Decode Modbus registers as a float or integer, and see the value in every byte and word order side by side.
Have the register values? Decode them here. To go from a number to registers, use the IEEE 754 float converter.
Value, ABCD order
2.004137
Check the values to see a result.
NaN or infinity usually means the wrong type or order, or a documented “no value” state in the register map.
The registers are laid out as bytes in the chosen order (ABCD is the first register high byte first) and read as an IEEE 754 float or a two’s-complement integer
In ABCD order the bytes are 40 00 43 C8, which as a 32-bit IEEE 754 float is 2.004137.
Tip: Check against a value you know, such as a voltage near 230 or 400. The order that gives it is the order for the whole device.
A Modbus register holds 16 bits, so a 32-bit float or integer takes two registers and a 64-bit value takes four. The Modbus specification fixes the byte order inside each register (high byte first) but not the order of the registers, so devices differ. Enter the registers as read and the decoder shows the value in all four orders side by side; the one that gives a sensible reading is usually obvious.
ABCD: A B from the first register, C D from the second
Big-endian ABCD is the most common. CDAB swaps the two registers (word swap), BADC swaps the bytes inside each register, and DCBA reverses all four.
1 sign bit | 8 exponent bits | 23 fraction bits
IEEE 754 single precision. The first bit is the sign, the next eight the exponent with a bias of 127, and the last 23 the fraction after an implied leading 1.
value = (−1)s × 1.f × 2(e − 127)
For 43 6A 00 00 the sign is 0, the exponent is 0x86 = 134, so the scale is 27 = 128, and the fraction 0x6A0000 gives 1.828125. 1.828125 × 128 = 234.
value = high word × 65536 + low word
An unsigned 32-bit integer joins the two registers. A signed integer uses two’s complement: if the top bit is set, subtract 232 (or 216 for one register). FF38 as a signed 16-bit integer is −200.
A meter returns 436A and 0000 for a phase voltage. Read big-endian (ABCD), the bytes 43 6A 00 00 decode to 234. Read with the registers swapped, the same data gives 2.4184e‑41, a clear sign of the wrong order.
Value, ABCD order 234 Open in the calculator
Another device returns 4000 then 43C8. In ABCD order that is 2.004137, which looks plausible but is wrong. With the registers swapped (CDAB) the bytes are 43 C8 40 00, which is 400.5. Check one register you can confirm, such as a line voltage, before trusting the rest of the map.
Value, CDAB order 400.5 Open in the calculator
Energy registers are often unsigned 32-bit integers. 0001 E240 is 1 × 65,536 + 57,920 = 123456. The register map gives the scale, for example 0.1 kWh per count, which would make this 12,345.6 kWh.
Value, ABCD order 123456 Open in the calculator
The registers for everyday readings in big-endian order, and what the same registers give if they are read word-swapped. A tiny number such as 2.4e‑41 is the usual symptom of the wrong order.
| Registers (hex) | Read as ABCD | Read as CDAB |
|---|---|---|
| 0000 0000 | 0 | 0 |
| 3DCC CCCD | 0.1 | −107605600 |
| 3F73 3333 | 0.95 | 4.1734392e−8 |
| 3F80 0000 | 1 | 2.278e−41 |
| BF80 0000 | −1 | 6.8697e−41 |
| 4248 0000 | 50 | 2.3777e−41 |
| 4366 0000 | 230 | 2.4178e−41 |
| 43C8 0000 | 400 | 2.4315e−41 |
| 447A 0000 | 1000 | 2.4565e−41 |
Almost always the wrong register order or the wrong data type. A float read in the wrong order tends to give values like 2.4e‑41 or 5.9e+25. Try each order here and compare with a value you can check on the device display.
Within a register, yes: the specification sends the high byte first. For values spread across two or more registers it says nothing, so each manufacturer chooses. The device’s register map should state the order.
The order of the two registers. In ABCD the first register holds the high half of the value; in CDAB it holds the low half. The bytes inside each register stay the same way round.
Read two consecutive registers in one request, for example function 03 with a quantity of 2, starting at the address in the map. The Modbus address converter turns a manual’s 40001-style number into the address to send.
Not a number. Some devices use it, or 0xFFFFFFFF, to mean a reading is not available. It can also mean the type or order is wrong, so check the map before assuming a fault.
Once the type and order are known, a ZMB reads the registers from an RS-485 device and sends them to the Gateway over the wireless mesh, with no data cable back to the panel.
