Choosing PMDX-424 or PMDX-126?

[OldePhart]

Building my first (and probably last) CNC using ClearPath Steppers. After more than 2 years I finally have the hardware done (dual motor gantry) and its time up build the electrics.  I originally assumed I would use the PMDX-126 and a smooth-stepper because Mach 4 was still not ready and the 126/SS was top of the line. The 424 is available now and Mach4 is also stable so I am wondering if I should reconsider.

The driving PC will not have a parallel port so I have to use USB or a Smooth-stepper. I was never a fan of USB as I thought it would be less reliable. Ethernet was meant for sustained traffic over hours and USB was not. However if that were really a problem, market forces would come into play and these boards wouldn't exist for long. So I must be wrong there.

I am also assuming that Smooth-stepper is not available/advisable/needed for the 424?

In addition I read that the 424 output pulses are independently controlled/timed separate from the control pulses, I guess thats the  "smart" part of Smart-Bob? Do I understand that correctly? How well does that work out and what made that necessary? Will that be a factor when driving the clearpath motors?

So I guess I need some guidance, when would someone select the 424 over the 126/SS combo?
I appreciate your input.

[12strings]

You had it right initially - it would be either the PMDX-126 and SS *OR* a PMDX-424.

The PMDX SmartBOB family use a USB transfer rate that is lower than is available via Ethernet.  But the amount of data needed to be sent/received is small enough that either interface has plenty of bandwidth.  USB has no inherent issues regarding sustained traffic that I am aware of.  USB configuration is easier than Ethernet as you don't have to muck with your PC's IP address (at least as you used to do with the SS, though I admit I haven't look at the SS setup programs in a LOOOOONG time, so take this with a grain of salt).

424 output pulses are independently controlled/timed separate from the control pulses,

I'm not sure what you mean here.  Both the PMDX-424 and the SS generate the step pulses themselves, using information sent by Mach4 (i.e. Mach4 tells them how many pulses to generate per time interval for each motor and the 424/SS calculate the timing and generate the pulses).

Since this is the PMDX forum I'm going to suggest the obviously biased answer of PMDX-424 :)

[OldePhart]

I'm not sure what you mean here.

From the 424 page...

3) The motion control pulse engine generates pulses independent of the host computer timing and provides real time control for smooth step pulse signals . . .

Perhaps the SS does the same thing, but it's that sentence that has my attention. I'm using the ClearPath motor and they have their own intelligence built into them ensuring that if I send 100 steps, by God it's gonna do 100 steps if it has to tears its way off the gantry to get there,  (and holy crap are they strong!).

So in any case, even with normal steppers, if the 424 generates pulses independent of the host timing, wouldn't that be a bad thing?  I guess I need an example of when that is beneficial.

[12strings]

Ahhhhhhh....  All that means is that, unlike Mach3 or Mach4 using the parallel port, when using a PMDX SmartBOB (any of them), or a SS, the step pulses are generated *ON* the SmartBOB (or SS), and it (mostly) does not matter what speed CPU you have in your computer, or how quickly it can service the high-speed interrupt needed to bit-bang the step pulses out the parallel port.  With the parallel port driver, some PCs have trouble generating a smooth pulse stream.

In more technical terms, the SmartBOB handles all of the "real time" (time critical) operations, not the PC.

Is that any cleared, or am I talking myself in circles?