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  • USB input to simple PLC

    Hi guys,

    Question here on how to tie an input that would be USB (output from device) to a simple PLC/pump that has 4 to 20 mA, 0 to 10 VDC, high speed digital pulse, or contact closure pulse as its inputs. I'm new to this type of automation, is there a converter that needs to be incorporated?

    Thanks


  • #2
    Can you rephrase the question? I'm not exactly sure what you want to do. Device with USB output connected to a PLC? What is the device?

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    • #3
      I'm sorry, Im still very new to this. Essentially, I want to connect a balance that has a USB to a metering pump that has digital I/O. The input to the pump (from their specifications sheet) is 4 to 20 mA, 0 to 10 VDC, high speed digital pulse. How would I connect to that or adapt from the USB to that? Also, if there is anything you recommend on me reading to familiarize myself more with how these things are hooked up and work, I would gladly read it. Thanks

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      • #4
        I do not know of a device to do that.
        I believe that you will need a different scale (balance).
        You will need a scale with either: Serial out, Ethernet out, or Analog out (4-20ma, 0-5/10v).
        You can then place a PLC between the scale and the metering pump that will convert/scale the signals.
        Circumstances don't determine who we are, they only reveal it.
        Jason Wolthuis

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        • #5
          USB is usually used to interface to a PC and there is probably PC software for your scale. As plcnut said, you will want a scale with an output designed to go into other units like PLCs sending either serial messages or a direct analog out. I find direct analog signals much easier to deal with in PLCs than taking in an ascii message from a scale, which rarely have good documentation on how to implement them.



          my own "hack" solution I used on some scales that only had USB output was to solder wires directly from the scale's LEDs in the unit that light up when the scale detects a "good", "over" or "under" weight product based on what I set in the scale during startup. Then I used those voltage signals from the LEDs to switch a solid state relay that converted to a 24v signal and sent that into my PLC. That way the PLC would get a good, over, or under signal and do logic based on that. Not nearly as good as a true analog signal, but at least I could know when the scale was at the weight I was looking for and then do logic based on that. The LEDs in these things are usually just a coiuple volts, which is why you have to use a SSR to change from that low voltage into a higher voltage the PLC can use as an discrete input signal.
          Last edited by MikeN; 04-04-2018, 12:36 PM.

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          • #6
            While I agree that the USB output is most likely meant for a connection to a "PC", a bare bones PC my be much cheaper than a new balance, though one would need some sort of output device there are undoubtedly USB to analog converters on the market. If there are LINIX drivers available one might be able to use a $30 Raspberry Pi

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            • #7
              OK, after looking at the specific scale, it also has an Ethernet out. As MikeN stated, should this scale be analog assuming everything is equal? This will be for a project that could possibly be implemented campus wide. Looking for the most efficient/simplest way.

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              • #8
                A BRX PLC should be able to talk to it over Ethernet.
                Can you post a link to the spec sheet for the scale?
                Circumstances don't determine who we are, they only reveal it.
                Jason Wolthuis

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                • #9
                  Sure, here is a link. The manual they offer is for all of their LA line of scales. I believe at the bottom of the page they offer an digital to analog converter. Is this something that would need purchased?

                  http://www.affordablescales.com/sart...p#.Ws5eJS7waUm

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                  • #10
                    cyaroschak, the problem you're going to run into is that it looks like the scale has internal calibration/offsets, etc. You probably don't want to be reading the raw signal coming from the scale because then you'd need to mimic the signal conversion functionality in your PLC program.

                    The scale does have an RS232 port which can be communicated directly with the PLC.
                    In your PLC you would do the following:

                    In a loop:
                    * Send a command to the scale - (it looks like something like \ESC\l\CR\LF will request the current readout from the scale)
                    * Read a response from the scale - (it'll probably be a string of characters something like "200.00")
                    * Convert the response to a number.
                    * Adjust the output to the pump (0-10V is the easiest to understand, I think) based on your formula.

                    You can buy the scale interface cable & connect it to your PC to play around with the commands through a terminal program (like putty). When you're ready to connect the scale to the PLC, you'll probably have to make your own cable (since the scale has a 25-pin DB25S-type connector on it where 6 of the pins are used for RS232 communication) and the click PLC has an RJ-11 6-pin connector). You'll probably need a $6 adapter like this one: https://www.altex.com/Modular-Adapte...D-P145444.aspx

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                    • #11
                      Originally posted by GP-FBD View Post
                      and the click PLC has an RJ-11 6-pin connector). You'll probably need a $6 adapter like this one: https://www.altex.com/Modular-Adapte...D-P145444.aspx
                      While the price of a CLICK is nice, if you were to use a PLC you might be better off with a BRX, there might be problems with a CLICK switching from send to receive fast enough to catch the reply, I think/hope a BRX will be better at that (damn well better be, this is the twenty-first century! 1)
                      While a BRX is expensive compared to a CLICK, it is still much less than the Sartorius Digital / analog converter.


                      1: Actually, the CLICK is a 21'st century product too, WTF Koyo!

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                      • #12
                        Originally posted by Tinker View Post
                        While the price of a CLICK is nice, if you were to use a PLC you might be better off with a BRX, there might be problems with a CLICK switching from send to receive fast enough to catch the reply, I think/hope a BRX will be better at that (damn well better be, this is the twenty-first century! 1)
                        While a BRX is expensive compared to a CLICK, it is still much less than the Sartorius Digital / analog converter.
                        !
                        BRX has buffered serial ports so there should be no issues with switching from send to receive and vice versa.

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                        • #13
                          Originally posted by Do-more PE View Post

                          BRX has buffered serial ports so there should be no issues with switching from send to receive and vice versa.
                          gotta love tech advancements!

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                          • #14
                            Originally posted by MikeN View Post

                            gotta love tech advancements!
                            There goes Host...dragging everyone kicking and screaming into the '80s.

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                            • #15
                              I appreciate the information, thanks guys.

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