Problem using two transmitters

My team finally finished building our robot modules and combined them together into a complete robot.

The way our 'bot is built demands that we use two transmitters.

Transmitter 1 should command motor ports 2, 3, 7 and 8.
Transmitter 2 should command all the other motor ports.

The trouble is, when we turn on the transmitters, #1 does everything and #2 does nothing.

We have a jumper on controller port 15 to use tank controls.

We’ve hacked a fix using SET PT (and turning down all motor ports not used by that transmitter), but that seems like a clumsy way to do it.

Has anyone else had problems/solutions with this?

FVT-42 The Hitchhikers

Our impression was Cindy was going to send out a second set of crystals, but that hasn’t happened yet. We have been using phone handset cord as a tether, which is a pretty good workaround.

Radio shack requires part numbers, which you can get by checking out:

There are lots of replacement parts listed, in particular crystals. I just used page
to verify they are in stock for me locally.

In regard to extra crystals…I think that will not help at the competition. There will be four teams playing at once and I believe each will have ONE set of crystals.

I can’t believe that the is only going to be 1 crystal per team.

I know that is what they have SAID, but it just doesn’t square up with the two driver part of the rules.

10 days left to figure it all out…

Joe J.

The way the manual is written, it sounds like we should be able to use two transmitters with one crystal.
But, untill we figure that out, here is our (temporary?) solution.

A guy on my team, Daniel, came up with this great hack:

Using EDIT PT, turn the response number thingie for motor ports 2, 3, 7, and 8 all the way down to zero on your second controller.
Then, use EDIT PT to turn the numbers for motor ports 4, 5, and 6 all the way down on the first controller.
And, of course, label the controllers with tape. :wink:

You can also change this on motor port 1 using SCALE, but you have to do this last, as anything you enter using EDIT PT will erase all SCALE stuff. It’s really kind of a pain.

Sorry, I’m not using all the technical terms. I’ll get Daniel to double-check this, but it should work.

I hope this helps, temporarily at least, the other teams who are having this problem.

I initially was sure that you needed 2 different crystals and 2 recievers to allow for 2 transmitters to work.

THEN, FIRST came out so confidently with their TEAMS WILL HAVE ONLY ONE CRYSTAL IN ATLANTA message, I read the manual again, and this time, I was sure that it was possible (especially reading Appendix E pages 11-20 or so).

THEN we tried to actually DO it… …no good… …I started scratching my head and it hit me: How does the reciever know which transmitter is #1 and which transmitter is #2? It can’t know – they’re both on the SAME FREQUENCY!

Armed with this insight, I read the manual again. I stumbled on page 6-21 (this is from memory, so I could be wrong). Basically, there is a note on this page that says words to the effect of “all multi-transmitter modes require multiple (different) frequency crystal sets.”

SO… …now I am back where I started, Multiple drivers (with multiple transmitters) require multiple frequencies.

I don’t know how FIRST is going to square this circle:

  1. They specifically talk about multiple drivers and transmitters in the rules, which I am now convinced that will require multiple crystal sets per team.
  2. They have specifically said teams will have 1 crystal set per team in Atlanta. By the way, even if we HAD multiple crystals sets, the rules as currently written only allow one receiver, so where would we plug the robot half of the second crystal set in.

What is a VEX team to do? Don’t know… …something has got to give.

…by the way, what did they do at WPI?

Did they require 1 driver/transmitter/receiver/crytal set or did they allow 2 per team?

Joe J.

Hi all,

The plot continues to thicken…

…I think have had an AH MOMENT… but first see the quote from this e-mail thanks to R Denton.

Subject: Solution to the VEX single-frequency problem with two transmitters

Hi, I’m the mentor for FRV-40, a group of mostly home-schoolers from
northeastern counties outside Atlanta. As you probably have read, FIRST
will assign only one frequency to control each robot, but gave no
guidelines for how to avoid signal conflicts between a team’s two
transmitters. Daniel, one of our student members, has figured out a
programming solution for using only one frequency while operating two

Use the EDIT_PT command to turn down the motor ports on each transmitter
that you do not want it to control. For instance, on our first
transmitter currently motor ports 4, 5, and 6 are turned down to zero
percent. On the second transmitter, currently motor ports 2, 3, 7, and
8 are turned down to zero percent. Motor port 1 can be turned down to
zero, but only by the SCALE command, and every time you use EDIT_PT that
command erases SCALE. Our team at least for the moment has chosen not
to use motor port 1; they can control it using SCALE but they don’t feel
it’s worth the trouble and they can get by without it. Daniel thinks
the overall solution is crude, but the more important point is, it works.

Our team also posted this solution on the VEX forum website,, (credit to
Daniel). This posting is at the VEX robotics section (it’s not the
official VEX forum).

Best wishes in your robot design, our crew is still sweating out some
critical details. See you in Atlanta.

– R. Denton

I PM’ed Colleen-t190 to give us the skinny on the WPI scrimmage. This is what she said:


As far as I know, but I’m going to double check with folks to be sure, teams used one crystal set because there is only one receiver on the robot side. There are two plug-ins on the field though such that two different transmitters can be plugged in per team.

Now do I know that this works correctly, no. I do not believe anyone tried using multiple transmitters. One of our teams had planned on it, but they were waiting for FIRST to send out the team update on how that would work (apparently they said it would be released Friday). It never got sent so they just made theirs work with one transmitter. It was mentioned to FIRST that the update never went out to which the response was “Oh… it didn’t?”. So I’d expect to see an update on it soon.

I’m heading to campus now and I will ask the people who were actually switching crystals (and will be working VEX staff in GA for that) what the good word is on the topic and let you know.


Now hear is what I am guessing they MAY be doing at in Atlanta… They may be tething teams up to the field and letting the field broadcast the signals. This solves 2 problems, 3 actually, no 4…

  1. it give them the chance to combine the signals from 2 transmitters with before sending them to a single common transmitter station set to the right frequency for your robot
  2. it solves the 4 pointy sticks the driver station problem
  3. it means 1 channel per team - so they would not run of crystal frequencies
  4. it lets them turn on and turn off your robot! – how were they going to do this with teams directly sending signals to their robots?

I think I have noodled it out. I HOPE FIRST will give us an answer soon. In the mean time, hopefully the work around by R Denton will help.

Joe J.

At the WPI scrimmage, they had the set of 8 crystals available from Radioshack plus the kit one and assigned each team a crystal for the entire scrimmage, because there were only 10 teams (don’t ask me how 10 teams ran on 9 crystals, I don’t remember, I assume they ran two teams on the kit freq and just switched them when they played each other). No teams used two receivers, although there were 4 disable ports available per side in case teams wanted to (you plug a cord into the tether port when you compete, that disables your controller until the match starts). I would assume at championship they will have to hand out the crystals as teams go on and off the field, which would mean there would be at least 8 different frequencies available for use. I’m not sure what the call is on allowing teams to use two controllers, that would have to come from FIRST.

I would assume teams will have to use tethers in the pit for championship, I believe a representative from FIRST said they were bringing 25 tether cables. We purchased a tether cable for every team at the scrimmage (50ft handset cords were $1 each at the local dollar store and make great tether cables), but didn’t end up needing them as every team was using a separate crystal set.


Another note - the crystal kit that they had a the scrimmage only had 8 crystal sets (one transmitter crystal, one receiver Crystal). They did not have 2 transmitter crystals for each team, meaning that they MUST give teams 2 frequencies and allow 2 receivers. It seems pretty crazy to me the way the rules read that teams are only allowed 1 reciever. Also, from what I’ve leared about FM (although its been a while), it isn’t the greatest idea to be running two transmitters modulated to the same frequency and just hope that you pick up the stuff you want to. Unfortunately I don’t get the updates because I’m not officially on a Vex team, so I have nothing to base my writings on other than the rules on the vex website and what I saw at the scrimmage.

Could someone report on how this was resolved?

This was really handled badly from a communications point of view in my opinion.

Basically, just as the VEX manual states, you need 2 crystal sets to make this work.

Once you have 2 crystal sets and 2 receivers on the RC side, it works pretty well*.

The field had a phone cord type affair that plugged into the tether port on the transmitters. These allowed the folks running the field to enable/disable the robots. I am not sure if this actually caused the transmitters to turn off all motors (by sending 127 to each motor) or if they just turned off the transmitters – I hope it is the second not the first, but I just don’t know.

Actually, I hope that there is some unknown “disable” command that gets sent because once we are able to program these things I think anything less than that may give a sneaky programmer an advantage**.

Radio Shack sells crystal sets with 4 pairs (4 channels) for about $50.

Joe J.

*Just make sure you have BOTH transmitters ON and that both transmitters have good battery states BEFORE you turn on your RC – very strange and potentially damaging stuff can happen if you don’t have a strong signal of the right channel number already on when the recievers start looking for a signal, they can lock onto other weaker signals on other channels.

**A programmer could detect this type of disable (by looking for 127’s on all channels for example) and then choose to keep doing whatever the driver was doing when it happened for an additional second or so. This would let the robot “coast” home after the bell or win that pushing match once the other guy is disabled or perhaps even keep slight power to the winch (to keep from falling for 10 seconds???)