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Cheap DRO scale specs

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  • Cheap DRO scale specs

    Anyone come across any specs on those cheap import DRO scales. The kind Cal Aero sells. They are 4 pin outputs, +V, gnd, Clk and data. I can't seem to find any specs on these things. Have read that they are "similar" to the Mitutoyo Digimatics, but think they differ. I am looking for the output protocol.


  • #2

    Can't help with your question, but I just asked a similar question on Kinchrome digital verniers, maybe they're the same ??



    <font face="Verdana, Arial" size="2">Originally posted by JasonW:
    Anyone come across any specs on those cheap import DRO scales. The kind Cal Aero sells. They are 4 pin outputs, +V, gnd, Clk and data. I can't seem to find any specs on these things. Have read that they are "similar" to the Mitutoyo Digimatics, but think they differ. I am looking for the output protocol.

    Kind regards



    • #3
      They use the SPC output common to many digital calipers, etc. It is an industry standard, but the readers and software cost way more than I'm willing to part with.

      I just mounted a set of these on three axis of my Bridgeport mill. They do great as a dirt cheap DRO. The only bad thing is they need to be mounted such that the user can read the scales directly. I've installed plexiglass shields to keep the chips and fluids off them.

      I'm hoping someone with a better understanding of the SPC output comes out with a freeware or shareware solution to hook them to a PC. I'd like to eventually set up a remote display and add some simple functions like over the counter DRO's have. I just am not computer savvy enough to do it.


      • #4
        Check out I have emailed the author, Matt Dralle, to see if he would be interested in helping with a remote DRO readout project, and will post his reply, if any.

        Rather than use a PC, I would be inclined to use a Basic Stamp ($49) and a 4 x 20 backlit LCD display ($99). These components could be packaged in a small box, and could provide readouts for up to 4 DROs for under $200.

        I would be interested in working on this project. I can design and make printed circuit boards (might not be needed), and have a dual-trace analog oscilloscope. The biggest problem will be determining the device's protocol. Does anybody have access to a storage scope?


        • #5
          I know it is SPC, but it is Chinese SPC. Which could mean anything.

          I wanted to get the protocol in order to make a remote display as well. I have scoped the Clk and data outputs. About a 76 KHz clock with some form of data. Hence the need for some documentation.I will spend many hours on the scope in order to figure this thing out, if I have to.



          • #6
            I did a little research on this topic few months back when I was thinking of using inexpensive digital calipers with SPC output as an expensive DRO for my machine but dropped the idea because I found that the cheap one will actually miss count. Play around with them and you'll notice that at certain range, no matter how finely you move the jaw you can't get it to stay at a certain reading. It will alway read one thou more or one thou less.

            To my surprise I found that Mitutoyo SPC (binary) and Starrett SPC (Ascii) protocols are different. The Mitutotyo interface seems to be more popular and it's one that I'm more familiar with. The 4 wire interface are for ground, clock, data, and request (no power because both sides are independently powered). The data is comprised of 13 nibbles (4bits), with LSB being transmitted first. Data can be read either on the rising or the falling edge of the clock (it's more like strobe). Only the 6th and the 11th nibbles contain the reading. The other nibbles are used for unit, preamble, sign, decimal place, etc. As far as I know the signals are TTL compatible so direct interface to uP or the printer port should be possible.

            I agree with alsinaj that a simple embedded display unit would make a lot more sense than using a PC. I would however avoid using LCD as display as they simply don't have the size, brightness and contrast necessary for the machining environment. LED or ELD display are better suited.

            If anyone wants to undertake this project, I would be glad to provide any help.



            • #7
              I was planinng on using an LED display (7 segment display). Driving these with a CD4511 driver/latch. Data to all displays using a 4 bit bus, and a 3:8 line decoder(HC138) to control the enable input of each latch.

              I have not seen any unstable data on the display yet. I would not complain too much about .001". It is still better than the .010-.015 " backlash in my lead screws.

              I have noticed some unstable data in the SPC data line. This does worry me, but if Cal Aero is selling a remote LCD unit for these things for $129 US, there must be a way of getting some decent data from these things.



              • #8
                This doesn't help any of your efforts at decoding the protocol, but I think it might be of interest to others reading the thread. Wholesale tool lists a three axis display for the scale units:


                They are asking $250 for the unit, which prices it out of consideration for me. After buying the three scales for around $270 on eBay, I might as well have gone for a Shooting Star DRO if I payed that much for a display.

                Also, using an old PC still seems like the best solution. For the price of the hardware you'd have to purchase to make a basic display and enclosure you could probably pick up an obsolete machine that has the computing power to run bolt circles, tool offsets, 10 digit keypad, etc.

                Of course I say this with the complete knowledge that I have nothing to contribute but this wish list of possibilites. Whatever direction the more capable contributors take this I'm interested to see what becomes of it and applaud your efforts.



                • #9
                  I bought one of those off of ebay myself with the same plan. It seems that there is a real blank wall out there when to comes to some company actually putting the SPC protocol up on the net.

                  I am finishing up a digital tach for the mini-lathe/mills that can be built for around $20, a lot less than the $70 they're going for now. Next I was going to build a DRO head based on the PIC line of miniprocessers. They cost about 1/7 what a basic stamp does and run code a 1000 times faster. I just needed to find a copy of the SPC protocol. I don't have a logic analyser yet which would make it much easier to hack the output of the dro.

                  I understand there is also a command set to send back to the dro's to zero them, change mode, etc...

                  Shopping list: anyone know where to get some cheap plugs/cables to hook these up? They don't look very standard.

                  Have you got the data stream mapped out? Can you post it? Signals, etc...