FNIRSI DPS-150: A Portable Programmable Supply, from Official Specs to Direct Serial Control

A palm-sized FNIRSI DPS-150 programmable supply (0–30V / 0–5A / 150W) for which the vendor only ships a Windows PC app. The community has fully reverse-engineered its USB serial protocol, so one line of pyserial reads telemetry, sets voltage and current, and switches the output. The first half of this page collects the specs and UI from the official manual (with product photos); the second half is the reverse engineering and the pitfalls we hit (DPS-150P / firmware V1.6). The code is packaged as tool/dps150.py.

Official resources

FNIRSI official product page · Official manual PDF (archived here) · FNIRSI website

Specs at a Glance (Official Manual)

Item	Value	Item	Value
Model / screen	DPS-150 · 2.8-inch 320×240	Output voltage / current / power	0–30V / 0–5A / 0–150W
Input voltage / current	DC 5.0–32V / 100mA–5A (supports PD / QC fast charging, can run off a power bank)	Full-load efficiency	High (as stated in the manual; see the PDF)
Setting resolution	voltage 10mV · current 1mA	Setting accuracy	voltage ≤0.1%±5mV · current ≤0.1%±3mA
Read-back accuracy	input voltage ≤0.2%±5mV · output voltage ≤0.1%±10mV	Load regulation	0.49%

The manual calls it DPS-150, but the actual unit is DPS-150P (this page uses the real model throughout).

Panel and Main Screen

The front has a 2.8-inch screen plus six buttons (the wheel slides left and right to adjust values):

The main screen is dense (matching the manual's numbered callouts): input voltage, internal temperature, run state, live V/A/W and accumulated energy (Wh/Ah), voltage setpoint (0.01V steps), current setpoint (0.001A steps), preset group number, panel lock state and output mode.

There are seven run states; any of them other than OK turns the output off automatically and sounds the buzzer:

State	Meaning
OVP / OCP / OPP	over-voltage / over-current / over-power protection
OTP	over-temperature protection
REP	reverse-polarity protection
LVP	under-voltage protection (the output cannot be turned on in this state)

Preset groups: the device stores 6 presets (M1–M6), each with its own voltage and current-limit setpoint; a short press switches between them — handy when powering several devices in turn.

Output modes: constant voltage CV or constant current CC.

Operating Essentials

• Adjusting values: click the settings key to enter the highlighted field, then slide the wheel left/right to change it; click again to switch between the voltage and current setpoints, short-press to save and exit
• Menu items (Power Settings / System Settings / About): the OVP/OCP/OPP/OTP/LVP thresholds are each configurable, plus language, standard ↔ industrial UI style, backlight brightness, volume and metering on/off
• Saving a preset: open the data group page → click to highlight a field → adjust with the wheel → short-press to save
• ⚠️ Once the PC app is connected the control panel locks itself (the lock icon turns white) and only unlocks when the serial session ends — the control tool we wrote later triggers this too

Advanced Features Built into the PC App

Besides basic monitoring and waveform logging (the voltage/current curves zoom with the wheel and can be saved or deleted), the official Windows app has three advanced "run the output like a program" features:

Feature	What it does
Sequence output	steps through preset V/I values on a timer, within a step range and loop count; supports manual single-stepping, pause/resume
Current sweep	voltage fixed, current sweeps from start to end as "step current + delay" (common in CC mode)
Voltage sweep	current fixed, voltage sweeps from start to end as "step voltage + delay" (common in CV mode)

Firmware Upgrade (Two Routes)

• USB-drive method: hold the upgrade key while plugging in the data cable; a USB drive shows up on the computer, copy the firmware in and the device upgrades itself
• PC app method: once connected, click "Firmware Upgrade → BOOT/restart"; the same USB drive appears for copying the firmware

⚠️ Note: firmware upgrade uses DFU mode. Don't confuse it with 0xC0 in our serial protocol — that's the "enter DFU" command byte, and sending it by mistake drops the serial port entirely (see the dangerous byte at the end).

Device Identification

It connects over the Micro-USB data port and enumerates as a USB CDC virtual serial port (the internal MCU is an Artery AT32):

system_profiler SPUSBDataType | grep -A6 "AT32 Virtual Com Port"   # 识别设备
ls /dev/cu.usbmodem*                                               # 串口设备节点

Protocol Overview (Community Reverse Engineering)

The frame is minimal — 1 header byte + command + register + length + data + checksum:

0xF1	Command	Register	Length	Data (0..N)	Checksum
Header	Cmd	Reg	Len	Data…	Sum

Checksum = (寄存器 + 长度 + Σ数据) & 0xFF (header and command are excluded); the device replies with header 0xF0; all values are IEEE754 float32 little-endian.

Command bytes:

Command	Purpose
0xA1	read register
0xB1	write register
0xB0	set baud rate
0xC1	session connect / disconnect
0xC0	⚠️ enter DFU flash mode — never send this (the CDC port disappears; replug to recover)

Key registers:

Register	Type	Meaning
0xC1	float32	voltage setpoint
0xC2	float32	current setpoint
0xDB	uint8	output switch (0/1)
0xC0	float32	input voltage
0xC3	3×float32	measured output V/A/W
0xDC	uint8	protection status (0=OK,1=OVP…6=REP)
0xDD	uint8	0=CC / 1=CV
0xFF	139B	full status dump (6 presets, protection thresholds, Ah/Wh)

Control Loop Measured

One command sets the values and turns the output on; the soft start ramps up in about 1.5 s:

Capability	Measured
Read model / firmware / hardware	DPS-150P · V1.6 · V1.0
Read telemetry	input 20.08V, temperature 30.5°C, output V/A/W, full 139-byte status
Write setpoints	5.000V / 0.500A, read back identical
Output on/off	on → 5.000V (soft start ~1.5s); off → discharges to zero in ~1.2s

Common operations:

python3 tool/dps150.py status                               # 读全状态
python3 tool/dps150.py set --voltage 12.0 --current 2.0     # 设 12V/2A（不开启）
python3 tool/dps150.py set --on                             # 开输出
python3 tool/dps150.py set --off                            # 关输出

Pitfall Archive

Symptom	Root cause	Solution
Register reads come back scrambled, only 12 bytes	the device pushes telemetry frames unsolicited (0xC3 etc.) into the response stream	filter the read-back by register number and drop non-target frames
Reading 0xE2/0xE3 on their own returns 0	the capability registers only carry values in the pushed frames	take max V/I from offsets 0x6F/0x73 of the full status dump
Setting 30V doesn't stick	buck architecture: the output ceiling is clamped below the input voltage (20V in → 19.88V max)	raise the input voltage if you need more output
Right after enabling, the voltage reads only a fraction of a volt	the soft start is still ramping	wait ≥1.5s before reading
Panel buttons stop working	connecting the PC app auto-locks the panel	end the serial session to unlock

One dangerous byte

0xC0 isn't a reboot — it enters DFU flash mode: the USB CDC port vanishes and you must physically replug to recover. The device has no soft-reboot command.

Reproduction and Sources

• Control tool: tool/dps150.py in the fnirsi-dps150 skill (only depends on pyserial)
• Protocol source: community reverse engineering nasheed-x/dps150_api (PROTOCOL.md, cross-checked against four implementations in JS/C/Python/Rust plus a decompiled official .NET app)
• Official resources: FNIRSI official product page · official manual PDF (all specs and UI images here come from that manual, archived on this site)
• For the full register map, the 139-byte layout and the preset group formulas, see the fnirsi-dps150 skill reference docs