Encoder Count Rate

How many steppers do you need to control? If you don't need more gpio than one 7i77 or 7i76 offers, it would be a little cheaper to use a 7i77+7i78 (or 7i85s costs the same) rather than a 7i76+7i77. Also if you only need 1 analog servo control you might be able to use the 7i76 encoder and analog out and forget the 7i77.

You can but its a bit overkill for one servo, if you can use the servo in step/dir mode you could use a 7I76 and a 7I85S

I am still in the process of selecting my servo so this may be an option. The thing I have had trouble understanding from reading the documents on these motors is how the loop is closed. Will this be done entirely by the drive and LinuxCNC will only control step and direction? Or can they be setup so that LinuxCNC is reading the encoder information and outputting step/dir appropriately. Also, if anyone has a good source for servo motors it would be appreciated.

How many steppers do you need to control?

I will be controlling 4 or 5 steppers. The 5th will only be used for infrequent positioning, so I may just put that on an entirely separate controller.

Also if you only need 1 analog servo control you might be able to use the 7i76 encoder and analog out and forget the 7i77.

Is the 1 spindle output and encoder feedback on the 7i76 able to be used in the same way as the servo outputs and encoder feedback as the 7i77?

I forgot to mention that the other reason I have been looking at the 7i76 and 7i77 is that they seem to be the most supported cards for LinuxCNC. Is this true or are the other cards just as easy to implement? Thanks.

squan09 wrote:

I will be controlling 4 or 5 steppers. The 5th will only be used for infrequent positioning, so I may just put that on an entirely separate controller.

That makes no sense at all There are lots of ways to control steppers using the controller you already will have.

That makes no sense at all There are lots of ways to control steppers using the controller you already will have.

Are you saying it doesn't make sense to separate the 5th axis?

it would be a little cheaper to use a 7i77+7i78 (or 7i85s costs the same)

If I went with this setup (it looks like I would want to go with the 7i85s to have the encoder feedback) I would only have 4 axis step/direction control, correct? So I could go with this setup and not have the 5th stepper, or I could go with the 7i76 which has 5 axis step/direction and try to use the spindle control with the servo motor.

You can but its a bit overkill for one servo, if you can use the servo in step/dir mode you could use a 7I76 and a 7I85S

Does anyone have experience using a servo with a step/dir drive? I am wondering if there is a limiting resolution(step size) as if I were using a stepper. The servo motor will be controlling a rotary table and I want to have a resolution of about .001 degree. Even with microstepping this would be difficult if not impossible to do without gearing on a stepper. Will I run into the same problem if using a step/dir servo drive?


Thanks again for all the input.

I am currently using a machine with step/dir controlled servos using a Mesa 5i25 plus a 7i85S. It works perfectly fine. I have the step resolution set to be equal to the encoder scale, so there is no loss in resolution, and the hardware step generation doesn't limit your max velocities. One thing I will say, if your servos are easy to tune with their software, there is likely little to be gained from adding encoder feedback, other than not needing to re-home the machine after an e-stop or other fault resulting in a shutdown. One of the main reasons I added the encoder feedback, was because I was having difficulty tuning the drives, and adding feed back to Linuxcnc let me use Halscope to tune the system better.