't have to pull wires from the two "child" panels into the "parent" panel. Am I making sense? Can the Micrologix units be configured this way? Thanks.

[Mike Lamond 1]

Hi Philip, The parent-child communication link was unique to the Modicon 110 Micro CPU line. It's separate from the Modbus port and connects a parent to one to four children at 125kbps by configuring registers to transfer. The MicroLogix 1000 has no equivalent, just a single RS232 port. You can create a substitute with three ML1000 PLCs by using a 1761-NET-AIC module on each one to create a DH485 network, but that brings your cost up into the range of an ML1100 or ML1400 with built-in Ethernet. These two PLCs also support on-line program editing, with the ML1000 lacks. With a four or five port Ethernet switch, you can plug in your programmer and watch all three programs at the same time. Ethernet is pretty much universal now, so that would drive my decision toward the ML1100 or ML1400, depending on the I/O count required. Mike

[PhilipD 0]

Thanks Mike, the information you gave is a BIG help. I do have another question though. If using three ML1100 connected with an Ethernet can unit 1 examine an input from unit 2? Or can unit 3 turn on an output in unit 1? Also, will each unit need it's own program? Or, will unit 1 be the master and units 2 & 3 act like remote I/Os? Again, thanks.

[Mickey 71]

can unit 1 examine an input from unit 2? Yes, by way of a message instruction you can monitor the data between all processors if networked together as recommended above. Or can unit 3 turn on an output in unit 1? Not directly, each PLC controls its own outputs. Also, will each unit need it's own program? Yes. Edited 21 Nov 2009 by Mickey

[PhilipD 0]

Thanks Mickey, Because there is so much going on between the Modicon modules its looking like the Micrologix might not be the best choice. Or I could use one unit and pull additional wires. Plenty of time to think about this -- thank goodness.

[paulengr 44]

A Micrologix PLC "can" control the outputs on another PLC using MSG instructions. You have to do it as an entire word. Simply configure a write to go to the output image table. However, this is rarely done. It is far better normally to add an indirect step so that you can selectively modify things in the future. The suggestion to pull lots of wiring sounds like a very bad idea. First, you risk the chance that all those new connections and wiring might have a mistake in it. Second, you will have to wade through it all while troubleshooting. Third, that is a LOT of labor cost which dwarfs the cost of the devices that would be added with a distributed system. There is nothing wrong with using a Micrologix PLC directly AS "smart" I/O. At under $500 for a 6 output/10 input/2 10-bit 0-10VDC analog input "combination I/O" block with the capability of doing expanded outputs/inputs, you can't really beat Micrologix 1100's as "smart I/O". The one exception is the Point I/O series if you need highly distributed low density I/O or the CompactBlock series for medium densities. Best thing to do is to get on the Rockwell shopping site so that you can quickly get list prices. Start pricing out various options and see what you can do with various combinations of equipment. One thing that I have found in general is that there are two areas to look at on such an upgrade. One is operator panels. Old style operator panels often have dozens of buttons, at roughly >$300 per I/O point by the time you count the button itself, wiring, I/O cost (per point), and labor. They are also very expensive in terms of maintenance costs. A simple touch screen display can replace all of that for a fraction of the cost. Another big area is in MCC's and other huge blocks of coils such as hydraulic or pneumatic panels. It is frequently better to look at more integrated options iin those areas with a single communication cable (and maybe power) attaching it. Even if you have to wire, you might be down to one or two armored cables and get rid of all that conduit and wiring. Doing this level of integration probably pushes you into using a CompactLogix processor but the troubleshooting cost reduction is worth it. Especially if you can get diagnostic I/O cards that automatically detect short and open faults at a cost less than putting in individually fused inputs/outputs. Also if you mess with drives, consider upgrading the communication capability in them. I just took several 1336 Plus II's out and replaced them with Powerflex 40's. Other than power wiring and an Ethernet connector, ALL the control wiring disappeared. In it's place, we got full diagnostics and can even troubleshoot remotely directly off the Ethernet port. It is a good deal more expensive but you can probably run an entire production LINE (not just individual machines) with a ControlLogix PLC as well. I've run entire plants that way. It's asking a bit much of a Micrologix to do that level of integration but no big deal with a ControlLogix PLC.

[PhilipD 0]

Paul, Thank you for your input on this. However, I must disagree with you on pulling new wires being a mistake. Each Modicon module has 16 inputs and 12 outputs so I would have to pull 28 wires for each unit. Each wire would be about 20' long. I would not have to put in new panels, buttons, switches or anything else so (depending on the PLC selected) this could be the cheapest and easiest way to go. There is also the option of doing nothing. This machine was down for about a week last month because one of the modules quit working. We found a module with no problem but we had to send it to Memphis to be programmed but it is a new unit and should last for a good while. My own view is that we should replace these units with a SLC-500 or Micrologix because we have the resources in-house to maintain them and replacements are pretty easy to find. I don't want to give the impression that I am a PLC programmer because I'm not. I have quite a bit of experience with the SLC-500 in troubleshooting mode. Hook up the laptop, find the problem and correct it. I have made a few changes to programs but I am way below the programmer standard. Again, thank you.

[paulengr 44]

The entire SLC and PLC-5 lines are on life support as per Allen Bradley. Should not be used for any new projects. If you do go down this route, it might be further down the road, but eventually you will be back in the same boat you are in with the Modicons, you've just delayed it by a few years. Although "nothing is forever", AB has made no noises about ending the Micrologix line and the 1400 model was just introduced last year, and the 1100 a couple years ago. So you should have at least 20 years before you'd have to worry about changing PLC's, they are off-the-shelf items at AB distributors, and they are very inexpensive. So for all those reasons, I would definitely not recommend a SLC-500. Second, you can't just plug a PLC in like you can with a relay and expect it to "go". On the other hand, programming PLC's is not nearly as big a learning curve as say learning C or Java, but they do have their own little tricks that can bite you if you don't know how to deal with them. For the record, I do understand and can appreciate box shops. My dad's farming partner ran the maintenance shop at a Union Camp operation in Michigan (the guy farmed as a part-time job on the side). If you were at a chemical plant or something like that, I'd be saying something completely different about pushing the RS-Logix 5000 stuff. But the "entry fee" to start using Logix 5000 including training, a bench unit, and licenses is over $10,000. You will need a major project to justify that kind of expense on cost alone.