Why use motor control boards?

Hello all. I've been envious of anyone with an RP machine for years now and am finally building my own repstrap. I've already got a functioning x axis with endstops connected to my duemilanove and running. My question is, Why do we need a separate board to control the steppers. I'm controlling the motor on my axis by just using 4 pins on the arduino and a darlington array. It seems to me that four or eight digital pins could be muxed and sent to darlington circuits to drive the motors and the parts total would be less.

I think the advantage of the IC stepper controllers is that they have adjustable current outputs. That allows the use of a high drive voltage to step quickly followed by a lower current to keep the steppers from overheating.

Ah, ok. But not entirely neccesary for a simple bootstrap system?

The print pauses as each g-code is read and step info is computed. Not long. Just long enough to cause a small blob of plastic to ooze out at the end of each move. Especially bad on short moves like with circles. Spoils the print.

YMMV

You are right , but using 4 arduino pins per motor is а little generously

Could you please present a scheme you use?
I can't uderstand how you control "upper" 12-v transistors by Arduino TTL.

I used the same idea for my design of a prototyping machine. It runs on an atmel mega325 processor which directly drives 3 unipolar and 1 bipolar stepper motors with TIP 120 darlingtons. There is ample time to read and process g-code from an sd card, drive an lcd and even run the motors on a simple pwm scheme. The only smd component on my hardware is the microcontroller.

You can find a video of my running machine on my webpage www.3dp-tools.com orders welcome :-)