Tools · Modbus Register Viewer

Modbus Register Viewer Modbus

Decode a Modbus register the way the device sees it.

Decimal, hex, binary, signed/unsigned, and individual bits, all synced as you change any one. Single Register lets you toggle bits directly or type a decimal or hex value. 32-bit Pair takes two consecutive registers and shows the int32 / uint32 / float reading under all four byte orderings vendors use — the one whose float reads plausibly is yours. New to Modbus? Read Modbus Basics for the protocol shape, then Modbus Decoding for the gotchas behind these tabs.

Inputs are seeded with an example — edit them to your numbers.

Input

Decimal (0–65535)

Hex (0x0000–0xFFFF)

Bit 15 = MSB (top-left) · Bit 0 = LSB (bottom-right) · click to toggle

On narrow screens the grid becomes 4×4 — the high-byte / low-byte boundary moves between rows 2 and 3.

Output

Decimal as

Decimal —

Hex —

Binary —

Input — two consecutive registers

Modbus transmits the first register's high byte first. Call the four bytes A B C D in transmission order — so Register 1 carries AB and Register 2 carries CD. The table on the right shows the 32-bit value under each of the four byte-ordering conventions vendors use.

Register 1 (bytes A B)

Decimal (0–65535)

Hex (0x0000–0xFFFF)

Register 2 (bytes C D)

Decimal (0–65535)

Hex (0x0000–0xFFFF)

Output — all four byte orderings

Order	int32	uint32	float (IEEE-754)
ABCD	—	—	—
CDAB	—	—	—
BADC	—	—	—
DCBA	—	—	—

ABCD is big-endian word + big-endian byte — the Modbus standard's stated ordering. CDAB is the "Modicon word-swap" form common on legacy gear. BADC swaps bytes within each register; DCBA reverses everything. When a float looks like nonsense at one ordering and plausible at another, the other ordering is the vendor's.

Input

5-digit address (vendor docs)

Table

Wire offset (0-based)

Two-way: type the address from the points list, or pick a table and type the offset off the wire.

Output

Wire offset —

Read with —

Why the off-by-one

The 5-digit number in a points list is a documentation convention: the first digit names the table, the rest count from 1. The request that goes out on the wire carries a 16-bit offset that counts from 0 — so "40001" is holding-table offset 0, and vendor address 43021 is polled as offset 3020 with FC03. Both shifts bite: forget the table prefix and you poll the wrong kind of point; forget the −1 and every value is one register off. Modbus Decoding walks the trap with worked frames.

Modbus essentials

A handful of subtle gotchas catch most people up on Modbus — less the protocol itself than what you have to know on top of it to interpret what comes back. The tips below cover the ones worth knowing before you start integrating.

Modbus is dumb on purpose — a register is just 16 bits, with no metadata about what those bits mean. The vendor's docs are load-bearing.

Register addressing offset. "40001" in vendor docs is holding register 0 on the wire — the 4xxxx / 3xxxx / 1xxxx / 0xxxx 5-digit numbering is a documentation convention, not a wire address. Off-by- one bugs live here.
Signed vs unsigned. A raw 16-bit value of 65535 is also −1 — same bits, different intent. Use the toggle on the Single Register tab to flip between interpretations.
Word / byte order. 32-bit floats and int32s span two registers, and four byte orderings exist in the wild (ABCD / CDAB / BADC / DCBA). The 32-bit Pair tab shows all four side-by-side; pick the one that produces a plausible value.
Read FC03 vs FC04. Holding registers (FC03) are read/write; input registers (FC04) are read-only. Write FC06 vs FC16. FC06 writes a single register, FC16 writes many. Wrong FC → illegal function exception.
Exception responses set the high bit of FC. A request with FC 0x03 that errors comes back as 0x83 (FC | 0x80) followed by a 1-byte exception code (0x02 = illegal data address, 0x03 = illegal data value, etc.).

Modbus Basics covers the protocol shape end-to-end — the data tables, the function codes, the request frame, and exception responses. Modbus Decoding is the companion page on the gotchas behind these tips — the 5-digit register trap, signed vs unsigned, the four byte orders, and scaling.

Function codes

FC	Hex	Function
01	0x01	Read Coils
02	0x02	Read Discrete Inputs
03	0x03	Read Holding Registers
04	0x04	Read Input Registers
05	0x05	Write Single Coil
06	0x06	Write Single Register
15	0x0F	Write Multiple Coils
16	0x10	Write Multiple Registers

Coils and discrete inputs are 1-bit; holding and input registers are 16-bit — this tool views one of those. Discrete inputs and input registers are read-only; coils and holding registers are read/write.

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