With apologies to Dave, i have answered this question a number of times. Not all D4 controls are equal! There was a early version that couls not do helical milling .
Later the control was improved to give extended features including helical milling.(true 3-D interpolation)
The difference is not software, but rather hardware. Later D4 controls have an extra circuit board and an improved NPP board.
For details see post #49:
First Time CnC, Contour 3 help needed!
As to drip feed, again this has been answered, the short version is that D4 can not do "drip feed" under any circumstances.
D4 can,with the correct software and hardware, do literally unlimited program sized work, but its not like any other control out there.
Here is hoe it works:
You first load the program into some proprietary soft ware on you computer.
Use the software to split the program into bite sized chunks that are file marked in a sequential manner , and hold the small program chunks in a file within the splitting program....
Enable the DMC feature (mode 13) Connect the RS232 cable (proper connection)
Then Switch to mode14 and call the split program from the control.
Computer loads the first several small programs from the splitting program into the control's memory.
You start running the program, same as when operating any stored program.
Machine executes the program...as it finishes each small segment, that segment is deleted from memory, and a new one is called up from the computer....its a sort of fill the bucket, empty the bucket thing.
Works fine and allows literally unlimited file size and unlimited blocks.
The nice thing is that because the memory holds more than one small segment, there is never any delay waiting for the file to transfer from the computer....the memory acts as a buffer.
Have successfully run programs with over 80,000 lines of code with zero issues using this setup. .....
For my money the biggest advantage that D4 has over D2 other than the helical milling, is the overall tool management and cutter comp differences.
D2 can only use cutter comp in the block its called, and all tool offsets are local.
D4 uses a global setup that applies cutter comp to length and diameter any time the tool is called.
D4 can run multiple programs off memory. D2 allows storage of one program only and requires tricks to overcome this limit.
D4 has "paramater" programming, the ability to use variables and math operations within the program (like an Excell statement)
As stated above, D4 can run an MPG , and has graphics , a speed/feed calculator and graphics for canned cycles like drilling and boring etc...
Oh and D4 has the RS232 interface allowing up and down loading of programs.
Cheers Ross