Modbus Glossary

The actual protocol addressing: addresses start from 0 in PDU requests.

The human-readable naming convention used in datasheets: 40001 = first holding register.

The complete Modbus frame including addressing and error-checking fields.

Pull-up and pull-down resistors on RS-485 A and B lines that define a known idle state when no device is transmitting.

The serial communication speed in symbols per second — must match exactly on master and all slaves.

A Modbus request sent to address 0 — all slaves receive and execute it, but none send a response.

A binary pattern used to isolate or set individual bits within a Modbus register.

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 range of values around a setpoint where no corrective action is taken — prevents constant small corrections.

A 1-bit read-only digital input — address range 10001–19999.

The real-world physical unit of a measurement after applying the scale factor — °C, kW, rpm, bar, etc.

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.

The slave accepted the request but needs more time to process it — used for long-duration commands.

The slave is busy processing a previous long-duration command and cannot accept a new request yet.

The slave detected a parity or checksum error in its own extended memory while executing the request.

A Modbus TCP gateway could not find a route to the target slave device.

A Modbus TCP gateway reached the serial bus but the target slave did not reply within the timeout.

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).

Returns a status word and event counter from a slave — a quick way to check if recent communications succeeded.

Reads device identification data including a run indicator and device-specific information bytes.

Reads the contents of a FIFO queue of holding registers from a slave — non-destructively returns up to 31 queued values.

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.

A communication mode where only one device transmits at a time — the mode required by Modbus RTU on RS-485.

A touchscreen panel or PC screen that displays Modbus data and lets operators interact with machines in real time.

The upper 8 bits (high byte) and lower 8 bits (low byte) of a 16-bit Modbus register — how bytes are ordered matters when extracting values.

A 16-bit read/write register — the most used data store in Modbus.

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 device that translates between Modbus RTU (serial) and Modbus TCP (Ethernet), bridging two network types.

The device that initiates all requests on a Modbus network — only one master is allowed per RTU bus.

The device that responds to requests from the master — sensors, meters, drives, PLCs in subordinate role.

The TCP/IP equivalent of a Modbus Master — initiates connections and requests over Ethernet.

The TCP/IP equivalent of a Modbus Slave — listens on port 502 and responds to client requests.

A wiring topology where multiple devices share the same pair of wires — the fundamental topology of RS-485 Modbus RTU.

An electricity or heat meter with a built-in Modbus RTU or TCP interface — exposes measurements like kWh, kW, V, A, and power factor as readable registers.

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.

An optional single-bit error check appended to each serial character — must match on all devices.

An industrial computer that controls machines and processes — usually a Modbus master or slave on the factory floor.

TCP port 502 is the IANA-registered default port for Modbus TCP — the port all Modbus TCP servers listen on.

How often the Modbus master reads a register or a set of registers from a slave device.

The function code plus data — the core payload shared across all Modbus variants.

The differential serial electrical standard used by Modbus RTU — supports up to 247 devices on one twisted-pair cable.

The older point-to-point serial standard — single-ended, shorter range, but still used for direct PC-to-device Modbus connections.

A differential serial standard like RS-485 but point-to-point — supports longer distances and higher speeds.

The maximum time the master waits for a slave to respond before declaring the request failed.

The number of times the master resends a request after a timeout before marking the slave as unresponsive.

A table in a device's datasheet listing every register — its address, type, data format, and meaning.

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).

The 1 or 2 idle-line bits that mark the end of each serial character frame.

Supervisory Control and Data Acquisition — the software layer that reads Modbus data plant-wide and displays it to operators.

A 16-bit register where each individual bit represents a separate ON/OFF status flag.

A multiplier that converts a raw integer register value to a real engineering unit value.

A target value written by the master to a slave device — telling it to reach or maintain a specific condition.

ASCII text packed two characters per register across consecutive registers.

A 120 Ω resistor placed at each end of an RS-485 bus cable to prevent signal reflections.

A 2-byte field in the Modbus TCP MBAP header used to match requests to their responses.

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.

Registers are swapped but bytes within each register stay in big-endian order.