Instrument Control (GPIB, Serial, VISA, IVI)

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Send Hex string over Modbus to SMC Actuator

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I am trying to send a hex string command to multiple holding registers via Modbus to an SMC traverse. I know I need to convert this string to a byte array, but I cant use the "string to byte array" VI because it interprets as ASCII.


I have attached the VI with the command inside, I need to pass it to Multiple Holding registers VI.










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Message 1 of 13

What would happen if you would change the display format of your string constants to hex display (right-click) and re-enter your data? Similarly, formatting your number as hex is probably incorrect.

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Message 2 of 13



I'm formatting as Hex initially because I want to be sure that I am sending the exact thing that is in the manual.


If I convert the string constant to Hex display and then paste the data in, what would I do with that output?


Also, The only part of the string that needs to be variable is the (3) Position input where 30000 (300.00 mm e.g) translates to 00 00 75 30.


Thank you for your help



EDIT: I see, now I can directly connect the hex string to the String to Byte array and it interprets it correctly. Can this Byte array then be directly sent to the Write Multiple Holding registers VI?



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Message 3 of 13

Sorry, I am currently not at a computer with LabVIEW, but there is a big difference between:


  1. Hex formatted string, i.e. a string containing exclusively ASCII characters 0..F
  2. A binary string where all values (0..255) are allowed and that looks correct if the display is set to hex display.

You want #2.

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Message 4 of 13



i understand now. I think I have gotten it to be in the correct format. So the move command is in two steps, Step 3.1 is to write the move settings, and 3.2 is to initiate the move. I have attached a snippet. However, nothing is moving. I should mention that I know I am communicating with it correctly because I am able to Home it through boolean (coil) flags.






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Message 5 of 13

This is still a horrible mess.


  • Please show the display style for all string diagram constant.
  • Your uninitialized shift register will grow forever because it contain stale data. It should be U8, so wire the default tile
  • You probably want to cast the position, not format it.


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Message 6 of 13

See if this can give you some ideas....




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Message 7 of 13

Haha, I wish I could say it wasn't a horrible mess. Just trying to get a proof of concept before I start streamlining. Yes, your example makes a lot more sense. That will result in a 1D array of  8bit bytes, but the Input of the Write Multiple Holding says it normally accepts 1D of 16-bit words as the register values. The controller I am using runs on 8 bit length.


Would U8 still be correct?


Thanks a lot for your help


This snippet didn't result in any movement.




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Message 8 of 13

Well, you can cast the string to an U16 array equally well. Don't forget to calculate the checksum and append it.

I did not validate the rest of your strings, so check the manual.

Notice the red coercion dot at the IO function? If you hover over it with the mouse, what does it say it expects?

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Message 9 of 13
Accepted by topic author Lazer723



So after much fiddling I've got it working. There were a few key things that needed to be known. The modbus Write Multiple Holding accepts only a 1D array of 16bit words (essentially two 8 bit bytes put together), simply casting my 8bit array to U16 would only pad each element with an extra 8 bits of zero, when what should be done is to combine each pair of 8 bits into a 16 bit word. So [11,22,33,44] would become [1122,3344] instead of [0011,0022,0033,0044].


I found it easier to go back to regular strings and loop Hex to 16bitNumber and form an array rather than reshaping the existing U8 array.


This works because the modbus VI actually does a lot of the heavy lifting, it splits each 16 bit word into two U8 bytes before sending it to the device. Also, I needed to remove any preamble to the hex string, (the number of words and number of data elements), and only give the exact values that are to be written to the registers, the modbus vi appends all necessary preamble, and indeed CRC bytes all by itself.

I found this out by opening up the modbus VIs and went digging.


So in the end here is the snippet that worked. Allowing me to set the speed and position and results in actuator motion. (All this is inside a larger VI that initialises the modbus master etc.)

Move working.png


Thank you for your help!


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Message 10 of 13