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Energy meter · Modbus register map
Eastron SDM120CT-40mA Modbus register map
Updated 16 October 2026
The SDM120CT-40mA is a single-phase two-wire DIN-rail meter that measures through external current transformers and reports over RS485 Modbus RTU. Readings are IEEE floats in input registers (function 04) covering voltage, current, powers, power factor, frequency, demand and import/export energy. Holding registers (function 03/16) hold the RS485 settings, CT primary, pulse outputs and demand period.
This page lists 31 Modbus registers for the Eastron SDM120CT-40mA — 10 writable — with the table, 0-based wire address, data type, word order, scale, units and access of each, re-typed from Eastron’s own free documentation (linked below) and checked against it.
- Manufacturer
- Eastron
- Model
- SDM120CT-40mA
- Equipment type
- Energy meter
- Registers
- 31 (10 writable)
- Transport
- Modbus RTU
- Default unit ID
- 1
- Default serial
- 9600 baud, 8N1
- Documented addressing
- Modicon 3xxxx/4xxxx (drop the prefix, subtract 1)
- 32-bit word order
- ABCD high word first (big-endian)
- Also known as
- SDM120CT-M-40mA, SDM120CT-M 40mA
- Source document
- SDM120CT-40mA user manual · V1.0 · rev. 2026-07-16
Integration notes: Eastron SDM120CT-40mA
Read two-register floats with function 04 from 0x0000 (voltage) through 0x0158; import/export kWh are at 0x0048/0x004A. Set the CT primary (40051) to match the installed CTs before trusting current, power and energy.
Minimum registers to map
Addresses are 0-based wire addresses. Enough to monitor and run the unit; map more as the job needs.
| Register | Name | Type | Why | Access |
|---|---|---|---|---|
| IR 0 | Voltage | FLOAT32 ABCD | Voltage | Read |
| IR 6 | Current | FLOAT32 ABCD | Current | Read |
| IR 12 | Active power | FLOAT32 ABCD | Active power | Read |
| IR 72 | Import active energy | FLOAT32 ABCD | Import kWh | Read |
| IR 74 | Export active energy | FLOAT32 ABCD | Export kWh | Read |
| HR 50 | CT1 | FLOAT32 ABCD | CT primary setting | Read/write |
Enable Modbus control & writes
Modbus RTU is always active on the RS485 port. Settings are written with function 16; the front-panel setup mode also sets the address and CT1.
Byte order & addressing
- Most settings, including baud code and address, are floats spanning two registers; the pulse constant and reset registers are 16-bit hex values.
- Six-digit register numbers (461457, 463761) map to wire addresses 0xF010 and 0xF910.
Network setup
- Defaults: address 1, 9600 bps, no parity, 1 stop bit (data bits are not stated in the manual).
- Baud options are 1200, 2400, 4800 and 9600 bps only.
- Enter the CT primary current (5-9999 A) in 40051.
Watch out for
- Current, power and energy are only correct once the CT primary in 40051 matches the installed CTs; this variant takes CTs with a 40 mA secondary.
Eastron SDM120CT-40mA Modbus registers
Address = 0-based wire address (the documented number is shown under it when it differs) · Order: ABCD = high word first, CDAB = low word first · Scale: engineering value = raw × scale.
| Table | Address | Name | Description | Type | Order | Scale | Units | Access | Values / bits |
|---|---|---|---|---|---|---|---|---|---|
| IR | 0doc 30001 | Voltage | Line-to-neutral RMS voltage. | FLOAT32 | ABCD | V | Read | ||
| IR | 6doc 30007 | Current | FLOAT32 | ABCD | A | Read | |||
| IR | 12doc 30013 | Active power | FLOAT32 | ABCD | W | Read | |||
| IR | 18doc 30019 | Apparent power | FLOAT32 | ABCD | VA | Read | |||
| IR | 24doc 30025 | Reactive power | FLOAT32 | ABCD | VAr | Read | |||
| IR | 30doc 30031 | Power factor | FLOAT32 | ABCD | Read | ||||
| IR | 70doc 30071 | Frequency | FLOAT32 | ABCD | Hz | Read | |||
| IR | 72doc 30073 | Import active energy | FLOAT32 | ABCD | kWh | Read | |||
| IR | 74doc 30075 | Export active energy | FLOAT32 | ABCD | kWh | Read | |||
| IR | 76doc 30077 | Import reactive energy | FLOAT32 | ABCD | kVArh | Read | |||
| IR | 78doc 30079 | Export reactive energy | FLOAT32 | ABCD | kVArh | Read | |||
| IR | 84doc 30085 | Total active power demand | FLOAT32 | ABCD | W | Read | |||
| IR | 86doc 30087 | Max total active power demand | FLOAT32 | ABCD | W | Read | |||
| IR | 88doc 30089 | Import active power demand | Printed as 'import active energy demand' but given in W. | FLOAT32 | ABCD | W | Read | ||
| IR | 90doc 30091 | Max import active power demand | FLOAT32 | ABCD | W | Read | |||
| IR | 92doc 30093 | Export active power demand | FLOAT32 | ABCD | W | Read | |||
| IR | 94doc 30095 | Max export active power demand | FLOAT32 | ABCD | W | Read | |||
| IR | 258doc 30259 | Current demand | FLOAT32 | ABCD | A | Read | |||
| IR | 264doc 30265 | Max current demand | FLOAT32 | ABCD | A | Read | |||
| IR | 342doc 30343 | Total kWh | What 'total' includes depends on the metering-mode setting (not mapped; see notes). | FLOAT32 | ABCD | kWh | Read | ||
| IR | 344doc 30345 | Total kVArh | FLOAT32 | ABCD | kVArh | Read | |||
| HR | 2doc 40003 | Demand period | 0 to 60 minutes; 0 updates demand every second. Default 60. | FLOAT32 | ABCD | min | Read/write | ||
| HR | 12doc 40013 | Relay pulse width | Pulse on-time: 60, 100 or 200 ms. Default 60. | FLOAT32 | ABCD | ms | Read/write | ||
| HR | 18doc 40019 | Parity and stop bit | RS485 framing. Default one stop bit, no parity. | FLOAT32 | ABCD | Read/write | 0 = 1 stop bit, no parity; 1 = 1 stop bit, even parity; 2 = 1 stop bit, odd parity; 3 = 2 stop bits, no parity | ||
| HR | 20doc 40021 | Modbus address | Node address 1 to 247. Default 1. | FLOAT32 | ABCD | Read/write | |||
| HR | 28doc 40029 | Baud rate | Default 9600. | FLOAT32 | ABCD | Read/write | 0 = 2400; 1 = 4800; 2 = 9600; 5 = 1200 | ||
| HR | 50doc 40051 | CT1 | CT primary current, 5 to 9999. Default 5. | FLOAT32 | ABCD | A | Read/write | ||
| HR | 60doc 40061 | Backlit time | 0 to 121; 0 keeps backlight on, 121 keeps it off. Default 60. | FLOAT32 | ABCD | min | Read/write | ||
| HR | 86doc 40087 | Pulse 1 output mode | Energy quantity driving pulse output 1. Default export active. | FLOAT32 | ABCD | Read/write | 1 = Import active energy; 2 = Import + export active energy; 4 = Export active energy; 5 = Import reactive energy; 6 = Import + export reactive energy; 8 = Export reactive energy | ||
| HR | 61456doc 461457 | Reset | Write 0000 to clear maximum demand. | UINT16 | Write | 0 = Reset maximum demand | |||
| HR | 63760doc 463761 | Pulse 1 output constant | Energy per pulse on output 1. Default 0.001 kWh/imp. | UINT16 | Read/write | 0 = 0.001 kWh/imp; 1 = 0.01 kWh/imp; 2 = 0.1 kWh/imp; 3 = 1 kWh/imp |
Notes on this map
- Word order: the manual states floats are sent big-endian with the high 16 bits in the first register, and its example decodes 43 66 00 00 as 230 V. All floats here are ABCD.
- Left out: metering mode (printed 463779 but with hex start address F9 20, which corresponds to 463777). The printed number and the hex address disagree, so the register is omitted until confirmed.
- Left out: scroll display time (463745), stored as BCD, which this schema cannot represent.
- Left out: serial number (464513), an unsigned 32-bit value with no stated word order.
- This map is for the SDM120CT-40mA only. The SDM120-M (direct connect) user manual on eastrongroup.com has no register table, so it is not covered here.
- The front-panel CT1 menu offers fixed primaries (5 to 600 A) while register 40051 accepts 5 to 9999; values outside the menu list have not been checked on a meter.
Check it against the live device. A register map tells you what the controller can expose; the unit in front of you may differ by firmware or options. Read a few registers and compare them with the unit’s own display; for a 32-bit value the float and byte-order decoder shows all four word orders at once, so a wrong word order or scale is obvious. New to this? Read what a register map is and why addresses are off by one.
Questions about the Eastron SDM120CT-40mA
What Modbus registers does the Eastron SDM120CT-40mA have?
This map lists 31 registers: 21 input registers, 10 holding registers. 10 of them can be written. Registers vary with firmware, options and configuration, so confirm against the device.
Are the Eastron SDM120CT-40mA Modbus addresses 0-based or 1-based?
The manufacturer's document numbers registers Modicon 3xxxx/4xxxx (drop the prefix, subtract 1). Every address on this page is the 0-based wire address you put in the request; the documented number is shown under it where it differs. Tables print Modicon numbers (30001, 40003, six-digit 461457...) beside the hex start address used on the wire. Wire address = printed number minus 30001 / 40001 / 400001; the manual's example reads 30001 at start address 0x0000.
What byte order does the Eastron SDM120CT-40mA use for 32-bit values?
Multi-register values in this map use ABCD (high word first (big-endian)). Set your client to this order for every 32/64-bit value in the table.
What are the minimum Modbus registers to integrate the Eastron SDM120CT-40mA?
Start with IR 0 Voltage (Voltage); IR 6 Current (Current); IR 12 Active power (Active power); IR 72 Import active energy (Import kWh); IR 74 Export active energy (Export kWh); HR 50 CT1 (CT primary setting).
How do I enable Modbus control of the Eastron SDM120CT-40mA?
Modbus RTU is always active on the RS485 port. Settings are written with function 16; the front-panel setup mode also sets the address and CT1.
Where is the official Eastron SDM120CT-40mA Modbus documentation?
The source is SDM120CT-40mA user manual (V1.0), revision 2026-07-16, published free by Eastron at https://www.eastrongroup.com/eastrongroup/2026/07/17/EastronSDM120CT-M40mAUsermanualV10.pdf.
More Eastron Modbus register maps
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Eastron SDM630-DI three-phase DIN-rail meter (100 A direct, two digital inputs, RS485 Modbus RTU), manual V1.02.
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Source: SDM120CT-40mA user manual · V1.0 · rev. 2026-07-16. Register numbers, types, scaling and units are reproduced as published; descriptions are our own.