RDTech UM series

RDTech UM24C

Status	planned
Source code	[1]
Connectivity	serial over Bluetooth
Features	measures USB devices; voltage, amperage, wattage, resistance, capacity, temperature, voltage over USB data lines (charging mode), color display (26x26mm, 128x128px)
Website	rdtech.aliexpress.com

What is it?

A ~$13 USB load meter; it measures various properties for USB devices including their voltage, amperage, wattage, resistance, capacity, temperature, data line voltage, and charging mode. It can track up to 11 groups of mAh/mWh capacity data, one of which is ephemeral (and disappears after replugging the device), nine of which are persistent until cleared, and one of which whose recording is only activated above a certain current threshold (and which can be recorded in parallel with any of the other 10 data groups). It also allows graphing the amperage and voltage over time, on the device's display itself, as well as rotating the display contents into any orientation.

Unlike most devices of this type, this one communicates through serial-over-Bluetooth; the manufacturer provides apps (for Android and Windows, downloads including device documentation here), but not protocol documentation nor source code.

Note that this is specifically about the UM24C - the UM24 is the version *without* Bluetooth communication, although it's unclear whether the serial pads are still exposed and functional on that model. On the C model, the Bluetooth board is a separate layer (using an off-the-shelf serial-to-Bluetooth module) that connects to the serial pads using pogo pins.

The manufacturer has indicated that the firmware is not designed to be upgradeable and doesn't provide updates; nevertheless, the SWIM pin for the on-board STM8 chip is exposed, as are the other necessary pins for STM8 debugging. It's unclear whether the chip will allow eg. dumping, though.

The load meter can be connected either by plugging it in directly using its USB male plug end, or by connecting it using a cable and the micro-USB port on top. These are functionally equivalent; in both cases, both power and data are passed through and measured in the same way. I've not observed any difference in measurements between these two modes of operation.

It's unclear whether measurement of data lines is accurate enough to theoretically be used as a logic analyzer, but given the strange stability of the values during testing (unlike the voltage on the power lines) and the low-end STM8 chip, I suspect it's not.

Protocol

Clearer documentation (as well as a JS implementation of the protocol) are coming soon, but below are my rough notes of the protocol, that I've figured out by reverse-engineering the Android application. The device won't auto-send any data; you're expected to send 0xf0 any time you want new data, eg. on a timed loop.

All data returned by the device consists of measurements and configuration status, in 130-byte chunks. To my knowledge, it will never send any other data. All bytes below are displayed in hex format; every command is a single byte.

Commands to send

Multiple commands may be sent at once; e.g. you could set the recording threshold to 0.28 A and rotate the screen by sending 0xccf2 immediately. An exception appears to be requesting the data dump; it doesn't seem to return the 130-byte response unless you wait a bit after sending other commands.

Byte	Type	Meaning
0xf0	device control	Request new data dump; this triggers a 130-byte response
0xf1	device control	Go to next screen
0xf2	device control	Rotate screen
0xf3	device control	Switch to next data group
0xf4	device control	Clear data group
0xb0 - 0xce	configuration	Set recording threshold to a value between 0.00 and 0.30 A (inclusive); add the value after the decimal point to 0xb0 (0.00 is 0xb0, 0.30 is 0xce)
0xd0 - 0xd5	configuration	Set device backlight level between 0 and 5 (inclusive); 0 is dim, 5 is full brightness
0xe0 - 0xe9	configuration	Set screen timeout ("screensaver") between 0 and 9 minutes (inclusive), where 0 disables the screensaver

Response format

All byte offsets are in decimal, and inclusive. All values are big-endian and unsigned.

Offset	Length	Type	Meaning
0	2	marker	Start marker (always 0x0963)
2	2	measurement	Voltage (in centivolts, divide by 100 to get V)
4	2	measurement	Amperage (in mA, divide by 1000 to get A)
6	4	measurement	Wattage (in mW, divide by 1000 to get W)
10	2	measurement	Temperature (in Celsius)
12	2	measurement	Temperature (in Fahrenheit)
14	2	configuration	Currently selected data group, zero-indexed
16	80	measurement	Array of 10 main capacity data groups (where the first one, group 0, is the ephemeral one) -- for each data group: 4 bytes mAh, 4 bytes mWh
96	2	measurement	USB data line voltage (positive) in centivolts (divide by 100 to get V)
98	2	measurement	USB data line voltage (negative) in centivolts (divide by 100 to get V)
100	2	measurement	Charging mode; this is an enum, where 0 = unknown/standard, 1 = QC2.0, and presumably 2 = QC3.0 (but I haven't verified this)
102	4	measurement	mAh from threshold-based recording
106	4	measurement	mWh from threshold-based recording
110	2	configuration	Currently configured threshold for recording (in centiamps, divide by 100 to get A)
112	4	measurement	Duration of threshold recording, in cumulative seconds
116	2	configuration	Threshold recording active (1 if recording, 0 if not)
118	2	configuration	Current screen timeout setting, in minutes (0-9, 0 is no screen timeout)
120	2	configuration	Current backlight setting (0-5, 0 is dim, 5 is full brightness)
122	4	measurement	Resistance in deci-ohms (divide by 10 to get ohms)
126	2	configuration	Current screen (zero-indexed, same order as on device)
128	2	marker	Stop marker (always 0xfff1)

Board pictures

Not great pictures, but hopefully they'll be useful.

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Links

• Direct AliExpress UM24C link (select "UM24C" under "Color")
• RDTech AliExpress store
• rdumtool - RDTech UM24C Bluetooth interface tool (Python 3)