What function code does the SDM120 use?

The SDM120 measurement values are input registers, read with Modbus function code 04 (not 03). Set-up parameters such as address and baud rate are holding registers, read with FC03 and written with FC16.

What format are SDM120 registers?

Every measurement is a 32-bit IEEE 754 float spanning two consecutive registers, high word first (big-endian word order). Start address and quantity must be even; the meter serves at most 40 values (80 registers) per request.

What Modbus settings does the SDM120 use?

The SDM120 is a Modbus RTU slave over 2-wire RS-485. Factory default is slave ID 1 at 2400 baud, 8 data bits, no parity; some batches ship with 2 stop bits. Baud and address are changed from the front button set-up menu or by writing holding registers.

Can I log SDM120 energy data to a file?

Yes. With ModbusManager Pro you can log any SDM120 register to a local Historian, view it as a trend chart and export to CSV for energy monitoring and billing checks.

// Device guide — Eastron

Eastron SDM120
Modbus Guide

Q: Do SDM120CT, SDM220 and SDM230 use the same register map?

The core measurement block is identical across the SDM120 family and the SDM220/SDM230 — voltage 30001, current 30007, power 30013, frequency 30071 and energies from 30073, all FLOAT32. Variant-specific extras such as CT ratio should be checked against the variant protocol document.

Read voltage, current, power, power factor, frequency and energy from an Eastron SDM120 single-phase energy meter over Modbus RTU. This guide gives the full register map, FLOAT32 decoding, the FC04 input-register detail that trips people up, the default 2400-baud settings — and how to get live data with ModbusManager on Windows. Covers the SDM120M, SDM120CT and the closely related SDM220 and SDM230.

Download demo workspace Get ModbusManager Pro

Windows 10/11 · Modbus RTU & TCP · one-time license

The everywhere single-phase meter

The Eastron SDM120 is probably the most installed single-phase DIN-rail Modbus energy meter in the world — one module wide, MID-certified variants, 45 A direct connection (or CT-operated as the SDM120CT) and a compact register map covering voltage, current, power and bidirectional energy. EV charger installations, solar export limiting, tenant sub-metering and heat-pump monitoring all lean on it. Its bigger siblings SDM220 and SDM230 share the same protocol style, and if you can read an SDM120 you can read the three-phase SDM630 too — same FLOAT32, two-registers-per-value pattern.

Ready-made SDM120 workspace

A free ModbusManager workspace that polls the whole SDM120 measurement block over FC04, decodes every value as FLOAT32 and shows a live dashboard — voltage, power and current gauges, power factor, frequency, import/export and total energy, plus a live power trend. Load it, set your meter’s slave ID and serial settings, and you are reading in under a minute.

Download SDM120 workspace

Open via File › Open Workspace. The dashboard uses the tag engine, which is a Pro feature — the poll windows and register grid work in Standard too. ⚠ Default poll assumes slave ID 1; adjust in the poll window if your meter uses another address.

Modbus settings

The SDM120 talks Modbus RTU over its 2-wire RS-485 terminals. Factory defaults below — all of them can be changed from the front-button set-up menu or by writing the holding registers.

Setting	Default / options
Protocol	Modbus RTU, 2-wire RS-485
Slave address	1 (range 1–247)
Baud rate	2400 default · options 1200–38400 depending on variant
Frame	8 data bits, no parity · 1 stop bit on most units, 2 stop bits on some batches — check the set-up menu if you get CRC errors
Read measurements	FC04 (input registers), values as FLOAT32 register pairs
Read / write settings	FC03 read, FC16 write (holding registers)
Request limits	Start address and quantity must be even; max 40 values (80 registers) per request

SDM120 measurement registers

Every value is an IEEE 754 FLOAT32 held in two consecutive input registers, read with FC04. Addresses below are the documentation form (3xxxx) with the on-wire hex offset — ModbusManager accepts either with the Base 0/1 selector.

Register	Hex offset	Parameter	Unit
30001	0x0000	Voltage	V
30007	0x0006	Current	A
30013	0x000C	Active power	W
30019	0x0012	Apparent power	VA
30025	0x0018	Reactive power	VAr
30031	0x001E	Power factor	–
30071	0x0046	Frequency	Hz
30073	0x0048	Import active energy	kWh
30075	0x004A	Export active energy	kWh
30077	0x004C	Import reactive energy	kvarh
30079	0x004E	Export reactive energy	kvarh
30343	0x0156	Total active energy	kWh
30345	0x0158	Total reactive energy	kvarh

Set-up lives in holding registers (FC03/FC16, also FLOAT32 pairs): 40021 / 0x0014 meter address, 40029 / 0x001C baud rate, plus relay-pulse and display options depending on variant. Write one parameter per message; some changes need a power cycle.

Read an SDM120 in three steps

Connect RS-485

Wire A/B from your USB-RS485 adapter to terminals 9 (B-) and 10 (A+), open ModbusManager › Connect › RTU, pick the COM port and set 2400 baud, 8N1, slave ID 1 (or your meter’s settings).

Poll FC04

Create a poll window: function code 04, start address 30001 (Base 1) or 0 (Base 0), quantity 80. The grid fills with raw registers — select the Float32 display column and real voltages and watts appear.

Or load the workspace

Skip the setup: open the free SDM120 workspace above. Polls, FLOAT32 tags and a live dashboard with gauges and a power trend are pre-configured — just match the slave ID and serial settings.

// why modbusmanager

A practical tool for energy meters

ModbusManager reads any SDM120 register directly, with built-in FLOAT32 decoding so you do not combine register pairs by hand, and a serial monitor to see every frame. The Standard edition ($49) covers polling, scaling and the serial monitor. The Pro edition ($119) adds drag-and-drop Modbus HMI software, a data logging Historian with CSV export for energy and load profiling, and an alarm system — all running locally on Windows, with no SCADA server or cloud account.

SDM120 Modbus — FAQ

I get garbage values like 1.4e-42 instead of 230 V — why?+

Your word order is inverted. The SDM120 sends FLOAT32 values high-word-first (big-endian, ABCD). If your master combines the two registers low-word-first you get nonsense. In ModbusManager select the Float32 column with big-endian byte order (the default) — or use the swap button if your gateway reorders words.

Do the SDM120CT, SDM220 and SDM230 use the same registers?+

Essentially yes. The core measurement block — voltage 30001, current 30007, power 30013, frequency 30071, energies 30073+ — is the same FLOAT32 layout across the SDM120 family, the CT-operated SDM120CT, and the SDM220/SDM230. Some variants add extra registers (demand values, CT ratio on CT models); verify extras against your variant’s protocol PDF.

Why does reading a single register return an error?+

Every value is a two-register FLOAT32, and the meter requires the start address and the quantity to be even. Reading one register — or starting mid-pair — returns an exception on most firmware. Always poll pairs: start 0x0000, quantity 2, 4, … up to 80.

How do I change the slave address or baud rate over Modbus?+

Write the holding registers with FC16 as FLOAT32: address 0x0014 sets the meter ID (e.g. float 2.0 for ID 2), address 0x001C sets the baud-rate code. One parameter per write. You can also do both from the front button’s set-up menu — hold it ~3 seconds until -SET- appears.

The meter doesn’t answer at all — what should I check?+

In order: A/B swapped (the most common one — just swap the wires), wrong baud (factory is 2400, not 9600), wrong stop bits (some batches ship 8N2), wrong slave ID, or a missing ground/termination on longer buses. ModbusManager’s Serial Monitor shows raw TX/RX so you can see whether anything comes back at all.

Is there a Modbus TCP version?+

The SDM120 itself is RS-485 only, but any Modbus TCP→RTU gateway exposes it over TCP with the same register map. ModbusManager’s “RTU over TCP” and plain TCP modes both work for gateway setups.

Eastron SDM120Modbus Guide