Help Me Understand Vexnet vs. Crystal

First let it be known that I am new all this robot stuff. I am trying to help out my son’s vex team and I want to understand some things that confuse me right now…At our last competition the arena was set-up where the transmitters were tethered to a ‘tower’ and matched freq crystals were put in the transmitter and the robots receiver. We had difficulties where the robots would not be user drivable after the autonomous prgm. The tournament was classified as “Competition Type = Vexnet”. This confuses me since crystals are also used. Here comes the real question…When programming and using the competition templates in easyC 2 what should we be using… the “Vexnet (WiFi)” setting or the “Crystal” setting? I asked this since we are not that experienced at programming and that we have been to tourneys where only crystals were used, no tethers and everything was fine. If anyone can help me sort this out I would greatly appreciate it! :confused:

Hi Johnny,

The Field hardware can run in two different modes:

  1. Crystal Only
  2. Crystal and VexNet

The difference is Crystals are run on a timed basis. Out of reset, when the processor sees your transmistter signal, it will run autonomous for the time you specified in the template (usually 20 secs), then run Driver control.

All the tether does is disable your transmitter. When the match starts, your transmitter is enabled for 20 seconds allowing your robot to run it’s autonomous (assuming you remembered to set the autonomous period for 20 seconds!). The field then disables the transmitter while the refs count the balls, then the field re-enables your transmitter and your program moves directly into driver control.

Yo can simulate this by simply turning your transmitter on/off (crystals only).

Note that if your battery connection is intermittent or the voltage dips to due heavy load, the process will reset and take you right back to the start of autonomous.

With VexNet, on the other hand , the field actually controls the transmitter AND the mode (Autonomous/Driver) so it can switch between them at any time. That’s why you put “0” seconds in the template for both modes - it doesn’t rely on timing. Though it has been my experience that you have to cycle through Driver to get back to Autonomous …

Hope this helps.

  • Rick F

Thanks for the reply…
"So they call the configuration of transmitters tethered to towers a “vexnet” competition??? …and they’re basically controlling the ability of the transmitter to work or not work??? Correct???

I am still confused on the mode timing for this type of setup…should it be the defaults for crystals, ie 20 and 254, or 0 for both???

Thank you again…

Hi Johnny,

Both setups are tethered to the post:

  1. Crystals via the curly phone hand set cables
  2. VexNet via the RJ45 cables

The tethers are what control the transmitters output (and mode for the VexNet)

For Crystals you want to specify (20) and (something bigger than 120)

This is the amount of time the processor will spend in each mode.

For VexNet, you specify (0) and (0) because it is not timing depnedant.

  • Rick

You say that the robot was not drivable after the autonomous - do you mean that even after they started the driver control period your robot did not work? Or that when it was paused (as rfolea said, when the transmitter was deactivated) the robot wasn’t working?

If it is the former, then it could have been field error. We’ve been to a couple competitions where the field wasn’t set up properly, resulting in certain robots not working. For example, couple weekends ago, anyone on crystals on one side of the field died immediately after the autonomous, whereas on the other side of the field, the tethers simply weren’t working. Also, if the crystal frequencies are too similar, there could be interference with the signals.

…after the autonomous was completed and scored and when the driver control time period started the robot wouldn’t move. We then installed the jumper in slot 6 to disable autonomous and then we could drive…but of course we had no autonomous action…Oh,let me state that we had to update to Master 10 just prior…so I’m thinking it was a code problem…I am trying to figure out how this all works so I can help the kids solve this problem…I am thinking something my be outta wack with the mode times in the programs…I wanna know how it should be programmed…Thx