Pololu TReX Jr User Manual

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TReX Jr before changing jumpers (other than the mix jumper, which can be changed at an time). This is not strictly
necessary, but it is the safest practice.

TReX Jr jumpers

• Channel-Mix (a.k.a. Single-Stick) Jumper: When this jumper is in place, the TReX Jr operates in single-
stick mode. Channel 1 is treated as turn left/right while channel 2 is treated as forward/reverse. When this
jumper is off, channel 1 directly controls motor 1 and channel 2 directly controls motor 2. This jumper may be
added or removed on the fly.

• Battery Elimination Circuit (BEC) Jumper: When this jumper is in place, the middle column of channel
input pins is connected to Vcc (5 V). You can use this jumper to power your RC receiver or analog controller
through your TReX Jr. If your RC receiver is powered via another source, you must leave this jumper off.

Warning:

The Vcc column is tied to the output of a linear voltage regulator, so its current output is

limited by thermal dissipation. The regulator will only be able to safely supply a maximum of 100 mA
when VIN is 12 V and 50 mA when VIN is 24 V (it has a 1-W power dissipation rating). This is typically
sufficient for powering an analog joystick or RC receiver, but it is insufficient for powering servos. If you
want to connect servos to some of your RC receiver channels, you must power your RC receiver
separately and disconnect the BEC jumper. Attempting to use the TReX Jr’s regulated Vcc line to power
servos can permanently damage the TReX Jr.

• Mode-Select Jumper: This jumper specifies the interface that is in control of the TReX Jr. When it shorts
the right two pins (as shown in the jumper image above), the TReX Jr is in RC mode, which means the TReX
Jr is expecting RC pulse inputs on its five input channels and that these RC signals will determine the motor
outputs. When the jumper shorts the left two mode pins, the TReX Jr is in analog mode; the TReX Jr measures
the analog voltages on the five input channels and sets the motor outputs accordingly. When there is no jumper
on the three mode pins, the TReX Jr is in serial mode and the serial interface controls the motors. The control
interface is determined by the location of the mode jumper when the TReX Jr is first powered up. Changing the
location of the mode jumper while the TReX Jr is running will have no effect.

• Learning-Mode/Firmware Upgrade Jumper: The TReX Jr can automatically calibrate itself for your RC
or analog controller by learning the neutral and extreme values for each channel you use. It also has the ability

Pololu TReX Jr User's Guide

3. Getting Started

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