Reference
Modbus Glossary
49 terms — every concept you'll encounter working with Modbus RTU and TCP, explained in plain English.
The actual protocol addressing: addresses start from 0 in PDU requests.
A numeric format where each decimal digit is stored in 4 bits within a register.
The arrangement of bytes across registers when storing multi-byte values.
The most significant byte is in the lowest address — the 'natural' network order.
A 2-byte cyclic redundancy check appended to every Modbus RTU frame.
A 1-bit read/write digital output — address range 00001–09999.
A 1-bit read-only digital input — address range 10001–19999.
The slave does not support or recognize the requested function code.
The register address or address range does not exist on the slave.
A data value in the request is outside the acceptable range.
The slave encountered an unrecoverable error while processing the request.
Read the ON/OFF state of one or more output coils.
Read the ON/OFF state of one or more digital inputs (read-only coils).
Read one or more 16-bit holding registers — the most commonly used Modbus function.
Read one or more 16-bit input registers (read-only, typically sensor values).
Set a single output coil to ON (0xFF00) or OFF (0x0000).
Write a 16-bit value to a single holding register.
Write ON/OFF states to a block of coils in a single request.
Write a block of 16-bit values to consecutive holding registers.
Modify specific bits of a holding register without changing others.
Read and write holding registers in a single atomic transaction.
Diagnostic function to test communication with a slave, including echo (sub-function 0x0000).
Read vendor, product name, and version strings from a Modbus device.
A 32-bit IEEE-754 floating-point number stored across two consecutive registers.
A 64-bit double-precision float spanning four consecutive Modbus registers.
The minimum silence on the bus that marks the end of a Modbus RTU frame.
A 16-bit read-only register — typically live sensor or process values.
A 16-bit signed integer stored in one register — range −32768 to +32767.
A 32-bit signed integer spanning two consecutive Modbus registers.
A 1-byte checksum used in Modbus ASCII frames.
The least significant byte is in the lowest address — reversed from network order.
A serial communication protocol standard for industrial automation.
Binary serial framing over RS-485 (or RS-232) with CRC-16 error checking.
ASCII-encoded Modbus frames with colon start and LRC error checking.
Modbus protocol encapsulated over a standard TCP/IP Ethernet connection.
The 6-byte header prepended to every Modbus TCP request and response.
The most significant word is first but bytes within each register are swapped.
An error reply from a slave indicating it could not execute a request.
The numeric location of a register within its register table.
The relationship between a 1-based datasheet address (40001) and its 0-based protocol address (0).
ASCII text packed two characters per register across consecutive registers.
The 1-byte device address identifying which device a request is intended for.
A 16-bit unsigned integer — the native data type of a single Modbus register.
A 32-bit unsigned integer spanning two consecutive Modbus registers.
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