Asked — Edited

Sevomovementpanel Vs. Horiz &Amp; Vert Servos?

I can control my servos, forward and reverse with the SMP. However, I can only drive them forward with the Horiz servo or Vert Servo.

Any ideas?

Thanks again


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Please provide more detail to your question so we can help you:)


I think, I see what is going on. I am using a VEX 393 3-wire motor from the modified servo control in the EZ-B. The pulse width from the EZ-B for Reverse is 0.5mS, independent of the value that I enter in the config window. VEX uses 1.0 mS for reverse. Their motor controller 29 units can deal with the narrower pulse width.But, their motors can not. It appears that I am stuck with an incompatibility issue.

I have a lot of VEX parts that I spent a fortune on. I am trying to help these guys complete their robotics project by giving them some of my parts, since I have parts but not cash. Hmmmmm.

Is there a way to stretch that reverse pulse width by a command?

Thanks again



Every control has a CONFIG button which opens a config window. Within the Modified servo config, you will find values to configure for each direction of both channels (left and right). The values are Forward, Reverse and Stop. A checkbox will send a PWM for STOP rather than sending no PWM (which is default).

Use the values in the CONFIG menu to select values that work for your hardware.:)


@Troop191Robotics Are you referring to the "Modified Servo" control or the "Modified servo Movement Panel" control?

I don't think it is really an issue with the EZ-B so much as the semi-proprietary nature of the VEX components.

With regards to what you might be running into, I did read somewhere the the VEX servos & motors (with their controller adapter) have a higher voltage range of 5.5 - 9 volts and thus may not work well with typical servo controllers without first supplying external voltage to the motor and running just the signal wire to the controller (and common ground between controller and servo power)... try that with your setup.


Good clarification Gunner, I am talking about the modified servo control NOT the Modified servo Movement Panel. Its config window only accepts values for Forward and Revers. Regardless of what value I put in for Reverse, I maesure 0.5 mS pulsewidth.

As to the operating voltage for the VEX motor controller, I am using a regulated 8 Vdc. Thanks, that's important. I monitor it with a panel meter. it is well behaved.

Thanks guys


Wow, do I need a spell checker!

Anyway, I just ran into the same issue when I tried to drive my Fergelli linear actuator in the servo mode. It would not reverse. So, I ran it in the PWM mode.

Just some more info


@Troop191Robotics I don't have any VEX servos to test this with, but based on my limited oscilloscope experience (AKA zipity squat:P) I can see the 0.5mS reverse and 2.5 forward pulse... so out of curiosity I used the PWM control and I can get to 1.0mS at about 5% setting.

Try playing around with the PWM and then possibly running your VEX servos using a scripted preset PWM setting and see if you can control them that way.

Disclaimer... I am really pulling this one out of my... hat:D and would be interested in your results.

@DJ, when I am playing around with the modified servo control and watching the scope, I do not see any changes when adjusting the config values... what are they supposed to do in this particular control?


Strange... I posted my above little bit then got an email of a new post a minute later and there is yours, but showing from about 30min earlier (yet I didn't see it when I posted)... Alas, you already figured out the PWM without little old me:) silly timewarps:P


Thanks Gunner, lots of good ideas as usual.

Just to be clear, that trick of switching to PWM worked for the Fergelli, since it has a built in motor controller withe 5 different modes of operation. The servos do not have that luxury. I ran them from a horizontal, and vertical servo script. That works. It is just from the Modified servo mode that I had no success.

We'll get there


@Troop thanks for that little feedback... you know, I never thought of running a modified servo with a H/V servo control, but after your comment I tried and not only does it work fine, but I can now get smooth speed control:) the modified servo control just goes all out and adding a servo speed controller just made the transition from forward to reverse a delayed and gradual ramp up to full speed. Guess I need to experiment more with unusual control combinations:)



The PWM Script Command PWM(port#,position) does not appear to work.

I test things using the sliders and when they work, I write script commands to execute them.

I used the PWM slider to drive my linear actuators. I monitored the waveform from the EZ-B and the dc voltage that was delivered to the linear actuator. All was in order.

Then I wrote script commands to do the same thing. PWM(D2, 0) for example. I saw a constant high, 100% duty, from port D2. It did not modulate the duty cycle, like the PWM slider did. So, of course it would drive the linear actuator to its stop and not come back. I can still control it properly from the PWM slider.

Any ideas?


I went to test PWM(D2, 0) to see if it was supposed to be all ON instead of all OFF... but I discovered my EZ-B Bluetooth module is dead:( so I can't assist much more, aside from theoretical guesses:P

I am sure you know this, but just in case... the script setting you used would lock the PWM at a single setting (like leaving the slider stationary in one spot)... you would need to change the position value to a variable and have some input adjusting that variable in a loop to act like a slider does.

Stop me now if you already know all that:P