Extrude3D,
Thank you for the reply.
Now I see the USB port! Hard to identify it from the top view
On the other part about the speeds of the processor, etc., I'm talking about the device that implements the Pulse Width Modulation (PWM) that drives the stepper motors. When stepper motors are 'stepped' a full step they normally turn 1.8 degrees. This is 200 steps per revolution. Sure they make other types of steppers but the 200 steps per revolution is the most common. Depending on your particular design in your machine, a 1.8 degree step can be pretty big (if you have no or little gearing). On my H1 for example, the stepper motors directly drive belts for both the X and Y axis with no gear reduction. A 1.8 degree step would mean the positioning of the X and Y axis would be rather crude. Luckily we can drive the steppers using half-stepping also. When half-stepping the driver board energizes both the A and the B coils of the stepper motor simultaneously and the motor goes 1/2 way between the two full steps, hence half-stepping. So this gives us twice the resolution as full stepping which is pretty good. Now, we can go even better if we use PWM on the steps. To implement PWM, at a very fast rate we turn on and off the A and B coils. Lets say we are on the A step and the B coil is off and the A coil is fully on. We can take a 'micro-step' if we very quickly pulse the A coil on and off but we mostly keep it on, say 15/16ths of the time A coil is on and 1/16th of the time A is off. Simultaneously we very quickly pulse the B coil also so that 1/16th of the time the B coil is on and 15/16th of the time it is off. We have just taken a 1/16th micro step. To make these PWMs 'smooth' the ONs and OFFs are perfomred VERY quickly - a lot faster than just 16 times as fast as say a full step.
So, in your printrboard you have a microcontroller that I believe is running at 16 MHz. I'm assuming that it has internal hardware the implements the PWM (I know other ATMEL microcontrollers do so this seems like a safe assumption) so it should be pretty fast with little impact on the microcontroller on the printrboard and probably NO impact on your MAC. This is a good thing, like the microcontroller in the keyboard that slaves its entire life away waiting for a key to be pressed so the real processor doesn't have to worry about processing these low-level hardware tasks.
In the H1 stock stepper motor driver there isn't a nice microcontroller that's handling the fast PWM for the steppers. What device is doing this? It is the PC (in my case) running linux. Now my 2 GHz PC is very fast compared to the printrboard's 16 MHz microcontroller, however, the printrboard's microcontroller has rather little else to do and has fast context switching and interupt handling and has PWM built into its hardware, whereas the PWM in the LinuxCNC software has to get the operating systems attention to accomplish anything and the OS is often busy with so many other tasks like performing memory swapping, talking to the hard disk, answering the internet where someone is trying to port sniff, and what the heck? some one just moved the mouse and exposed a widow that I now have to fill in and Now What?! Some LinuxCNC application wants me to toggle my bits on the parallel port.... (the LinuxCNC application gets no respect from the OS). When all is said and done, the very fast PC just might not have enough umph and fast context switching in order to implement the PWM signals in software smoothly enough. In the LinuxCNC program there is a utility that helps you measure how long your PC takes to toggle the port pins and will tell you whether the PC is good enough or just too damn slow.
If I were a betting man I would place my money on a microcontroller board like the printrboard to implement smoother microstepping than my PC, however, I went with the stock H1 which implements in the PC's software and I've seen prints from others that look great so I'm not throwing stones. On other forrums I've seen people saying bad things about the microcontroller based driver boards but I couldn't tell if they were just biased against them or had real technical points. Since the printrboard and the H1 stock driver boards are the two opposite design approaches as to the PWM signals, I was hoping to see if we could take a biased look at the pros/cons of the methods and determine if my next machine should be a printrboard-like approach or a H1 stock driver-like approach.
I hope I have not rambled on and on about stuff you already knew, I just wanted to be sure an explained my question without assumptions.
Thanks,
Z
