Micro Serial Servo Controller with USB to Serial Adapter


I’m using the following USB to Serial Adapter: pololu.com/catalog/product/391

with the following servo controller: pololu.com/catalog/product/207

and I’m using the Pololu Serial Transmitter Utility (PSTU) on a windows machine.

Here is what happens:

  1. Connect serial adapter to laptop and hit connect in the PSTU.
  2. Apply power to servo controller.
  3. Yellow light on.
  4. Issue a command using PSTU
  5. Yellow light on, Red light blinking.

The servo controller user guide says that that the serial in / serial out lines are 5V lines. The Usb to Serial adapter is a 3.3v part (but 5v tolerant). The guide also says that those lines expect uninverted serial signals.

Until I can test this later tonight, my ideas are:
a. The 3.3v signals are too low for the micro servo controller.
b. The 3.3v signals from the usb to serial adapter are inverted.
c. Something else and I should use the full serial connection (which would require buying some other usb to serial adapter).

If someone knows the cause of the problem, please let me know.


The USB adapter should be fine with the servo controller, and that you are getting any response at all is an indication that voltage levels are likely not the problem. Are you sure you’re sending the right commands at the right baud rate for the protocol you’re using? You can try sending one byte at a time and seeing when the error occurs.

- Jan

Hi Jan, thanks for the response.

I’d been using a single power source for both the servo power and servo controller power.

I had the battery connected at the servo power pin and had shorted the Vcc = Vs pin.

After I posted my message, I spent about 30 more minutes reading through other posts in this forum and the general trend is that using separate supplies is best. So, I have the servo controller powered from the USB vbus and the battery solely for the servos.

That results in a working servo. I can control things in Mini SSC II mode and i’m experimenting with Pololu mode now.

Hi zboot,

I recently did exactly the same thing you’re talking about using a similar UART to USB device (revolution-robotics.com/inte … e_uart-usb) and got it working.

The module I used was a UART level converter so I wired it into the logic-level serial i/o port of the servo controller. Don’t forget to connect the ground line of the serial converter to the gnd of the servo controller. The lack of common ground can cause erratic behavior.

I jumpered the vcc and vs jumpers like you did and powered the entire thing from a single 5V supply. I had some issues where I had to power cycle it in the beginning, but everything seems to be running reliably now.

One thing to pay attention to is the fact that the servo controller has different sets of serial inputs (RS232 vs logic level). You might double check to make sure you were using the right set. The REVO uart to usb converter I used is also speced at a 3.3V signal level and it works great with the servo controller on the logic lines. Sounds like you might have connected it to the RS-232 inputs?

Hope this helps,