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  • PLC help

    I am a novice in the area of PLC' well as many other areas. Can someone knowledgeable in the use of these devices give me a few pointers? I am looking for a small inexpensive outfit to control I/O's between a couple of home brewed CNC machines and devices controlled by relay. I am told that allen bradley makes an inexpensive unit with software, but would like some help deciding what to purchase. Educational perpose, not industrial.


  • #2
    I'm not familiar with AB's smaller controllers, I believe they're discontinuing the micrologix though - in favor of a newer line that will use the rslogix 5000 programmng suite. Nothing inexpensive about that.

    For small, limited I/O applications, I've had very good experiences with the Koyo brick plc's. Depending on your requirements, they have models as cheap as 100 bucks (DL5's which I think have something like 8 in and 6 out). The next step up is an expandable brick that goes for something in the low 200's depending on AC or DC, relay outs and stuff like that (DL6's).

    Their basic 'demo' software is free and will get you going with fairly simple programs. I think it's limited to 100 lines of logic, which for a small simple app is PLENTY.

    Get a count of how many inputs and outputs you need to control - that will determine the size of PLC you need. Always buy something that will take a couple extra beyond your initial estimate cause you alwayse forget one or two. A plc that's expandable, like the AB SLC series (outrageously expensive) or the Koyo DL6's will give you added flexability there cause you can add cards to accept more inputs and outputs - plus they can be a different type than the rest of the controller. So if you have a PLC with DC I/O you can add an AC input card down the line.

    The next thing you need to consider is whether you're looking at AC or DC inputs and outputs. I recomend relay outs because they're more flexible as long as you're not doing any super high speed switching (in the kHz range).

    Most PLC's come in flavors enough to cover any need... AC, DC, Relay, and any combination you can dream of.

    PLC's are a BIG subject that could cover many pages, but this bit should get you started. If you have any specific questions, feel free to PM me.


    • #3
      Matt, when you say "educational" are you after something off-the-shelf to run educational CNC, or are you after something to teach logic design and programming from the ground up?

      If the latter, I suggest you have a look at the picaxe series from Rev-Ed. They are designed with educational environment in mind.
      Just got my head together
      now my body's falling apart


      • #4
        Do you really need a PLC

        How complex is the project you are talking about here? If you only need a few I/O you might be able to do it just as cheap with a couple relays. Most PLC's require you also but some software to do the programming. You might look in and search for PLC. There are a couple that will work on a PC (kind of large gun for the job but who doesn't have several of them sitting around) or you could create an application in some other computer language and use the parallel port to interface with he real world.

        Personally - I would recommend that anyone looking at starting a project with a PLC look at the CP1L series from Omron - lists for $250 - only need to install a USB driver to program - uses standard USB cable - it helps to use Binary mode instead of BCD. The software lists for $795 - includes all products in that price - PLC, HMI and special functions - and all the PLC's program using the same instruction set. Can get a demo version of the CXOne software that runs for a month (or even longer if you reset the date on your computer...) I have also used the PLC Direct DL6 PLC - Software is cheaper - you have to get used to octal numbering systems, software is cheap, I got annoyed with the math in the application I was using it in. I have used Allen Bradley also - nice stuff, but can be pricey. Siemens - way too expensive, Software is way overpriced and is plain difficult to use.

        Omron also makes a small "Programmable Relay" PLC they call the ZEN - that might be the way to go -

        Don't know where you are located, but I could give you a contact number for an Omron vendor I used to use.

        NO - I do not sell, obtain commisions or otherwise benefit from recommending this stuff. I have just found it easy to use and very cost effective.


        • #5
          Hi Matt,

          I have done a lot of PLC work and I agree completely with Robbie. PLC's are like the Ford / Chevy argument... Just about everyone has their favorite brand and the ones they hate for various reasons. Personally I dislike Allen Bradley mainly due to the price of the equipment and software. AB stuff works very well but you pay for it. I like the stuff from Automation Direct (formally KOYO). They have a great line with everything from very basic low cost, all the way up to highly advanced higher cost. The software is reasonable and like Robbie said they offer a demo software package for free. The demo software is only limited by the size of the programs you can create. All of the PLC functions are available so you can learn. I think they still have "starter packages" available which have the PLC, cables, software and test I/o (switches and lights) so you can get started for a fair price. Also, all of Automation Directs manuals are available for download free so you can read about their stuff and get some ideas.
          The first thing you need to do is define what you want to do, the I/O count needed, the voltage of the I/O, output type (relay or Solid State, I agree relays are more versatile for lower speed), and expansion capability if needed. For small dedicated applications I really like the "bricks", DL05, DL06 or DL105 series units. They are very simple and easy to use. They can even drive display units if needed. The DL05 and DL06 even have some expansion capability. I have used these units for everything from very simple pump station control all the way up to very advanced process line control with hundreds of I/O points. I have been very pleased with both the performance and the reliability (not to mention the profit ). Take a look at the offerings and jump in with both feet, you will learn a lot and hopefully have fun doing it.

          Good Luck,

          Normal disclaimer: I am in no way affiliated with Automaton Direct, just a very satisfied customer / user of their products.

          Happily working on my second million Gave up on the first


          • #6
            I am trying to control a handshaking activity between an Eshed robot, a couple of mills and possibly a homemade Router. The idea is to have the robot load/unload the different machines and not crash into each other because they are communicating through the PLC. I have background using basic stamps and could easily handle doing this with a stamp, but I want to learn PLC's. As this will be teacher financed, low cost is a primary consern. I would estimate that 6 DC input and 6 DC output would do fine to start. Most of the devices will be "low" 12-24 voltage DC devices.



            • #7
              Notes from a old electrician.

              Take a note page, lay out your IO in logical progression to you.. ON the first page of the plc program, Make a dummy rung and use every contact and coil, name them all in the notes..
              That way as you write the program and enter a coil or contact point the name pops up there when entered showing logic mistakes immediately..

              Think "If", "NOT", "then".. some plc programs have done away with relay logic all together for boolean logic.. Icon based logic that engineers understand easier than a bunch of old school relay contacts.

              TURNING ON SOMETHING? you can poke/push/move a binary number into a output card writing to them just like a old timey sequential drum switch. Does wonders for making the next guy crazier than ever, don't document it for even more fun and laughs at the young guys. I took a six hundred page program and did it with forty pages of this move logic..

              I wrote a batch file for plc starting up, reloading, and which cables to use for which plc for DIamond mills where I was a overworked tech.. SOON.. the $8 a hour man could reload the plc programs as easy as me.. I got laid off. I don't hold a grudge thou.. you think? (I'm still pissed)

              I got a couple old AB-slc's here.. NONE are 232 or USB compatible.. ya are welcome to one.. they are cheap on ebay.. use a hand held programmer (off ebay).. OR a pic and expensive software.. I also got one here I got a tad over $1500 in.. ya can have it too, for $1500...
              Excuse me, I farted.