Modbus guides
Practical Modbus guides: register addressing, exception codes, frames, word order, timeouts and RTU over TCP, with frames captured from a real server.
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Modbus register addressing: 40001, 0-based and off-by-one errors
Why register 40001 is address 0 on the wire, how to tell 0-based from 1-based documentation, and a quick test that settles it on a real device.
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Modbus exception codes 01 to 0B: what they mean and how to fix them
What each Modbus exception code means, how to spot one in a frame, the usual cause of 01, 02, 03, 04, 0A and 0B, and how a real server deviates from the spec.
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Modbus TCP timeout or connection refused: a troubleshooting checklist
Work through a Modbus TCP failure layer by layer: refused, timed out, connected but silent, gateway errors. Each symptom tells you what to check next.
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Reading 32-bit floats over Modbus: ABCD, CDAB, BADC and DCBA
Why two Modbus registers decode to nonsense, what the word order names mean, and how to find the right order on a device, with real values captured from a server.
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The Modbus TCP frame, byte by byte: MBAP header and PDU
A real Modbus TCP request and response dissected field by field: Transaction ID, Protocol ID, Length, Unit ID, function code and data, plus the size limits.
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Modbus function codes: a reference with limits and example frames
The Modbus function codes you need most: 01 to 06, 15 and 16, with quantity limits, real request and response frames, and the less common codes.
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Modbus Unit ID: when it matters, and why 0, 1 and 255 all turn up
What the Modbus TCP Unit ID does, what the implementation guide recommends for direct devices and gateways, and how to find the right one when a device stays silent.
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Modbus RTU over TCP vs Modbus TCP: which does your converter speak?
The frame differences between Modbus TCP and RTU over TCP, transparent and gateway converter modes, how to tell which one you have, and the traps of each.
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How to test Modbus TCP without a PLC: three ways
Test a Modbus TCP client or SCADA screen without hardware: a Python simulator with a working script, the FieldKit server on an iPhone, and when to use each one.
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Modbus testing tools compared: Modbus Poll, mbpoll, QModMaster, apps
Which Modbus tool fits which job: Modbus Poll, mbpoll, QModMaster, pymodbus and iPhone apps, including FieldKit. Platforms, protocols, prices and their limits.