Modbus troubleshooting

First, tell what kind of result you have:

Result	Meaning	Page
{"ok":true,...,"status":"exception"}	A Modbus exception response: the server rejected the request.	Exception responses
{"ok":false,"error":"..."}, a C++ or .NET exception, ... failed: in the GUI	A service error: the command itself failed.	Service errors and timeouts
"status":"no_response"	Intentional silence after a broadcast or in listen-only mode	Serial communication problems
"status":"success" with odd values	An address, table or value interpretation problem	Wrong address

By symptom

Symptom	Go to
Timeout	Timeouts; on a serial line, No response at all
Closed	Transactions
Io(ConnectionRefused, ...), Io(AddrInUse, ...), Invalid TCP address	Connection
Invalid endpoint configuration	Configuration
CRC or LRC errors, rising communication_errors	CRC and LRC errors
Security(...), Config(...), certificate verify failed	Security connection problems
Exceptions 01, 02, 03	Exception responses
Exceptions 0A, 0B	Gateway exceptions

Narrow it down

1. SDK: does {"action":"capabilities"} answer, with the transports and Security you need set to true?
2. Configuration: does configure succeed?
3. Connection: does connect report connected as true?
4. Transaction: check functions 06 and 03 on 127.0.0.1 on the same computer first, then widen to the real device.
5. Meaning: do the values match the manual, and is the result’s source wire? A local result isn’t communication.

What to collect before contacting support

• The results of capabilities, state, diagnostics, trace and project_export
• The failing request’s PDU and unit, its result or error message, and the process exit code
• A diagnostic log recorded with PDCOMM_LOG=debug
• The OS, the SDK version (manifest.json) and, for serial devices, the device model, adapter and wiring

Diagnose failures and report them explains how, and includes a request template. Never send private keys or raw customer data.