Serial-to-Modbus Bridge (ESP32 + RS485)

USB Serial bridge for dual-channel Modbus RTU DC power supplies (e.g. SK120X-style modules): 36 V / 6 A per channel. Uses ESP32 or ESP32-C3 and RS485.

No WiFi, no web UI—just a text-based serial protocol for direct computer control.

Features

USB Serial — Text-based command/response protocol
Dual channel — Two independent outputs (Modbus slave IDs 1 & 2), 36 V / 6 A per channel
Fast streaming — Continuous DATA output at configurable rate (50-1000ms)
Board abstraction (HAL) — One codebase for ESP32 and ESP32-C3

Hardware Setup

MCU: ESP32 or ESP32-C3 dev board
RS485 transceiver (DE/RE optional; enable via build flag)
Two Modbus RTU power supply units (slave IDs 1 and 2)

Pin mapping

Board	UART	RX	TX	RS485 DE/RE
ESP32 (e.g. devkit)	Serial2	16	17	4
ESP32-C3	Serial1	20	21	3

Baud rate: 115200 (both USB CDC and Modbus).

Software Setup

Build & upload (PlatformIO)

# Build both environments
pio run

# Build for a specific board
pio run -e esp32dev
pio run -e esp32-c3-devkitm-1

# Upload
pio run -t upload

Open Serial Monitor (115200) after upload. The device sends !READY v1.0.0 on boot.

Serial Protocol

The ESP32 streams data continuously. GUI/scripts send commands to control behavior.

Output format (ESP32 → Host)

DATA <ch> setV=<v> setA=<a> outV=<v> outA=<a> outP=<p> outE=<e> inV=<v> temp=<t> time=<s> cap=<c> ovp=<v> ocp=<a> opp=<p> status=<s> cvcc=<m> out=<0|1> mppt=<0|1>

ch: Channel number (1 or 2)
Values are raw register values (see Register Map below)
On read error: DATA <ch> ... ERR

Commands (Host → ESP32)

Command	Description
`WRITE <ch> <reg> <val>`	Write single register. Response: `OK` or `ERR`
`LINK`	Copy Ch1 settings (V, I, output, MPPT) to Ch2. Response: `OK` or `ERR`
`STOP`	Stop streaming. Response: `OK stopped`
`START`	Resume streaming. Response: `OK started`
`POLL <ms>`	Set poll interval (50-1000ms). Response: `OK poll_ms=<ms>`
`POLL`	Query current poll interval. Response: `OK poll_ms=<ms>`
`RAW <ch>`	Dump all 32 registers. Response: `RAW <ch> <v0> <v1> ... <v31>`
`PING`	Connectivity check. Response: `PONG`
`REGS`	Print register map
`VERSION`	Firmware version. Response: `OK 1.0.0`

ch: 1 or 2
reg/val: decimal or hex (0x prefix)

Example session

!READY v1.0.0
DATA 1 setV=1200 setA=3000 outV=1198 outA=1500 outP=1797 outE=0 inV=2400 temp=381 time=0 cap=0 ovp=3700 ocp=6100 opp=25000 status=0 cvcc=0 out=1 mppt=0
DATA 2 setV=1200 setA=3000 outV=1195 outA=1480 outP=1768 outE=0 inV=2400 temp=378 time=0 cap=0 ovp=3700 ocp=6100 opp=25000 status=0 cvcc=0 out=1 mppt=0
> STOP
OK stopped
> WRITE 1 0x0000 1400
OK
DATA 1 setV=1400 ...
> POLL 200
OK poll_ms=200
> START
OK started

Register Map

Reg	Name	Unit	R/W
0x0000	setV	V * 100	R/W
0x0001	setA	A * 1000	R/W
0x0002	outV	V * 100	R
0x0003	outA	A * 1000	R
0x0004	outP	W * 100	R
0x0005	outE_H	Wh (high)	R
0x0006	outE_L	Wh (low)	R
0x0007	inV	V * 100	R
0x000D	temp	°C * 10	R
0x0009	time_H	sec (high)	R
0x000A	time_L	sec (low)	R
0x000B	cap_H	Ah*1000 (h)	R
0x000C	cap_L	Ah*1000 (l)	R
0x000E	OVP	V * 100	R/W
0x000F	OCP	A * 1000	R/W
0x0010	OPP	W * 100	R/W
0x0012	out	0/1	R/W
0x0013	status	flags	R
0x0014	cvcc	0=CV, 1=CC	R
0x001F	mppt	0/1 (exp)	R/W

GUI Application (psu-gui)

Standalone SDL2 control panel for the PSU with:

VFD display — Voltage, current, power readouts with green-on-black style
Bar meters & temperature — Voltage/current bars and temp strip (full GUI)
Scope traces — Real-time voltage/current waveform per channel
Controls — OUTPUT toggle, voltage/current setpoints, keypad (full GUI)
Dual vs single — psu_gui / psu_gui_toolbar (two channels); psu_gui_single / psu_gui_toolbar_single (channel 1 only)
TRACKING — Link Ch1 settings to Ch2 (dual full GUI only)
Demo mode — Runs with simulated data if serial port unavailable

Screenshots (canonical copies in docs/images/psu-gui/; duplicates in psu-gui/screenshots/). More detail: psu-gui/README.md.

Dual full GUI (psu_gui)

Single full GUI (psu_gui_single)

Dual toolbar (psu_gui_toolbar)

Dependencies

# Ubuntu/Debian
sudo apt install libsdl2-dev libsdl2-ttf-dev

# Fedora
sudo dnf install SDL2-devel SDL2_ttf-devel

# Arch
sudo pacman -S sdl2 sdl2_ttf

Also needs a monospace font (DejaVu Sans Mono or Liberation Mono).

Build & Run

cd psu-gui
make

# With ESP32 connected
./psu_gui /dev/ttyUSB0

# Demo mode (no device)
./psu_gui

# Compact toolbar-style GUI (dual channel)
./psu_gui_toolbar /dev/ttyUSB0
./psu_gui_toolbar

# Single-channel variants (hardware channel 1 only)
./psu_gui_single /dev/ttyUSB0
./psu_gui_toolbar_single /dev/ttyUSB0

GUI Controls

OUTPUT button — Toggle output on/off
VOLTAGE/CURRENT inputs — Click to edit, type value, press SET or Enter
TRACKING — When enabled, copies Ch1 settings to Ch2
REFRESH — Force status poll

Toolbar GUI (psu_gui_toolbar): Compact strip with large V/A readouts, ON/OFF and CV/CC status. SET opens a modal to edit setpoints (click outside, CANCEL, or Esc to close)

Code Structure

src/main.cpp — Serial protocol, DATA streaming, command parser
src/hal/ — Board HAL: UART/RS485 pins, init
src/modbus_psu.h / modbus_psu.cpp — Register map, Modbus read/write
psu-gui/ — SDL2 GUI application
- main.c — Full dual-channel GUI
- main_single.c — Full single-channel GUI
- main_toolbar.c — Compact dual-channel toolbar
- main_toolbar_single.c — Compact single-channel toolbar
- serial_port.c/h — Linux serial port abstraction
- psu_protocol.c/h — PSU command/response parsing

Scripting Example (Python)

import serial
import time
import re

ser = serial.Serial('/dev/ttyUSB0', 115200, timeout=1)
time.sleep(2)  # wait for ESP32 boot

# Wait for READY
while True:
    line = ser.readline().decode().strip()
    if line == '!READY':
        break

# Stop streaming to send commands
ser.write(b'STOP\n')
ser.readline()  # OK stopped

# Set voltage to 15.00V on channel 1
ser.write(b'WRITE 1 0x0000 1500\n')
print(ser.readline().decode().strip())  # OK

# Resume streaming and read data
ser.write(b'START\n')
ser.readline()  # OK started

# Parse DATA lines
for _ in range(10):
    line = ser.readline().decode().strip()
    if line.startswith('DATA'):
        m = re.search(r'outV=(\d+)', line)
        if m:
            print(f"Output voltage: {int(m.group(1)) / 100:.2f} V")

ser.close()

Build Options

RS485 Direction Control

By default, RS485 DE/RE control is disabled. To enable:

# In platformio.ini, add to build_flags:
build_flags =
    -DUSE_RS485_DIR=1

Notes

MPPT — Experimental; support depends on PSU firmware.
Values are raw register values (setV 1200 = 12.00 V, setA 3000 = 3.000 A, etc.)
Single-threaded Arduino environment; modbus_psu_meaning() / modbus_psu_interpret() use static buffers.

Resources

SK120X DC Power Supply (Amazon)
Modbus library: lahirunirmalx/ModbusMaster

License

MIT — use, modify, and share.

Navigation Menu

Repository files navigation

Serial-to-Modbus Bridge (ESP32 + RS485)

Features

Hardware Setup

Pin mapping

Software Setup

Build & upload (PlatformIO)

Serial Protocol

Output format (ESP32 → Host)

Commands (Host → ESP32)

Example session

Register Map

GUI Application (psu-gui)

Dependencies

Build & Run

GUI Controls

Code Structure

Scripting Example (Python)

Build Options

RS485 Direction Control

Notes

Resources

License

About

Releases

Packages

Contributors

Languages

Name		Name	Last commit message	Last commit date
.vscode		.vscode
bin		bin
docs/images/psu-gui		docs/images/psu-gui
include		include
lib		lib
psu-gui		psu-gui
src		src
test		test
.gitignore		.gitignore
LICENSE		LICENSE
README.md		README.md
platformio.ini		platformio.ini

Latest commit

History

Folders and files

Repository files navigation

Serial-to-Modbus Bridge (ESP32 + RS485)

Features

Hardware Setup

Pin mapping

Software Setup

Build & upload (PlatformIO)

Serial Protocol

Output format (ESP32 → Host)

Commands (Host → ESP32)

Example session

Register Map

GUI Application (psu-gui)

Dependencies

Build & Run

GUI Controls

Code Structure

Scripting Example (Python)

Build Options

RS485 Direction Control

Notes

Resources

License

About

Topics

Resources

Stars

Watchers

Forks

Releases

Packages

Contributors

Languages