Frequently Asked Questions

While some of the following have been embellished for entertainment value, most are real questions that I have received.
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Question: What does 'STMD' mean?

Answer: It is an acronym for STepper Motor Driver. I couldn't invent a cool-sounding name if my life depended on it.
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Question: Can you help me modify the design to run a motor at 5 amps per phase?  

Answer: No. Go buy a Geckodrive.
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Question: Is it possible to parallel the LMD18245T for increased current capacity?

Answer: No. Go buy a Geckodrive.
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Question: I just got done building my board and programming my microcontroller. The documents say the 'alive' LED should be on for 1/2 a second and then off for 1/2 a second. Mine is on and off for more like 4 seconds. Why?

Answer: Since you programmed your microcontroller yourself, you need to verify your fuse settings. In particular, make sure the CLKDIV8 fuse is unprogrammed (logical 1). This is not the default setting of a new Mega48. In the default setting (programmed, logical 0) your microcontroller is only running at 1/8th of the intended speed.
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Question: What happened? Now my 'alive' LED has stopped flashing and is illuminated all the time!

Answer: The microcontroller firmware has detected a fault. This could be a watchdog timeout, etc. The appropriate fault code has been logged in EEPROM and you can read it out with an AVR programmer. Cycling the power should clear the fault and allow for normal operation. The fault code will still be retained in EEPROM.
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Question: Hmmm... now my 'alive' LED is flashing very quickly. What is wrong?

Answer: Check the voltage being supplied to the board; it should be greater than +8 Vdc at all time. If you are using a 'weak' supply it is possible for the voltage to sag as the microcontroller turns the LED on. This sag resets the microcontroller and the LED turns off as the output pin is tri-stated. When the LED turns off, the voltage rises and the microcontroller begins to run again. And so the vicious cycle continues...
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Question: In your parts list, R20 is missing. In the assembly instructions it is listed as being optional. What is the purpose of this 5 kohm potentiometer?

Answer: R20 is a 'feature' that I never implemented. It has no functionality and should be ignored.
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Question: I just looked at some pictures of assembled boards on your website and I noticed that you used some test points. I do not see these test points in the parts list?

Answer: Some early boards I built used test points so I could clip oscilloscope probes to them during debug. The points add about $2-3 per board, so I haven't been including them in the parts list. In fact, with a multimeter, it is easier to do spot checks without the test points installed because the empty hole prevents the probe lead from slipping away as easily. If you want some test points, check out Digikey #5005K-ND. This is a red test point but other colors are available.
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Question: The STMD requires step & direction signals to operate. I want the motor to rotate as soon as it is powered up. Is this possible?

Answer: Yes. Turn on the 'Test Mode' DIP switch. Now when power is applied the motor will rotate based on the direction signal. The enable signal is still required, so you must wire it high if you aren't using it.
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Question: OK, that worked, but how can I adjust the speed?

Answer: There is a spot on the board for a potentiometer (R20) that could be used as a user-adjustable speed control, but the open-source firmware will need to be modified to take advantage of this. Either do it yourself or ask me for a quote.
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Question: I hooked up a motor that internally had a shorted winding. Now my board won't work. How can I tell if the LMD18245 drivers chips are damaged?

Answer: I have been able to detect most damaged LMD18245's with an ohmmeter.

  1. Turn off power.
  2. Disconnect all 3 sets of terminal blocks from the board.
  3. Use an ohmmeter and verify that none of the terminals for the motor connections (A+, A-, B+, B-) are shorted to the motor power terminals (V+ or V-).
I have also seen a damaged LMD18245 take out the microcontroller.
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Question: I need the board to walk, talk, and quack like a duck? Is this possible?

Answer: Sure, just ask me for a quote. Seriously, the microcontroller's program memory is only 40% utilized, so many custom features and functions can be added. 
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Question: Is it possible to modify the design to have 'step clockwise' and 'step counter-clockwise' signals instead of 'step' and 'direction' signals?

Answer: Yes.  I have developed special firmware that does this.  Note that the 'test' mode of operation will only allow the motor to rotate in a single direction as the 'direction' input is now another step signal.
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Question: Do you have a single board that can control 3 or 4 stepper motors?

Answer: No, I prefer a more modular design. Otherwise, if the magical smoke gets let out of one driver channel and that channel is not repairable (burnt traces, etc.) you may have to trash the working driver channels as well. Modularity is much better from a replacement cost standpoint.
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Question: Nice product. Do you have any servo motor drives?

Answer: No.


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Last Updated: Dec. 28th, 2007

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