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A Modbus register map is the table a manufacturer publishes to tell you which value sits at which address inside a device. Every energy meter, heat pump and inverter ships one, and every map looks slightly different. Once you understand the model behind them, though, you can read any register map on any brand without guessing. This guide teaches that model.
You will learn the four register types Modbus defines, how a datasheet address like 40001 maps to the address on the wire, how a 32-bit value hides across two registers, and how to turn a real Eastron SDM630 map into working polls. Written for installers and integrators, not for software developers.
Key takeaways
- Modbus defines exactly four register types: coils and discrete inputs (1 bit each) and input and holding registers (16 bit each).
- The 1-based Modicon notation (40001) and the 0-based wire address (0x0000) are the same register: drop the leading digit and subtract 1.
- A 32-bit value spans two registers, and the word order (big-endian or little-endian) is not fixed by the standard, so the wrong order returns a plausible but wrong number.
What is a Modbus register map?
A Modbus register map is a manufacturer-supplied table that lists every readable and writable data point in a device, together with its address, data type, unit, and access rights. It is the Rosetta Stone between the raw 16-bit values on the RS485 or TCP link and the temperatures, voltages and setpoints they represent.
The Modbus standard itself, defined in the Modbus Application Protocol Specification V1.1b3 maintained by the Modbus Organization, fixes only the transport and the four data blocks. It says nothing about what any given address means. That is why every device needs its own map, and why the map is the first document you request before commissioning a new device. For the wider context of how Modbus works, see our complete Modbus guide.
The four Modbus register types
Modbus organises all data into exactly four tables, and every address you ever read belongs to one of them. They differ on two axes only: size (a single bit or a full 16-bit word) and access (read-only or read-write).
The same four blocks appear in the specification's data model, section 4.3. Each block has its own address space of 0 to 65535, so address 0 exists four times, once per block. The function code you send is what tells the slave which of the four tables you mean.
Coils and discrete inputs: the single-bit blocks
Coils are single read-write bits, typically a relay, a pump enable, or an on/off command. You read them with function code 01 and write them with 05 (single) or 15 (multiple). Discrete inputs are single read-only bits, typically an alarm contact or a run/stop status, read with function code 02. In HVAC and refrigeration you meet these less often than registers, but alarm and status monitoring lives here.
Input and holding registers: the 16-bit blocks
Input registers are 16-bit read-only values, read with function code 04. They carry measured data: a voltage, a temperature, an instantaneous power. Holding registers are 16-bit read-write values, read with function code 03 and written with 06 (single) or 16 (multiple). They carry setpoints, configuration and, on many devices, measured values too. Roughly 80 percent of what an installer reads sits in these two blocks.
How do you find the register address?
Here is the single biggest source of off-by-one errors on the job. Datasheets quote addresses in two different notations, and mixing them up sends you one register too high or too low on every read.
The wire address, also called the PDU address, is what actually travels in the Modbus frame. It is 0-based, so the first holding register is address 0x0000. The Modicon notation, the classic five-digit "4xxxx" style, is 1-based and hides the register type in the leading digit: 40001 is the first holding register, 30001 the first input register, 10001 the first discrete input, 00001 the first coil.
Drop the leading digit and subtract 1: 40001 becomes 0x0000.
To convert, drop the leading digit and subtract 1. So datasheet 40001 is wire address 0x0000, and 40100 is 0x0063. Some modern datasheets skip Modicon entirely and quote the 0-based hex address directly. Neither is wrong; you just have to know which convention the sheet in front of you uses before you type a single address into your tool.
Reading values that span more than one register
A single register holds 16 bits, which tops out at 65535. Any value larger than that, and every floating-point number, spreads across two or more consecutive registers. A 32-bit float or int32 takes two registers; a 64-bit value takes four.
The catch is that the Modbus specification does not define the order of those words. A manufacturer is free to send the high word first (big-endian) or the low word first (little-endian, sometimes called a word swap). Read them in the wrong order and you get a plausible-looking but wrong number, often off by a huge factor or negative.
Eastron, Carlo Gavazzi and most European energy meters use big-endian word order. Some Asian brands and many PLCs default to little-endian. The datasheet almost always states it, usually as "MSW first" or "float, ABCD/CDAB". When it does not, and the value is clearly wrong by a factor, try the swapped order first before you suspect the wiring.
A worked example: reading the Eastron SDM630 map
The Eastron SDM630 is a three-phase DIN-rail energy meter that exposes its measurements as 32-bit floats in input registers, read with function code 04. Here is the top of its map in wire (PDU) notation.
| Wire address | Value | Type | Unit |
|---|---|---|---|
| 0x0000 | Voltage L1 to neutral | Float32 | V |
| 0x0006 | Current L1 | Float32 | A |
| 0x000C | Active power L1 | Float32 | W |
| 0x0034 | Total system active power | Float32 | W |
| 0x0048 | Total active energy import | Float32 | kWh |
To read voltage L1, you send function code 04 with start address 0x0000 and quantity 2 (two registers, because the value is a 32-bit float). The meter returns four data bytes, which you decode as a big-endian float. According to the Eastron SDM630 Modbus protocol datasheet, every measurement register follows this same pattern, so once you have decoded one you have decoded them all.
Notice that the SDM630 puts measurements in input registers (function code 04), while a device like a heat pump often puts the same kind of data in holding registers (function code 03). Always check the register type column of the map, not just the address. If you would rather skip the byte-level work entirely, the SDM630 is a ready-made template that maps every register for you.
Why the same address means different things on every brand
Because the standard fixes no meaning per address, register 0x0000 is a voltage on one meter, a firmware version on the next, and a setpoint on a third. There is no universal map, and no gateway can auto-detect what a register means. This is the single fact that surprises installers moving from BACnet, which carries self-describing object names, to raw Modbus.
Take one concrete case. Register 0x0000 returns voltage L1 to neutral on an Eastron SDM630, the flow temperature setpoint on a typical heat pump, and the device firmware version on an inverter. Same address, three meanings, and only the device's own register map tells you which one you are reading.
The one real exception is SunSpec, a standardised Modbus register model that more than 100 solar inverter and meter manufacturers implement, so a single map fits many brands. Outside SunSpec, treat every device as unique and get its map before you start. To discover which devices and addresses are even present on a bus, run a Modbus network scan first.
Troubleshooting register map problems
When a value comes back wrong rather than missing, walk these four in order before you touch the wiring.
| Symptom | Probable cause | Fix |
|---|---|---|
| Value is one register off | Modicon vs wire address mixed up | Subtract 1 and drop the leading digit, or add it back |
| Value off by a large factor or negative | Wrong word order on a 32-bit value | Swap high and low word (big-endian vs little-endian) |
| Read fails with exception 02 | Address outside the device's map | Read only addresses the map lists; some ranges are gaps |
| Read fails with exception 03 | Too many registers requested at once | Function code 03 and 04 read at most 125 registers per request |
For a bus that answers nothing at all, the problem is addressing or the physical layer rather than the map. Start with our RS485 wiring guide and Modbus RTU explainer.
This is where the ModbusCloud Gateway fits. The Gateway reads your Modbus RTU and TCP devices into a cloud dashboard using ready-made register templates, so you pick the device instead of decoding its map by hand. It handles the addressing offset and the word order for you.
What are the four types of Modbus registers?
Modbus defines coils and discrete inputs, which are single bits, and input and holding registers, which are 16-bit words. Coils and holding registers are read-write; discrete inputs and input registers are read-only. Each type has its own function codes and address space.
What is a Modbus register map?
A Modbus register map is a manufacturer table that lists every data point in a device with its address, data type, unit and access rights. Because the standard fixes no meaning per address, each device needs its own map to translate raw values into real measurements.
How do I find a Modbus register address?
Check whether the datasheet uses Modicon notation (1-based, five digits like 40001) or the wire address (0-based hex like 0x0000). To convert Modicon to wire address, drop the leading digit and subtract one, so 40001 becomes 0x0000.
What is the difference between input and holding registers?
Both are 16-bit registers. Input registers are read-only measured values, read with function code 04. Holding registers are read-write and hold setpoints, configuration and often measurements too, read with function code 03 and written with 06 or 16.
How many bits is a Modbus register?
A Modbus register is 16 bits, or two bytes, holding a value from 0 to 65535. Coils and discrete inputs are single bits. Values larger than 65535, and all floating-point numbers, span two or more consecutive registers.
Why does my Modbus value read as garbage?
A 32-bit value spans two registers, and the Modbus standard does not fix their order. If the device is big-endian and your tool assumes little-endian, or the reverse, the words combine wrongly and the value looks off by a large factor. Swap the word order to fix it.
Can you write to a Modbus holding register?
Yes. Holding registers are read-write. Write a single holding register with function code 06 and multiple registers with function code 16. Input registers and discrete inputs are read-only and cannot be written. Always confirm the access column in the register map first.
Ready to stop decoding register maps by hand? The ModbusCloud Gateway reads your Modbus devices into a cloud dashboard using ready-made templates, handling the addressing offset and word order automatically. See how a complete monitoring chain looks for your installations.
