Vexnet Keys

As vex has recently moved from the stone age of crystals to super-fancy wifi (!!!), most people were happy

HOWEVER, there is one problem; the keys often fail and short out

this is rumored to be an electrical problem, and if so, is it dangerous to have anyone touch the robot?

and, furthermore, why does it take so long to connect? sometimes it feels like your robot is having a BSOD

does anyone have a solution to this?


Thanks for providing the daily humor !

We have been using VEX keys & cortex systems for a while. They seem to be fairly robust but sometimes after massive firmware changes in RobotC , we have noticed erratic behavior. We are hoping these are just software programming problems that will go away in due time. :eek:

We have been using this system all season, and lately our link is dropping out during matches. The last 3 competitions, we lost link 2 or 3 times in matches, but we havent been able to find the problem or solution. We are going to try a new USB extension cable next. If anyone has any suggestions, they would be appreciated. As a side note, normally the robot just dies and sits for about 20 seconds, then relinks. The last time we lost link, the robot remembered the last command it received and drove backwards into another robot and we could not stop it.

Thank You for the information, we normally have a freshly charged backup battery, but have not looked into the amperage we are pulling. We will look into that. Thanks Again.

Chuck pretty much summed it up
a tip about back-up batteries:
if they are NOT fully charged and you plug in a low-dead back up battery, then the vexnet will not link AT ALL
(as in you cannot even control the robot)
this has happened to many teams that used the 9v battery last few competitions
the fix for that is of course to unplug the backup batt

long story short: its better to have NO 9v batt then a dead one…

Do any teams suggest a clip on Amp-meter to watch current usage ?

Maybe VEX should consider making a current sensor since drawing too much current this year is a big problem with the new motors as well.

I use a $3 (on sale) DVM from harbor freight, works up to 10A.
Make a Battery extension cable, and put the DVM in series with the positive line.

I picked up one of these with my SparkFun Free Day winnings, though I haven’t had a chance to play with it yet. My idea was to monitor current load in software and have the robot automatically reduce power when the load exceeds 4A or so.

It would be very cool if IFI came out with a smart power expander or smart motor controller that attached via I²C or UART and had an ammeter like this built-in.


  • Dean

After extensive testing by borrowing a lot of vexnet keys from other teams, we found that apparently two of our vexnet keys are duds. They take a long time to connect with any other vexnet key, and then the connection cuts out quickly, with or without backup battery, with or without extension USB cable. Unfortunately it looks like we’re going to have to buy two new vexnet keys, which is a problem because they’re reasonably expensive and we can’t afford them.

If they are still under warranty (three months), see if IFI will replace them for free. Even if it has been over three months, it may be worth your time to ask since the defect probably occurred long before you fully diagnosed the problem.


  • Dean

It’s been less than three months, so we’ll definitely be asking IFI. Thanks!

One thing we did that seemed to help was attach the key to the cortex with a usb gold plated extension cord.


This has happened to my team (84) at a few of competitions, and including a couple of other Downingtown Vexmen teams. At worlds, while waiting to start the match, one of the members of the opposing team could not link up, then after many attempts to fix this, the field control people (tech support?) plugged in new keys and it worked instantly. Apparantely, faulty keys are not so uncommon and Vex should look to try to fix this, whether it is from dropping them or over-using/heating them, etc… It is very frustrating and can take a while to figure out the problem. In our case, the key situation got very bad and when we could link up, nothing could be downloaded. No firmware (joystick, main CPU, or the other one) , no code , nothing. Not even the IFI loader would work at first.