Reference

Glossary

114 terms. Press G anywhere in the course to pop open the quick drawer.

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0x reference: Coil reference range (00001-style) — table 0 in the Modicon convention.
1x reference: Discrete-input reference range (10001-style) — table 1 in the Modicon convention.
3x reference: Input-register reference range (30001-style) — table 3 in the Modicon convention.
4x reference: Holding-register reference range (40001-style) — table 4 in the Modicon convention.
8-E-1: 8 data bits, even parity, 1 stop bit — Modbus RTU default character framing.
8-N-2: 8 data bits, no parity, 2 stop bits — alternate Modbus RTU framing when parity is disabled.

A

AB-style: Allen-Bradley 1-based addressing convention (e.g. F8:0 / N7:0) sometimes mapped onto Modbus by gateways.
ABCD: Word order label — registers concatenated in transmitted order, high word first.
ADU: Application Data Unit — the PDU wrapped with transport addressing and integrity (RTU adds addr + CRC; TCP adds MBAP).
ASCII: Modbus ASCII framing — hex-encoded bytes between ':' and CR/LF, with an LRC byte.
audit log: Persistent record of who wrote what and when — required for safe operation in flat OT networks.

B

BADC: Word order label — bytes swapped within each register, words in transmitted order.
baud: Symbols per second on a serial line. Modbus RTU commonly uses 9600, 19200, 38400, 57600, 115200.
biasing: Pull-up/pull-down resistors that hold an idle RS-485 line in a defined logic state.
big-endian: High-order byte first. Modbus registers are big-endian within a 16-bit word.
bitfield: Status word where each bit represents a discrete flag (enabled, fault, remote, etc.).
block read: Single FC 03/04 covering many registers — far more efficient than per-register polls.
broadcast: Slave address 0 in RTU/ASCII writes — every server acts, no one replies.

C

CDAB: Word order label — two registers swapped before assembly (low word first).
client: Modern Modbus terminology for the device initiating requests (formerly 'master').
coil: 1-bit read/write item in the Modbus data model. Conventionally a discrete output.
coils: 1-bit read/write items — discrete outputs.
Concept: Schneider Electric's legacy PLC programming environment — origin of much of the 4xxxx Modicon addressing convention.
CRC: Cyclic Redundancy Check. Modbus RTU appends a 16-bit CRC (poly 0xA001, init 0xFFFF), low byte first.
CRC-16: The 16-bit CRC used by Modbus RTU. Polynomial 0xA001, initial value 0xFFFF, appended little-endian.

D

daisy chain: RS-485 wiring pattern where each device taps the trunk and the trunk continues to the next device — no star or branch.
DCBA: Word order label — fully reversed byte order across the two registers.
discrete input: 1-bit read-only item in the Modbus data model.
discrete inputs: 1-bit read-only items.
documented address: 1-based address printed in vendor manuals (e.g. 40001). Convert to PDU before sending.
DTM: Device Type Manager — vendor software component that knows how to configure a device. Often Modbus-backed under the hood.

E

EDS file: Electronic Data Sheet — vendor file describing a device's registers and meanings. Common in EtherNet/IP, sometimes reused for Modbus.
endianness: Byte and word ordering. Modbus registers are big-endian; multi-register integers/floats may be word-swapped (CDAB vs ABCD).
engineering units: The physical units a scaled register represents (V, A, °C, kWh).
engineering value: Raw register transformed into a unit-bearing physical quantity.
enum: Register where the integer value maps to a named state (0=Off, 1=Local, 2=Remote).
even parity: Parity bit set so the total number of 1-bits in the byte is even. Modbus default.
exception: Server response with the function code OR'd with 0x80 plus a one-byte exception code (illegal function, illegal address, etc.).
exception code: One byte in an exception response. Common: 01 illegal function, 02 illegal address, 03 illegal value, 04 server failure.

F

fail-safe biasing: Bias resistors sized so an idle/floating RS-485 bus reads as logic high — prevents lost start bits.
FC 01: Read Coils — fetch 1..2000 coil states.
FC 02: Read Discrete Inputs — fetch 1..2000 input states.
FC 03: Read Holding Registers — fetch 1..125 16-bit registers.
FC 04: Read Input Registers — fetch 1..125 input registers.
FC 05: Write Single Coil — set one coil to ON (0xFF00) or OFF (0x0000).
FC 06: Write Single Register — write one 16-bit holding register.
FC 15: Write Multiple Coils — write up to 1968 coils in one transaction.
FC 16: Write Multiple Registers — write up to 123 contiguous holding registers in one request. Device-level atomic behavior is not guaranteed by the protocol.
FC 17: Report Server ID — vendor-defined server identification string.
FC 22: Mask Write Register — atomically AND/OR bits in a holding register.
FC 23: Read/Write Multiple Registers — combined read+write in one transaction.
FLOAT: IEEE-754 32-bit single-precision float, spanning two consecutive registers.
function code: One byte identifying the operation. Read codes 01–04, write codes 05/06/15/16. 0x80+ marks an exception response.

G

gateway: Device that translates between Modbus TCP and Modbus RTU/ASCII, routing by Unit ID.
gateway path unavailable: Exception 0A — gateway can't route this request (wrong unit ID, dead RTU bus).
gateway target failed to respond: Exception 0B — gateway routed the request but the downstream device timed out.
ground reference: Common signal-ground wire on an RS-485 bus. Required when devices share no other reference.

H

holding register: 16-bit read/write register — the workhorse for analog setpoints and configuration.
holding registers: 16-bit read/write registers.

I

ICC card: Industrial Communications Card — option module that adds Modbus to a drive or motor controller.
IEEE 754: Floating-point standard. Modbus floats are typically single-precision (4 bytes / 2 registers).
illegal data address: Exception 02 — the request targeted a register the server doesn't expose.
illegal data value: Exception 03 — the request was syntactically valid but the value is out of range.
illegal function: Exception 01 — the server doesn't implement this function code.
input register: 16-bit read-only register, typically used for measured values.
input registers: 16-bit read-only registers.
INT16: Signed 16-bit integer (-32768..32767).
INT32: Signed 32-bit integer spanning two consecutive registers.
inter-character timeout: 1.5-character window inside an RTU frame. Exceeding it aborts the frame.

L

length field: Two-byte MBAP field — number of bytes that follow (Unit ID + PDU).
little-endian: Low-order byte first. Modbus does NOT use little-endian within a register.
LRC: Longitudinal Redundancy Check. One byte two's-complement sum used by Modbus ASCII.

M

MBAP: Modbus Application Protocol header used in Modbus TCP. Seven bytes: transaction ID, protocol ID (0), length, unit ID.
MBAP header: Modbus TCP 7-byte header: transaction ID, protocol ID (0x0000), length, unit ID.
Modbus Security: TLS-secured Modbus TCP profile (Modbus/TCP Security). Adds mutual authentication and encryption on a separate port (802).
Modicon notation: Legacy 1-based reference numbering (40001 etc.) used by Schneider/Modicon docs and many vendors.

N

node: A single addressable device on a Modbus bus (one Slave Address / Unit ID).

O

odd parity: Parity bit set so the total number of 1-bits in the byte is odd.

P

parity: Optional extra bit per byte for error detection. Modbus default is even parity with 1 stop bit.
PDU: Protocol Data Unit — the transport-independent core of a Modbus message: function code + function-specific data.
PDU address: 0-based register address that appears on the wire. Documented 40001 → PDU 0.
poll rate: How often a client requests data from a server. Faster polls increase bus load and latency.
port 502: Default TCP port for unencrypted Modbus TCP.
port 802: Default TCP port for Modbus/TCP Security (TLS).
protocol ID: Two-byte field in MBAP, always 0x0000 for Modbus TCP.

R

read-back: Reading a register immediately after writing it to verify the device accepted the value.
register block: Contiguous group of registers read in a single request to reduce overhead.
register map: Vendor-supplied table listing register addresses, data types, scaling, and meaning.
register number: Ambiguous — some vendors mean documented 4xxxx number, some mean PDU offset. Always restate.
RS-232: Short-range single-ended serial. Point-to-point, no multi-drop. Mostly legacy for Modbus today.
RS-422: Four-wire differential serial, point-to-point or one-driver-many-receivers.
RS-485: Two-wire differential serial bus, multi-drop up to 32 unit loads, terminated 120Ω at each end.
RTU: Remote Terminal Unit framing — binary over serial with CRC-16 and inter-character timing (3.5 char silent interval).

S

scaling: Converting a raw register value to engineering units via a multiplier, offset, and unit (e.g. raw/10 °C).
sentinel value: Reserved raw value meaning 'invalid/unavailable' (commonly 0x7FFF, 0x8000, 0xFFFF).
server: Modern Modbus terminology for the device responding to requests (formerly 'slave').
server device failure: Exception 04 — unrecoverable error inside the server.
silent interval: 3.5-character idle time that delimits RTU frames. Below 1.5 chars indicates the same frame.
slave address: RTU/ASCII byte identifying the server (1–247). 0 is broadcast.
stop bit: Idle bit(s) marking the end of a UART character. Modbus uses 1 (with parity) or 2 (no parity).
stub: Branch line off the main RS-485 trunk. Keep derivations short and verify the allowed length against the official design case plus the cable and device documentation.

T

TCP: Modbus TCP — PDU wrapped with the MBAP header, carried over TCP port 502. No CRC, TCP handles integrity.
telemetry: Read-only measurements polled from a device for monitoring or historian storage.
termination: 120Ω resistor at each physical end of an RS-485 bus to absorb reflections.
transaction ID: Two-byte counter the client uses to match TCP responses to requests.

U

UINT16: Unsigned 16-bit integer (0..65535).
UINT32: Unsigned 32-bit integer spanning two consecutive registers.
unit ID: One byte in Modbus TCP identifying the target on the other side of a gateway. Often 0xFF or 1 when no gateway is present.
unit load: RS-485 standard load unit. Standard transceivers are 1 UL; the bus supports 32 UL (more with 1/8-UL parts).

V

vendor map: The register documentation produced by a device manufacturer.

W

Wireshark: Open-source packet capture tool. Has built-in Modbus TCP dissector and a Modbus RTU plug-in.
word swap: Reversing the order of two 16-bit registers when assembling a 32-bit value.
write allowlist: Defensive practice — restrict writeable registers and writers explicitly, deny by default.
write-readback: Verify that a write took effect by reading the register back immediately afterwards.