Worlds VEXNet dropouts

I’ll just say that our team had zero issues with Vexnet at Worlds (technology field) and we have never had an issue with the connection being dropped during a match at an event this year.

The field plug in for power was great as well, no risk of controllers dying during the match due to low batteries!

We did have issues this year with tripping our thermal breakers, but this was because we were running a 1:3 gear ratio and were running too heavy for that gear ratio. After switching for a 5:3 (using four 393’s on the drive), this eliminated the problem. Similar to FRC, we used the best available motors on our drive.

It’s been my experience that we are too quick to blame wifi connections when the underlying root cause for no movement is actually something else. I’m not saying there are no wifi issues but I used to think our problems were wifi related until we started to discover the real causes.

We have found that loose power connections and overheating problems were the most common contributors to our problems. The Molex connectors that are used on leads for the battery and the Cortex are not necessarily meant for high cycle use and tend to fatigue over time. This results in the flickering of power to the Cortex when the wires are wiggled or the robot is jarred from an impact. We are now looking at an intermediate connector (only for practice) better suited for high cycle usage. Paul, this might be something that would be a nice addition for a future product. Half of our batteries (mostly the older ones) will flicker power to Cortex when the connector is bumped.

The other issue is overheating of the motors and/or Cortex. Careful design and testing can overcome some of this and provide you with some idea of how much margin you have when you enter a competition. Usually I see gear ratios too high or bearings/wheels/motors are not well aligned.

Coach/Mentor Team 1069A, B, C, D, E

we wrapped the ziptie around the cortex and key similar to the attached image so it would not be able to wiggle out. we did not use traction wheels on and we were not fighting the pin. they are 269s. they were used directly attached to the wheels

our bot was not too heavy for the motors as it never overheated within 5 minutes other than at the fields at worlds

My team was having a lot of random dropouts and other problems such as download failures, weird blink codes, and random code failures.
We tried moving the VEXnet key up to the top of the robot, but it did not resolve the dropouts.
All of these problems were resolved when we got a new cortex. Our old one was an NC-1, and we switched to an NC-3+

Just curious, what was the gear ratio on your drive and which motors were you using to power it with? Also how many motors were you using on your drive?

we had 4 large omni wheels directly driven by 1 269 motor per wheel

That’s four small drive motors at a reasonable speed. How much did the robot weigh?

Our 177C team definitely had field related Vexnet issues. After two matches of not moving and lots of apologies from the field tech we quit using the field setup and had nomore issues. I also question why the manual says no metal within 2 inches of the vexnet key if you put it on an extension but the field key was directly behing a piece of metal frame.

No it wasn’t. The key had clearance above the metal bracket it was mounted on and below the metal frame perimeter.

We are investigating all claims regarding the remote power and key this week and next week.

The bottom line is that we want to fix any issues that we find. The problem with the competition environment is that there is no time to dig into every problem a team has on the field. Every time a robot has a problem on the field, the team immediately blames the field. At the VRC World Championships we were able to show many teams the issues within their robot. However, there were a few times that the techs could not get to the root of the problem and we are investigating those here in Greenville. If there were problems with the field hardware or VEX firmware we will find them.


our robot weighed around 14 pounds. we had no overheating issues while driving countless consecutive practice matches at our school so im not sure what changed but the increased friction im sure played into it.

thank you so much for looking into the issues. my guess is that the static spray and the field vexnet key were a portion of the problem. i personally cannot see how the key attached to the field did not severely overheat during the day. we have had vexnet keys get very warm and get poor connections while driving around with the key in the joystick so overheating the keys should be looked into i think. if nothing else maybe ventilate the keys more or have a fan to keep them cooler.

In our case (177c) the tech on the science field came to us because 2 or 3 robots in a row did not run in that position on the field. We had a rematch but the same thing happened. We then stopped pluging into the field and the issue went away.

Other things the tech said included that the instuctions given ahead of time as to which cord to plug in first were wrong and to remove all velcro from the keys because that was bending the usb connection and breaking the solder joint. He said to use tape to hold the key in. I don’t know if that is legal or not.

I am glad you are trying to fix the issues. We have similiar issues as a team. One side of the drivetrain stops working on our other robot (177) during competition, but works fine as soon as the competition is over. The problem is hard to solve because we can never reproduce the problem on the practice field.

As far as the vexnet keys on the field, I only saw one up close since we only have students on the drive team but it didn’t appear to me to meet the dimensions given in R7 hii (Gateway).

all of our issues that we had at the competition fields at worlds we were not able to reproduce at the practice fields there.


This problem can not be caused by the field or Vexnet. The competition port can only disable/enable teleop/autonomous and all Vexnet communications are over a single serial communications channel.

Partial operation of a robot must be due to a problem internal to the robot or the joystick.

It is extremely likely that you are tripping an internal over-current protection.



That sounds exactly like our situation. We had practiced for hours on a full field at home and had been to a regional competition and never had any problems with overheating in our drive motors, but at worlds we did.

Specs: 4 High Strength Motor 393s geared internally for speed in a holonomic configuration

We checked every axle for friction by taking off the motors, but they were extremely smooth.

Next year, we will be designing our robot accordingly and hopefully we can get some anti static spray for our field.

we looked into this but this is not why vexnet would drop. this only explains not being able to move partially into driver control. a robot from our school got stuck in a single case from the autonomous period throughout all of driver control. this only happened on the field

ya apparently the static spray is a big deal. its going to be interesting what they will recommend to deal with these issues

I just posted a picture of the field Vexnet key in the Gallery.

So I was wrong, the key was not directly behind the metal field frame, but it was very close. I still say it would not pass the rule about keeping metal 2" from the top of the key. Gateway <R7>h

i doubt that it would pass the rule too

I hope VEX will be able to fix vexnet dropouts by Worlds. Although I never went Worlds this year I know that Vexnet drop out plagued my team at Worlds in Clean Sweep.
Big Thank You to Vex for the huge improvement in vexnet dropout. From watching the webcast this year I could tell only a few robots had vexnet issues. Luckily Vex fixed it at the field by taking a look at the dead robot.

Vexnet dropouts really hurt teams especially great teams, so I hope vex will be able to fix problems. It would suck this year if either your robot or your partners robot stops working at worlds. Teams who get to worlds work so hard.

Just wondering what if there were bad keys on the field which lead to your robot not moving? Which the field people don’t catch until the next match, and you lose this match. Will the match be replayed or will they carry on with schedule?

Ideally, they would replay the previous match, however since you leave the field pretty promptly after the match, you may never find out that they fixed the key or something, so you would not know to request a replay, and it wouldn’t surprise me if they forget to do that. In short, ideally they would, and they should replay the match; but in reality it is much less likely that it would end up happening.

What if a robot passes in front of the VEXNet key? What if it stays there? Wouldn’t that definitely violate the two inch rule?