VEX 75MHz Crystal Radios

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VEX Robotics Transmitter and Receiver Pair

VEX 75MHz Crystal Radios

The VEX 75MHz Transmitter is used as a remote control for the VEX Robotics system. It sends radio signals to a small 75MHz receiver attached to the robot. The receiver can be used with either the VEX Microcontroller (V0.5), the VEX Cortex Microcontroller or with the VEX Signal Splitter. The Transmitter and Receiver must have matching crystals inserted so they operate on the same radio channel and are only compatible with the VEX Microcontroller.

Operator Controls

The VEX 75MHz Transmitter allows an operator to control 6 channels remotely. Channels 1-4 are analog signals operated by two joysticks on the front of the transmitter. It is possible to utilize a 2nd Transmitter and Receiver so you can use up to 12 channels of input, with two different human operators. The joysticks each control two analog channels; one for the X and one for the Y axis. They have springs which cause the joysticks to return to their center positions when released. Additionally, the joysticks have small "trim" controls which can be used to adjust the center position of each channel.

Each input on the transmitter is labeled with a number that ranges from 1-6. Using Default Code, these numbers correlate with the PWM outputs on the VEX microcontroller. Ex: Channel 1 is the input generated by moving the right joystick in a left and right motion. This would control a motor connected to Motor Output 1.

Channels 5-6 are digital channels operated by two pairs buttons on the back of the transmitter (a.k.a. "back buttons"). With no buttons pressed, these channels send a neutral, or centered, signal. When one of a channel's buttons are pressed, the channel will send a full CW or full CCW signal. If both of a channel's buttons are pressed, then the signal returns to a neutral value.

In addition to these channel controls, there is a power switch, and a programming interface. This consists of a small LCD and a few buttons to navigate through menus and select options. These controls may be used to enable such features as "Tank Steering" or "Arcade Steering", as well as reverse and scale each channel. While not in use to program the transmitter, the LCD constantly monitors the transmitter battery voltage. For full details on programming the transmitter, see the "Transmitter Menus" section of the VEX Inventor's Guide (link provided below).

VEX Microcontrollers allows 2 Transmitters to control one brain. (i.e. 12 operator channel inputs control 8 motor outputs.) By utilizing a 2nd Transmitter and Receiver you can use up to 12 channels of input, with two different human operators. Some robots have multiple functions that are cumbersome to control using only one operator.

Crystals, Channels, and Frequencies

The VEX 75MHz radio control system includes ten crystal pairs, allowing ten radio links to operate in close proximity without interfering with each other. The crystals all operate in the 75MHz-76Mhz band set aside by the FCC (in the U.S.) for use with radio-controlled "model surface craft" such as R/C cars and boats. There are 30 channels within this frequency range which match the channel numbers marked on the VEX crystals.

Power, Tether, and Antenna

The VEX 75MHz Crystal Radio requires a 9V power source, which may either be eight AA batteries installed into the provided battery holder, or a VEX 9.6 Volt Transmitter Battery pack. The power connector to the transmitter is a standard "Tamiya" power connector.

The back of the transmitter has "tether port" which has three uses:

1. A tether cable (telephone handset cord) can be used to directly connect the transmitter to a VEX Microcontroller or signal splitter, which is useful during prototyping or during competitions when radio transmissions are strictly controlled.
2. The tether port can be used to disable the actual RF transmitter inside the remote control, which is important during competitions to ensure all robots start and stop at the same time.
3. The tether port is used to attach the VEXnet Wifi Upgrade.

The VEX 75MHz Crystal Radio has an integral telescoping antenna. When retracted, it fits entirely inside the case of the transmitter. When fully extended, it measures approx 33 inches (84 cm) long. Replacement antennas are available from the VEX Store and can be easily installed by unscrewing the old antenna and screwing the new antenna into place.

The RF Signal

This transmitter generates a 6 channel PPM pulse train, which is FM modulated in the 75 MHz band. This signal is fairly standard in the R/C hobby industry, and is suitable for operating radio controlled "surface" models (in contrast to the 72 MHz band, which is used for operating radio controlled aircraft). The raw (unmodulated) PPM pulse train is available at the Tether Port on the back of the transmitter. For a full description of this signal, visit the 6 Channel PPM Signal page.

Documentation and Links

• VEX Control Products (VEX Store)
• VEX 2008 Inventor's Guide - Control (4.1M pdf, 06-27-08)
• VEX 2008 Inventor's Guide - Appendix D - Control Configurations (2.6M pdf, 06-27-08)
• VEX 2008 Inventor's Guide - Appendix E - Transmitter Menus (3.1M pdf, 06-27-08)
• VEX 2008 Inventor's Guide
• List of R/C Frequencies and Channels

CMU Robotics Academy Resouces

• Troubleshooting Radio Control (Teaching ROBOTC for VEX Print)
• Remote Control Operation Lessons (VEX Curriculum 2.0)
• How are Signals Sent Lesson (VEX Curriculum 2.0)
• Using ROBOTC with the Radio Control Transmitter (Teaching ROBOTC for VEX)
  

ROBOTC Sample Code