On a sunny day (Sat, 22 Dec 2018 11:38:04 +0000) it happened Folderol
 wrote in :

>On 22 Dec 2018 10:53:49 +0000 (GMT)
>Theo  wrote:
>
>>A. Dumas  wrote:
>>> Theo  wrote:
>>> > There's a variety of firmware for running such controllers on Arduino and
>>> > similar, doing closed loop control.
>>>
>>> ###>>>>> "as this is Raspberry newsgroup" <<<<<###
>>
>>Yes, and?  You attach the Arduino to the Pi and communicate over USB or
>>serial (subject to voltage levels).  You don't want to do realtime control
>>on a Pi running Linux, but a $3 Arduino is a suitable coprocessor for the
job.
>>
>>Theo
>
>Well even without that, a pair of these:
>https://uk.rs-online.com/web/p/motor-driver-ics/9062978/
>
>will do the job with just 4 GPIO lines from the Pi
>
>Or with the simplest of wiring, just one with 2 GPIOs :)
>
>Been there, done that, got the T shirt - but was using the userport from a BBC
>Model B + 2 741s and 4 transistors :)

Indeed, here raspi driving a stepper via GPIO, uses an ebay stepper motor
driver:

http://panteltje.com/panteltje/xgpspc/testing_steering_stepper_motor_IMG_5563.J
PG
More info:
 http://panteltje.com/panteltje/xgpspc/index.html

But I changed to a Microchip PIC in a separate box doing the stepping.
is connected via ethernet to the raspberry:
  http://panteltje.com/panteltje/pic/step_pic_udp/

For RC type servos I have some PIC code on my site.
Not a good idea to directly drive that from a raspberry, timing is problematic,
but driving a PIC via RS232 works (PIC asm):
  http://panteltje.com/panteltje/pic/step_pic-0.5.tgz

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