Overview

For my self-balancing robot I wanted to control two DYNAMIXEL motors straight from an ESP32, without buying a ROBOTIS controller board. The catch is that DYNAMIXEL motors talk over one data wire, and the ESP32’s UART expects two (TX and RX).

It turns out you can make it work with a jumper wire, one line of setup code and a small check in the driver. No extra chip. Here’s how I did it.

The problem

A normal UART uses two wires, one per direction. A half-duplex bus uses one wire shared by the controller and several devices, which take turns.

A normal UART has separate wires for sending and receiving. A half-duplex bus uses one wire for both, so everyone has to take turns: the ESP32 sends a command to one motor ID, stops, and then that motor replies on the same wire.

Two things get in the way:

  1. The ESP32’s TX pin holds the line high all the time, so it would block the motor’s reply.
  2. The motors use 5 V signals, but the ESP32 is a 3.3 V chip.

The trick

The ESP32 TX pin GPIO43, set to open-drain, is bridged to RX pin GPIO44. The joined wire goes through a BSS138 level shifter to the 5 V DATA wire shared by two motors.

  1. Jumper TX to RX. Connect GPIO43 (TX) to GPIO44 (RX), so the two pins become one wire.
  2. Set TX to open-drain. Now TX can only pull the line low. When it’s not sending, it lets go, and the motor can use the wire to reply.
  3. Throw away the echo. Since TX and RX are joined, the ESP32 hears everything it sends. After each command, the code reads those bytes back, checks they match, and drops them. Whatever comes after that is the motor’s reply.

For the voltage, I used a BSS138 level shifter module between the ESP32 and the bus. It just converts 3.3 V ↔ 5 V. Its built-in pull-up resistors also pull the line high when nobody is talking, which open-drain needs.

A nice side effect of step 3: if the echo doesn’t come back, I know it’s a wiring problem and not the motor.

Building the board

Front of the finished control board with the ESP32, level shifter, regulator and MPU6050
My finished board on 50 × 70 mm perfboard.

The board has:

  • ESP32-S3 Nano (bottom)
  • BSS138 level shifter (top left)
  • 5 V regulator (top right), to run the ESP32 off the battery
  • Motor socket (right edge, the upper white connector). The second motor is daisy-chained: the socket goes to motor 1, and motor 1’s spare port goes to motor 2.
  • Switch connector (right edge, the smaller lower one) for the power switch
  • MPU6050 (middle). Only needed for the robot; skip it if you just want the motors.

I put every module on female headers so I can swap one out if it dies.

Part Qty Notes
Waveshare ESP32-S3-Nano 1  
BSS138 level shifter module 1 Only channel 1 is used
Pololu D24V10F5 5 V regulator 1  
DYNAMIXEL XL430-W250-T 1–2 With their 3-pin cables
JST B3B-EH-A 3-pin header 1 Motor socket
3S LiPo battery and power switch 1 each  
5 A inline fuse 1 Optional
Perfboard, pin headers, wire –  
U2D2 adapter 1 To set motor IDs from a PC

Full control board schematic Full schematic, drawn in EasyEDA. Click to enlarge, or download the PDF.

These are the connections that actually matter for the single wire:

From To
ESP32 TX1 (GPIO43) ESP32 RX0 (GPIO44), the jumper
ESP32 TX1/RX0 Level shifter LV1
Level shifter HV1 Motor socket DATA (pin 3)
ESP32 3V3 and regulator 5 V Level shifter LV and HV
All grounds Together
Full pin list
Module Pin Goes to
ESP32 TX1 (GPIO43) + RX0 (GPIO44) Joined, then to level shifter LV1
ESP32 VBUS Regulator 5 V out
ESP32 3V3 Level shifter LV, GY-521 VCC
ESP32 A4 / A5 (GPIO11 / 12) GY-521 SCL / SDA (optional)
ESP32 VIN Not connected
Level shifter HV Regulator 5 V out
Level shifter HV1 Motor socket DATA (pin 3)
Regulator VIN Battery +, after the switch
Motor socket Pin 1 / 2 / 3 GND / battery + (about 12 V) / DATA
Everything GND Common ground

Check the level shifter’s printed labels. The pin order is different between brands.

Full wire list
From To Wire
Battery + (optional fuse) → switch connector → VBAT 20–22 AWG red
Battery − GND 20–22 AWG black
VBAT Regulator VIN, motor socket pin 2 20–22 AWG red
GND Regulator GND, motor socket pin 1 20–22 AWG black
GND ESP32 GND, level shifter GND, GY-521 GND 26 AWG black
Regulator 5 V Level shifter HV, ESP32 VBUS 26 AWG orange
ESP32 3V3 Level shifter LV, GY-521 VCC 26 AWG purple
ESP32 TX1 ESP32 RX0 (short jumper) 26 AWG blue
ESP32 TX1/RX0 Level shifter LV1 26 AWG blue
Level shifter HV1 Motor socket pin 3 26 AWG cyan
ESP32 A5 / A4 GY-521 SDA / SCL 26 AWG pink / yellow

Motor 2 plugs into motor 1’s second port with a normal DYNAMIXEL cable.

Back of the control board showing the solder traces
Back of the board.

A few things I learned while building it:

  • Keep the TX–RX jumper and the wire to LV1 short.
  • Run the motors straight from the battery, not the 5 V regulator. Each motor can pull about 1.4 A.
  • Join all the grounds.
  • Turn the battery on before plugging in USB. Otherwise the motors can lock up with a low-voltage error. It happened to both of mine once, and they needed a restart.
  • Use a 3S battery, never 4S.

I tested everything on a breadboard with one motor before soldering anything:

The circuit built on a breadboard
Breadboard version.
Breadboard circuit connected to the LiPo battery and a motor with wheel
With the battery and a motor.
Driving the motor and reading the serial monitor.

Setting up the motors

New DYNAMIXEL motors all ship as ID 1 at 57600 bps. If two of them are on the same wire, they both answer at once, so each one needs its own ID first.

I used DYNAMIXEL Wizard 2.0 with the U2D2 adapter, one motor at a time:

Setting Left motor Right motor
ID 2 1
Baud rate 1 Mbps (1000000) 1 Mbps (1000000)
Return delay 0 0

Get the source code

All the code is on my GitHub. Download it, build it with ESP-IDF and flash it.

Get the source code on GitHub →

The code

How it works:

  • A normal UART on GPIO43 (TX) and GPIO44 (RX) at 1 Mbps.

  • TX is set to open-drain, so it lets go of the wire when it’s not sending:

    gpio_od_enable(TX_PIN);   // TX lets go of the wire when idle
  • Everything sent comes back on RX (the echo). The code checks the echo, then reads the motor’s reply.
  • Log output goes over USB, since GPIO43/44 are used for the motors.

Testing

Before plugging anything in, check the board with a multimeter:

Check Expected
Battery + to GND (continuity, no battery) No beep
ESP32 TX1 to RX0 (continuity) Beep
Regulator output (battery on, only regulator plugged in) 5.0 V

Then flash hw_test from the repo. It checks the wiring and both motors and prints PASS / FAIL.

Results

Test Result
Speed 1 Mbps
Ping + read one motor 1.00 ms
Read both motors + send both speeds ≈1.2 ms

What I’d try next

  • A stronger pull-up (1 kΩ from DATA to 5 V) for a sharper signal.
  • A 74LVC2G241 buffer chip with a direction pin, for higher speeds.

Troubleshooting

Problem Check
Echo error TX–RX jumper missing, or LV1 not connected
Motor doesn’t answer Battery on? Level shifter powered? Common ground? Right ID and baud rate?
Motor shows error 0x01 Low-voltage lockout. Restart the motor, and turn the battery on before USB
ESP32 resets when the motors work hard Weak 5 V supply or low battery

Files

Where it’s used

The Two-Wheel Self-Balancing Robot

Two-Wheel Self-Balancing Robot

This board runs both wheel motors 250 times a second so the robot can balance by itself.

View the project →