connecting a pololu A4988 to drive a second stepper for Y-axis (slave to onboard driver)

[Michael Rosplock]

Hello, 

quick question, so I have a 3d printer with an X5 smoothieboard. its dual extrusion with 3 axis of control (using all 5 onboard drivers). I have a pololu A4988 driver to drive this extra motor to make up a dual drive Y axis. I know that I should be able to connect the logic pins between these two drivers and they should do the exact same a the same time, BUT I can't figure out how to nomenclature from the smoothie board (STn, DIRn, ENn, GND) to the pololu A4988 (STEP, DIR, VDD, GND, SLEEP, RESET). It seems like it should be pretty easy but I really can't figure it out. The ground makes sense, step, and dir seem self explanatory, but I'm getting goofed up by the ENn on smoothie and where VDD should come from to go to the A4988. I'm assuming that the stepper drivers on the smoothie board are set at 1/16th microstepping (can't find anything anywhere that can verify this) so i set the A4988 to 1/16 by grounding S1, S2, and S3 to GND on the A4988 (a jumper between GND and all the S's on a breadboard). 

here's a picture of a wiring schematic for the A4988 

here's the picture that smoothie lists for external drivers: 

I know I can just stick the leads of the second motor into pinouts from the onboard stepper driver, but I'm afraid i could burn out the onboard driver, as the motors are rated up to 2A each. This is more of an electrical engineering question, but if i hook the two motors in parallel will it draw double the current, or will the smoothie board limit the current for the pair to what i have it set to in config (1.8A)? giving me .9A per motor? As long as the pair has enough to drive the axis and keep up with microstepping i'd be happy to do it this way, I just would like a second opinion before going this route, as I think actually using a second driver is the better option. 

I'm suprised no one has done this with an A4988, or if anyone has, nobody has posted anything about it online that I could find. If someone could help me that would be so awesome, 

Thanks, 

[Triffid Hunter]

> The ground makes sense, step, and dir seem self explanatory, but I'm getting goofed up by the ENn on smoothie and where VDD should come from to go to the A4988.

connect step to step, direction to direction, enable to enable, ground to ground, vdd to 3.3v, and vmot to vbb.

> if i hook the two motors in parallel will it draw double the current, or will the smoothie board limit the current for the pair to what i have it set to in config (1.8A)? giving me .9A per motor?

The drivers always limit current (they have to for microstepping), you'll get 0.9A per motor. This is how the Z motors on most repraps are wired. It works fine.

[Michael Rosplock]

ok so I do need to use the enable, so that makes sense, so where do i get the 3.3v off the smoothieboard? 

is it the part labeled VBB in big letters or is it the pins by the pinouts for the stepper logic (3 circular pins by the 2nd drivers pinouts)?

[Michael Rosplock]

Tried following your suggestions and I get nothing from the Y axis when its all connected. If I pull the step or dir pin from the board I can get the original motor to move in one direction, whether i hit a negative direction or not. If I pull both of them the first motor works as normal. Just an interesting observation, I'm not sure if that's of any help.

 I may just end up driving both with the one stepper driver, are there really any reprecussions from this? It seems like the only problem i might run into is not having enough torque while microstepping. 

[Triffid Hunter]

http://i.imgur.com/F2vcM9V.png

This header has GND, 3.3v and VBB, specifically for external drivers.

There's a couple of diagrams at the bottom of http://smoothieware.org/smoothieboard that show this

[Michael Rosplock]

ok so that all seems fine. I have all of those plugged in and I'm still getting the same behavior... Is there something I'm missing with how these work? 

My friend told me that bars above the pin name mean it needs to be grounded, should I be running ground to all of these? also do the two grounds need to be referenced to each other with a big resistor between them? Seems like it should be working now, could it be something with the enable pin needing to be high or low? 

--
You received this message because you are subscribed to a topic in the Google Groups "Smoothieware Support" group.
To unsubscribe from this topic, visit https://groups.google.com/d/topic/smoothieware-support/WxvFNJVSNME/unsubscribe.
To unsubscribe from this group and all its topics, send an email to smoothieware-sup...@googlegroups.com.
For more options, visit https://groups.google.com/d/optout.

[Triffid Hunter]

On 1 June 2015 at 02:47, Michael Rosplock <rosp...@umn.edu> wrote:

ok so that all seems fine. I have all of those plugged in and I'm still getting the same behavior... Is there something I'm missing with how these work? 

must be
 

My friend told me that bars above the pin name mean it needs to be grounded, should I be running ground to all of these?

overbar means the signal is inverted, ie !enable means ground = enabled, 5v = disabled.
 

also do the two grounds need to be referenced to each other with a big resistor between them?

absolutely not. they must be connected directly
 

Seems like it should be working now, could it be something with the enable pin needing to be high or low? 

it must be low to enable the driver. smoothie sets the enable signal low when the motor is commanded to move, and sets it high on M84