How can I send negative numbers from the NI Modbus VIs?

[Jimbo81]

 I have tried Altering the format of the numerics in all the Vis and my hardware is still receiving the positive version (65535 - the negative number).  I know that the device I am connecting to can receive negative values because bundled with it is an example application.  I can't however deconstruct this to see how it was done!

Any ideas

Thanks

Jimbo

[Khalid]

Hi,

Is it the device that's reading the number "incorrectly"?  Or your Modbus VIs?  Technically, the negative number does get stored as the result of 65536 minus the_negative_number.  So, if I were to write -3 to a Modbus register, it actually gets the value of 65533.  It's up to the device, and possibly your reader VIs, to interpret this as a signed number and display a -3 (and not 65533).  This is because 65533 (b1111111111111101) can represent either 65533 or -3 (two's complement of +3 using 16 bits), depending on how we interpret it.  This last part is the key:  it depends on how the number is interpreted.

I hope I am not confusing you even more.  You should try and find out if any specific registers are used for negative numbers and use those.  Very likely, the device will interpret only those registers as signed numbers. 

Good luck!

-Khalid

Message Edited by Khalid on 12-01-2006 02:22 PM