This protocol hub collects 35 pages categorized under Modbus, including reference material and task-oriented support content.
Knowledge Base
Byte Order
Byte order (endianness) overview covering big-endian vs little-endian, multi-register data, and the relationship to word order.
Coil
Modbus coil data point overview covering discrete read/write behavior, function codes, and addressing conventions.
Discrete Input
Modbus discrete input data point overview covering read-only bit access, function codes, and comparison to coils.
Modbus Function Codes
Modbus function code reference covering FC01–FC16 for reading and writing coils, discrete inputs, holding registers, and input registers.
Holding Register
Modbus holding register overview covering 16-bit read/write data, multi-register values, function codes, and addressing.
Input Register
Modbus input register overview covering read-only 16-bit data, function codes, and comparison to holding registers.
Register Map
Modbus register map overview covering vendor documentation, address tables, data types, and integration best practices.
Modbus RTU
Modbus RTU overview covering serial RS-485 transport, framing, CRC error detection, and pre-commissioning requirements.
Modbus TCP
Modbus TCP overview covering Ethernet transport, MBAP header, Unit ID usage, and comparison to Modbus RTU.
Modbus Slave ID / Unit ID
Modbus slave ID and unit ID overview covering device addressing for RTU serial and TCP networks, broadcast behavior, and common configuration mistakes.
Word Order
Modbus word order overview covering 16-bit word arrangement in 32- and 64-bit values, and relationship to byte order.
Modbus
General Modbus wiki page covering concepts, addressing, data representation, and integration pitfalls.
RS-485
RS-485 serial communication overview covering differential signaling, daisy-chain topology, termination, and protocol usage.
Articles
Modbus RTU Pre-Commissioning Checklist
Step-by-step checklist to verify RS-485 wiring, serial settings, and device connectivity before deploying a Modbus RTU configuration. Prevents the most common commissioning failures.
How to Read a Modbus Register Map
Step-by-step guide to interpreting Modbus register maps from third-party device manufacturers — addressing conventions, data types, register grouping, and validation techniques.
Modbus Addressing & Register Reference
A practical reference for Modbus register addressing: function codes, register ranges, 0-based vs 1-based conventions, page-based addressing, and common offset pitfalls.
Modbus Broadcast Address: Node 0 vs Node 255 Explained
Technical reference for Modbus broadcast addressing — how Node 0 works per the specification, why some devices use Node 255, and how to handle broadcast write commands in multi-device RS-485 networks.
Modbus Commissioning Site Toolkit Checklist
Essential tools, adapters, and software to bring to site for Modbus RTU and TCP commissioning — a field technician's packing list.
Modbus CRC & LRC Error Checking: Calculation, Troubleshooting, and Packet Diagnosis
How CRC-16 and LRC error detection works in Modbus RTU and ASCII — calculation methods, common causes of checksum failures, and step-by-step packet diagnosis with Wireshark and serial tools.
Modbus Data Types & Byte Order Reference
How to handle 32-bit floats, byte order (endianness), multi-register values, and scaling factors in Modbus integrations. Includes a device byte-order lookup table.
Modbus Device Discovery & Network Scanning Guide
How to discover unknown Modbus devices on RS-485 and TCP networks — scanning address ranges, identifying baud rates, mapping registers, and building a device inventory from scratch when documentation is unavailable.
Modbus Exception Codes & Error Handling Reference
Complete reference for Modbus exception codes (01–0A), error response format, diagnostic function codes, and troubleshooting guide for common exception scenarios in RTU and TCP integrations.
Modbus Function Code Deep Dive: FC01–FC06, FC15–FC16, FC23
Detailed reference for the most commonly used Modbus function codes — request/response byte structure, parameter requirements, practical examples, and which function codes to use for each data type.
Modbus Gateway Data Validity & Offline Device Handling
How Modbus gateways should handle stale data when a downstream device goes offline — reliability concepts, BACnet comparison, and QuickServer strategies.
Modbus Protocol Variants: JBUS, Enron, and Daniel
Overview of non-standard Modbus variants — JBUS register numbering, Enron Modbus extended data types, and Daniel Modbus — and how to handle them in protocol conversion.
Modbus Register Addressing: Modicon, PDU, and Page-Based Conventions
Complete guide to Modbus addressing conventions — Modicon 5-digit notation, PDU 0-based addressing, page-based schemes, and how to resolve the off-by-one errors that cause most integration failures.
RS-232 Serial Wiring & Handshaking Bypass for Modbus
RS-232 pinouts, cable wiring, and handshaking bypass jumper schemes for Modbus serial connections — DB9 configurations, null modem wiring, and common pitfalls.
RS-485 Physical Layer Wiring & Termination Reference for Modbus RTU
Complete RS-485 physical layer specification for Modbus RTU networks — wiring diagrams, termination resistor placement, polarization, cable selection, maximum distances, connector pinouts, and grounding best practices.
Modbus RTU Multi-Device Daisy Chain Wiring Guide
How to wire, address, and commission multiple Modbus RTU devices on a single RS-485 daisy chain — topology rules, addressing best practices, repeater placement, and troubleshooting for 2 to 32+ device networks.
Modbus RTU vs ASCII Transmission Modes Explained
Technical comparison of Modbus RTU and ASCII serial transmission modes — framing, error checking, timing, performance differences, and when to use each mode in building automation integrations.
Modbus Scaling & Data Conversion Guide
How to convert raw Modbus register values to engineering units — integer scaling, multipliers and divisors, floating-point registers, temperature conversion, and configuring scaling in protocol conversion gateways.
Modbus TCP MBAP Header & Message Format Reference
Complete reference for the Modbus TCP message format — MBAP header fields, Transaction ID management, Unit Identifier usage, TCP connection handling, and differences from Modbus RTU framing.
Modbus to BACnet Protocol Conversion Guide
End-to-end guide for converting Modbus RTU or Modbus TCP devices to BACnet/IP or BACnet MS/TP using a QuickServer protocol gateway.
Modbus Troubleshooting Guide
Symptom-based troubleshooting for common Modbus integration issues: no communication, wrong data, intermittent timeouts, and garbled values.
Wireshark for Modbus TCP: Capture, Filter, and Diagnose
Step-by-step guide to using Wireshark for Modbus TCP troubleshooting — capture setup, display filters, packet analysis, and hub vs switch considerations.