One note: The Brains has a bug in it, in where it does NOT support Bit-of-Word, so until that is fixed you will have to send out an entire word or in, for a discrete bit, then use Bit of word to send or recieve that bit. i.e. b1200.0, b1201.0 (and your watching the zero bit, for the dicrete going to and from Mach3).
I dont understand your question about the Y axis, especially since your Brian you show, does not have a Y axis DRO any where in it, just the X.
Are you wanting to send Axis Position out TO the PLC?, or, are you trying to bring a position in and display it? (note you will not be able to overrite the position DROs since they are updated by Mach3, anything you "Write" to them, would only be considered and Offset at that time).
If you going to do "Floating Point", or "Double words", you will need to break up your Double using "Modulo" or the like in the PLC, or in VB (or plug in), to seperate the interger from the remainder and send each in its own word, to be recombined at the End device be that the PLC or Mach3. It CAN be done, and I have done it for a CNC Tram table but it kinda sucks to do.
Look at your RS Logics docs for how your PLC processes Real or Floating point numbers.
scott