Modbus message two's complement

[AdkAndy]

I am using Lookout to control a variable speed drive via Modbus ethernet.  If I send a number from 0 to 20000 to address 400002, it adjusts speed in the forward direction from 0 to 100%.  I need to send the "two's complement" of that number to control reverse speed.  Simply sending -20000 is not producing reverse direction on the drive.  How do I send a two's complement of a number?

[JimLight]

I need to use Lookout to control a variable speed drive by using two's complement for the negative direction.  It appears, the Lookout data member does not allow you to enter a negative number to write to Modbus 40002 or 40003.  Any way around this?

[JimLight]

Were you able to find an answer to the 2's compement problem?

[Mike@Forshock]

Use the float datamembers

[JimLight]

Thanks for this.  Can you be a bit (pardon the pun) more descriptive as to how a floating point number ends up in Modbus 40003 as two's complement?

[Mike@Forshock]

I myself do not have the time to make an example, but this could help point you in the right direction.

http://www.cs.cornell.edu/~tomf/notes/cps104/twoscomp.html

http://planetcalc.com/747/

http://twos-complement.calculator.io/

[AdkAndy]

Basically what I discovered, behind the math geeks' mumbo jumbo, is that any binary number greater than half of the range is the equivalent of a negative number.  So for a 16 bit integer, which has a decimal range of  0 to 65535, any number greater than 32767 is a negative of a number below 32766.  This of course limits the decimal range to -32766 to +32767.  

If I want to transfer the value +10000 the integer value would be 10000.  If I want to transfer -10000 the integer value would be (65535 - 10000) which is 55535.

If I want to transfer the value +25535 the integer value would be 25535.  If I want to transfer -25535 the integer value would be 40000. 

Let me put it a more intuitive way.  Positive numbers count forward from zero.  Negative numbers count backwards from 65535, or whatever the top decimal number of the binary range is.

[Mike@Forshock]

Glad you got it figured out.

And thanks for posting a solution

[JimLight]

After some googling of 2's complement and significant trial and error, I managed the same result.  I like your explanation but one still has to shake his head a few times while scaling this math.  I am working with a ABB ACS 600 drive.  ABB was no help in this matter.  I will let them know the solution.  Thanks

[AdkAndy]

That's funny.  The original question was from when I was working with an ABB ACS 550 drive, and I found ABB to be no help on this question as well.  In every other question for which I have called their tech support, they have been excellent, but this one issue was a dead end.  "Just send over the two's complement"  "What is that?"  "I don't know, just send it over"