vexnet and linetracker: vexnet not starting

today we encountered a really weird problem, and I was wondering if anybody has had similar problems. Today we noticed that our microcontroller was turning on, however, the vexnet was not booting up. At first we thought it was a problem with the microcontroller, and we switched it out. After setting all the pwms in, The same problem persisted. I then thought: maybe it might be the line trackers that we put in that day? I decided to pull out one of the line trackers(we have 3 in the IO ports), and wala, the vexnet turned on. This was very weird though. The vexnet would boot when there is 2 linetrackers plugged in, but not 3. Also, when there is 3 plugged in, it will not boot, but if I plug one out while its on it turns on, and if I plug it back in, everything seems fine. I’m assuming for somereason the power or signal is not going to the vexnet.

Does anybody have a solution to this problem? we have switched out the linetracker that has thought to be ‘broken’ as well, but I will switch out all of them soon. Thanks.


FWLIW, it sounds like a power problem to me. If you have a voltmeter handy, I suggest watching the nominal 5V (power pin on a Digital/Analog port, *not *a motor/servo port) while trying to start with and without the line tracker.

Does it matter which of the three line trackers you unplug? Or in which port each of them is connected? If there’s on “special” line tracker that’s causing the problem, it may well (for reasons yet to be determined) be drawing excessive current, causing the voltage to sag below what Vexnet needs to get started.

If unplugging any line tracker will do, it could still be power. In that case, however, unplugging something else should do the same trick. How much power a line tracker draws and what else (remember, motors/servos do not get their power from the same bus) would draw the same power, I don’t know.

If it’s not a power problem and it doesn’t matter which line tracker you unplug, you may very well have stumbled upon a software bug, in which adding a third line tracker causes a resource conflict with the Vexnet. I’m going to leave chasing that theory to someone who’s much more familiar than I am with the relevant code.

Happy Hunting,

I believe it was because of a faulty tracker, because we decided to switch out all 3 of the trackers with brand new ones and it worked fine. I did however try unplugging them one at a time to see if I can root out the cause, but I decided that would be a waste of time. If for any reason you want me to go back and see if there was a particular tracker that was the problem for future reference, I will.


No, but thanks for asking! :slight_smile:
I don’t have any particular interest in knowing which of *your *trackers is wonky. You might, however, want to figure it out at some time so you know which ones are usable and which are parts sources. It may be possible to find the faulty one(s) by checking the impedance between the power and ground pins, rather than by seeing which one causes Vexnet not to boot.