The DTSU666 bidirectional, three-phase electricity meter and data retrieval via Modbus on the ESP32

The DTSU666 is an electronic, three-phase energy meter designed for mounting on a DIN rail within a switchboard. Thanks to its built-in RS485 interface and support for the Modbus RTU protocol, this meter is well suited to reading real-time data on voltages, currents and power, allowing for easy integration into your own projects. This is exactly what I’ll be demonstrating here, using a ready-made ESP32 board.

I bought the DTSU666 from a Polish online shop for around 250 zł. The device occupies 4 standard DIN modules (72 mm wide) in a switchboard and weighs just under 400 grams. It is also fitted with a backlit LCD display, which allows you to conveniently view the current voltage, currents and energy metered on individual phases without the need for any additional apps, as well as to configure Modbus. The pack also included a Polish-language manual.

Connecting the DTSU666 is fairly straightforward – even the sticker on the side lists the pins, including the RS485 port and the pulse output.

The RS485 port is used here for communication via the Modbus RTU protocol, which is widely used in industry. RS485 itself is merely a physical layer based on a differential signal, providing high resistance to interference and ranges of hundreds of metres. From the microcontroller’s perspective, it is handled in exactly the same way as a standard UART serial port (RX and TX pins); however, the signal is not directly compatible. To bring it all together, you need a suitable hardware converter (e.g. based on the popular MAX485 chip) or a ready-made board that already has such a transceiver built in. Here, I decided to use the previously featured LilyGO T-CAN485 – an ESP32 for learning industrial communication, RS485 and CAN buses .

We simply connect pin A to A and pin B to B. We do not ‘cross-connect’ (from RX to TX) as with UART, because RS485 is a bus to which we can connect multiple devices in parallel. Communication is managed by a single master device – in this case, our ESP32, which polls the rest of the network. The other devices are known as slaves (in this case, the DTSU666 meter), which simply listen and respond to commands from the master. For longer cables, it is also worth remembering to connect a terminating resistor (usually 120 Ω) at both ends of the bus, which will prevent signal reflections.

Modbus is a widely adopted standard for data transmission between machines, which enables the standardised exchange of information through simple frames. The most important concept of this protocol is that all variables read and written are stored in so-called registers. Registers have different types and addresses, which can be found in the documentation. From the table below, we can see that, for example, phase voltages start at register 0x200A, are 32-bit (2 registers) and are of type float, i.e. floating-point.

RS485 can operate with various port and baud rate settings, whilst Modbus supports various slave IDs. These settings must match between the meter and the firmware. On the meter, they can be changed via the UI:

Now you’ll need the firmware. The simplest way is probably to write it in PlatformIO, which is an extension for the Visual Studio Code environment. It makes it easier to manage external libraries and automates the download of the appropriate compilers for our board. The project configuration is saved in its entirety in a single platformio.ini file, which greatly simplifies transferring code between different computers.
I started with the basics, namely setting up over-the-air (OTA) programming and a simple website:
How to programme the Arduino-style Wemos D1 (ESP8266) board? ArduinoOTA in PlatformIO
Next, I added the ModbusMaster library to the project:
https://github.com/4-20ma/ModbusMaster
My platformio.ini:

With the library now added to the project, querying the meter is essentially just a matter of calling a single function. Below is a short code snippet showing how to read the 32-bit voltage value for phase L1 (address 0x2006). Please note that after a successful read, we need to combine two 16-bit registers into a single

float

and divide the result by 10 – exactly as required by the manufacturer’s documentation.

The whole process during testing:

Results on the web:

In the same way, you can access other values from the documentation, such as phase current (from 0x200C), total active power (0x2012), power factor (0x202A), total energy consumed and supplied (from 0x101E) or even the mains frequency (0x2044).

Full register table:

Address	Code	Parameter description	Data type	Length (words)	Read/Write
0000H	REV.	Software version	Signed	1	R
0001H	UCode	Programming code (1–9999)	Signed	1	R/W
0002H	CLr.E	Energy reset (1: clear energy)	Signed	1	R/W
0003H	net	Selection of mains configuration (0: 3-phase 4-wire, 1: 3-phase 3-wire)	Signed	1	R/W
0006H	IrAt	Current ratio (1–9999)	Signed	1	R/W
0007H	UrAt	Voltage ratio (0.1–999.9)	Signed	1	R/W
000AH	Disp	Alternating display time (s)	Signed	1	R/W
000BH	B.LCD	Screen backlight duration (minutes)	Signed	1	R/W
000CH	Endian	Reserved	Signed	1	R/W
002CH	Protocol	Protocol selection (1: DL/T645, 2:n.2, 3:n.1, 4:E.1, 5: o.1)	Signed	1	R/W
002DH	bAud	Transmission speed (0:1200, 1:2400, 2:4800, 3:9600)	Signed	1	R/W
002EH	Addr	Modbus address (1–247)	Signed	1	R/W
2000H	Uab	Phase-to-phase voltage Uab (V, ×0.1)	float	2	R
2002H	Ubc	Ubc phase-to-phase voltage (V, ×0.1)	float	2	R
2004H	Uca	Phase-to-phase voltage Uca (V, ×0.1)	float	2	R
2006H	Ua	Phase voltage Ua (V, ×0.1)	float	2	R
2008H	Ub	Phase voltage Ub (V, ×0.1)	float	2	R
200AH	Uc	Phase voltage Uc (V, ×0.1)	float	2	R
200CH	Ia	Phase A current (A, ×0.001)	float	2	R
200EH	Ib	Phase B current (A, ×0.001)	float	2	R
2010H	Ic	Phase C current (A, ×0.001)	float	2	R
2012H	Fri	Total active power (W, ×0.1)	float	2	R
2014H	Pa	Active power, phase A (W, ×0.1)	float	2	R
2016H	Pb	Active power, phase B (W, ×0.1)	float	2	R
2018H	Pc	Active power, phase C (W, ×0.1)	float	2	R
201AH	Qt	Total reactive power (var, ×0.1)	float	2	R
201CH	Qa	Reactive power, phase A (var, ×0.1)	float	2	R
201EH	Qb	Reactive power, phase B (var, ×0.1)	float	2	R
2020H	Qc	Reactive power, phase C (var, ×0.1)	float	2	R
202AH	PFt	Total power factor (×0.001)	float	2	R
202CH	PFa	Phase A power factor (×0.001)	float	2	R
202EH	PFb	Phase B power factor (×0.001)	float	2	R
2030H	PFc	Phase C power factor (×0.001)	float	2	R
2044H	Freq	Mains frequency (Hz, ×0.01)	float	2	R
101EH	ImpEp	Total active energy consumed (kWh)	float	2	R
1020H	ImpEpA	Active energy consumed, phase A (kWh)	float	2	R
1022H	ImpEpB	Active energy consumed, phase B (kWh)	float	2	R
1024H	ImpEpC	Active energy consumed, phase C (kWh)	float	2	R
1026H	NetImpEp	Net active energy consumed (kWh)	float	2	R
1028H	ExpEp	Total active energy supplied (kWh)	float	2	R
102AH	ExpEpA	Active energy supplied, phase A (kWh)	float	2	R
102CH	ExpEpB	Active energy supplied, phase B (kWh)	float	2	R
102EH	ExpEpC	Active energy supplied, phase C (kWh)	float	2	R
1030H	NetExpEp	Net active energy supplied (kWh)	float	2	R

You can also take a look inside the meter:

The integrated circuits are unmarked; the board is coated with a layer of varnish for protection. The internal layout appears to be quite elaborate; MYN12-821K varistors can be seen, used for surge protection, along with 5(80)A\2mA current transformers.

In summary, integrating the DTSU666 meter into your own projects relies on handling standard serial communication via an RS485 converter. Thanks to the libraries available for the ESP32, we can handle the Modbus RTU protocol by calling a single function. This enables automated monitoring of network parameters without being locked into a single manufacturer’s closed software ecosystem. We can now freely process the received data and send it on to a display, a database, or perhaps even to Home Assistant.
Do you use energy meters with a Modbus interface?