This lower-power, smaller, and lower-cost version of the TReX is the ultimate general-purpose motor controller for small motors: it has a wide operating range (5 to 24 V), delivers 2.5 A continuously per motor, provides current sensing on the two main channels, and it features the TReX triple interface that allows you to control motors from a serial line (RS-232 or TTL), radio control (RC) receiver, or analog voltage source such as a potentiometer.

The TReX Jr Dual Motor Controller, a lower-power version of the TReX Dual Motor Controller, is a versatile DC motor controller designed to seamlessly blend autonomous and human control of small robots. The TReX Jr can control two bidirectional and one unidirectional motor via three independent control interfaces: radio control (RC) servo pulses, analog voltage, and asynchronous serial (RS-232 or TTL). Using the compact MC33887 motor driver allows for a single-board design that reduces cost while maintaining a small package. As an added benefit, the use of the MC33887 motor driver allows operation to 24 V.

The TReX Jr uses five input channels to receive the RC or analog control signals. When operating in RC or analog mode, the five channels function as follows:

Channel 1: motor 1 speed and direction or, if in mix mode, turn left/right
Channel 2: motor 2 speed and direction or, if in mix mode, go forward/reverse
Channel 3: auxiliary (unidirectional) motor speed
Channel 4: can be used to enable “flipped mode”, which allows invertable robots to be controlled as normal when they are inverted
Channel 5: determines whether the motors are controlled by the channel inputs or the serial interface; this channel allows you to switch between autonomous and human control at will

The serial interface can switch instantly with one of the other two interfaces, allowing mixed autonomous and remote control. For example, a robot could be configured to run autonomously most of the time, but a human operator could override the autonomous function if the robot gets stuck or into a dangerous situation. If the serial mode is selected as the primary interface, high-resolution measurements of all five channel input signals (be they RC pulses or analog voltages) are made available to the autonomous robot controller, allowing for complex and unlimited mixing of operator control and sensor input. For example, the TReX would be a great motor controller for a remotely controlled balancing robot.

Pololu TReX Jr Dual Motor Controller with labels

Technical specifications

Dimensions	1.75" x 1.75" x 0.5" (0.3" height without connectors)
Supply voltage	5 – 24 V
Primary motor outputs	2 bidirectional, 5 A peak, up to 2.5 A continuous (may be combined to control 1 bidirectional, 5 A continuous)
Auxiliary output	1 unidirectional, 10 A continuous
Speed control	128 steps in each direction
Control interfaces	RC servo pulses, analog voltages, or serial commands; master interface set by mode-select jumper
Input channels	5 (RC servo pulses or analog voltages)
RC pulse measurements	12-bit resolution, 1 us accuracy
Analog measurements	10-bit resolution
Serial interface	both RS-232 and TTL (logic level), bidirectional
Baud rates	supports 11 common baud rates ranging from 1200 to 115,200 bps
Expanded protocol	Allows multiple Pololu servo and motor controllers on one serial line
Compact protocol	Allows both motors 1 and 2 to be simultaneously set with just three bytes

RC/Analog control features

Battery Elimination Circuit (BEC) jumper lets the TReX Jr optionally power your RC receiver or analog controller
mix-mode jumper allows for single-stick motor control of differential-drive robots
automatic calibration for your particular RC or analog controller (the TReX Jr can learn the channel ranges)

Options accessable through the serial interface

all five high-resolution channel input values are available
remapped channel input values show what the TReX Jr would do if it were in control
optional 7-bit cyclic redundancy checking to ensure command/data validity
calibration values can be explicitly read and set
each channel can be reversed and parabolically scaled
current limit, acceleration, and maximum motor speed settings
the bidirectional motors can (individually) be set to coast or variable brake when in the deadband
upgradable firmware

Safety features

“safe-start” requirements prevent accidental motor activation at power-up
optional automatic motor shutdown on serial error, timeout, or input channel noise

Note: The TReX Jr does not require use of the serial interface to function; it will work right out of the box as an electronic speed control (ESC). You will not have access to the full suite of features the TReX Jr provides if you do not make use of the serial interface, though.

24" (600 mm) female-female RC servo extension cable.

Servo cable accessories

The TReX Jr has male 0.1" headers; for direct connection to an RC receiver or serial servo controller, we recommend ordering one 6" female-to-female cable, 12" female-to-female cable, or 24" female-to-female cable per channel you intend to use (the TReX Jr can use up to five channels).

Dimensions

Size:	1.75" x 1.75" x 0.5"
Weight:	15 g

General specifications

Motor driver:	MC33887
Motor channels:	3¹
Control interface:	non-inverted TTL serial (2-way); RS-232 serial (1-way), RC servo pulses, analog voltage
Minimum operating voltage:	5 V
Maximum operating voltage:	24 V
Continuous output current per channel:	2.5 A²
Peak output current per channel:	5 A³
Continuous paralleled output current:	5 A
Current sense:	0.5 V/A⁴
Maximum PWM frequency:	10 kHz
Minimum logic voltage:	2.7 V
Maximum logic voltage:	5.5 V
Reverse voltage protection?:	Y

Notes:

1

Two bidirectional, one unidirectional.

2

On primary channels.

3

On the primary channels.

4

Current readings are converted to a digital number from 0-255.

Documentation and other information

Pololu TReX Jr User’s Guide (Printable PDF)

User’s guide for the Pololu TReX Jr motor controller

Sample C# Program for Communicating with the TReX and TReX Jr(Printable PDF)

This sample C# program (written for Microsoft Visual C# 2008) shows how you can use your computer to send and receive data from the TReX and TReX Jr motor controllers.

File downloads

TReX Configurator utility for Windows version 100608 (295k zip): If you have the ability to connect your TReX to a computer (via serial port or USB-to-serial adapter), you can use this program to monitor the channel inputs, control the motor drivers, and configure its various settings. This version supports TReX and TReX Jr firmware versions 1.3 and below. Please manually uninstall older versions of the TReX Configurator before installing this version.
TReX Jr command documentation (100k pdf)
TReX Jr configuration parameter documentation (125k pdf)
TReX Jr (DMC02) Firmware version 1.2 (161k trx): This firmware fixes a bug that affected the 38,400 and 28,800 bps baud rates, and it adds a configuration parameter that lets you optionally insert a 1 ms delay between the last byte of a received command packet and the first byte of the TReX Jr’s transmitted response. Without this delay, slower controllers, such as the Basic Stamp, can miss the first byte of the TReX Jr’s response.
MC33887 motor driver datasheet (1MB pdf)
Drill guide for Pololu TReX Jr Dual Motor Controller DMC02 (85k dxf): This DXF drawing shows the locations of all of the board’s holes.