Modbus tutorials built for commissioning and troubleshooting
Clear answers to the issues that cost engineers time in the field: wrong register offsets, RS-485 wiring, byte order, exception codes, timeouts, function-code selection and simulation.
Quick answer: Most Modbus problems come from five causes: wrong serial settings or IP/port, a wrong slave ID, a register offset (40001 vs address 0), the wrong function code (FC 03 vs FC 04) or the byte order of 32-bit values. Start with the timeout checklist if nothing answers, or the exception-code guide if the device replies with an error.
Start here: if you are new to Modbus, read Learn Modbus. If you already have a device that will not communicate, go directly to the timeout checklist or exception-code guide.
CRC, register-address and Float32 checks
Use quick calculators before or during field troubleshooting.
How does Modbus work?
Modbus TCP vs RTU
Framing, wiring, speed, topology, RTU-over-TCP gateways and Modbus UDP.
ReferenceModbus function codes
FC 01/02/03/04/05/06/15/16 with practical read/write examples, plus FC 08, 17, 22, 23 and 43.
Addressing40001 vs address 0
Why documentation and software can appear one register apart—and how to correct it.
Why is my Modbus RTU link unreliable?
RS-485 wiring & termination
Daisy chains, polarity, 120 Ω termination, shielding and common grounding mistakes.
DiagnosticsFix Modbus timeouts
A systematic checklist covering serial settings, slave IDs, TCP ports, wiring and polling.
ErrorsException codes 01–04
What Illegal Function, Illegal Data Address and related errors mean in practice.
Why do register values look wrong?
Float32 & byte order
ABCD, DCBA, BADC and CDAB for 32-bit and 64-bit values, plus strings in registers.
Engineering unitsScaling & units
Turn raw register counts into volts, amps, temperature, power and other engineering values.
WorkflowRegister maps in ModbusBB
Import aliases, formats, scale factors and units, or start from the built-in device library, so a device map is reusable.
How can you test without the real device?
Use ModbusBB as a slave/server
Serve editable or generated values, inject exceptions and delays, and test a PLC, SCADA client or gateway without the real field device.
Product docsModbusBB documentation
Connections, poll tabs, workspaces, traffic monitor, device tools, watchdogs, CLI and licensing.
Command lineModbus command-line tool
Script reads, writes, polls and scans with JSON/CSV output, hex trace and exit codes.
Tool selectionModbus master software comparison
See how ModbusBB approaches master testing, simulation, trends and CLI workflows.
Use the guides with a real Modbus tool
The free trial includes the complete ModbusBB feature set.