On Wed, 27 Jun 2012 03:38:52 -0000, you wrote:
>
>>
>> I don't believe the USB interface is capable of fast enough throughput
>> to handle direct stepper phase control, but step-direction commands
>> might work. (flow control would still be an issue).
>>
>
>
>I bet it is. The widely available FTDI USB to serial chips will do 250kbps, just how fast does it need to be? That said, I'm not sure why you'd want to do direct phase control, there are some nice stepper driver IC's out there. I played with some TI driver ICs that have built in switchmode current limiting, and some fancy microstepping modes.
USB doesn't work well for stepper control. It's not routed through
the appropriate layers in the operating system to give you a real time
way of controlling the mill.
Fast is not the problem, it's that the messages do not arrive reliably
(in terms of time), nor are they interpreted necessarily in real time.
How to use USB would be to take the entire file, download it to
something smart, then let that execute and decode the file.
Other people have tried (lots of discussion in the DIY-CNC group about
this), and that's about the only way that USB will work.
The traditional way is to either have dedicated hardware for
step/direction timing (I mean interpreting the G-code and then
generating the pulses), or a windows PC specially modified to bypass
windows's erratic timing, or a DOS machine, or a tweaked Linux
machine.
Harvey