Servos and the Smoothieboard


In the previous post my VIX-IH drives were programmed to behave like an average stepper driver.  This makes configuration very simple and allows them to be used with any of the standard 3d printer controllers.

There are a huge number of software and hardware options for controlling a 3d printer but I have decided to use the Smoothieboard.  The major reasons for my choice are that it uses a 32-bit micro controller, has a good set of features, and the code is very clean.  Smoothie can also support high step rates which is important as my servos are fast.

You might remember the servo drive wiring diagram from the previous post, here is a revised version showing how the Smoothieboard fits in.  As promised, the servos make use of the standard STEP/DIR interface and require only one extra pin to activate the homing feature.


The only complication I have not yet addressed is that the Servo drives take differential inputs.  Differential inputs are more resistant to electrical noise but require two inputs for each signal, which is why there is a STEP+, STEP-, DIR+, and DIR- input on the VIX-IH drive.  A differential input is read as high when the + input is at a higher voltage than the - input, and low when the reverse is true. 

If you leave the STEP- pin unconnected or at 0V you will have no way of lowering the STEP+ pin below that threshold and the drive will be unreliable.  The simplest way to work around this is to apply half the logic voltage to the negative inputs, which is 1.6V.  This way the STEP+ pin can be easily set lower or higher than the STEP- pin by applying the usual 0V or 3.3V.  You could also achieve higher noise resistance using dedicated differential line drivers and higher voltages but that will not be necessary for this printer.


To keep everything neat I broke out the required Smoothie pins using right-angle headers and put together a nice shielded cable.

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