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docs/TESTING.md

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+# Validation
+
+Run `make test` on Linux with a C compiler and Python 3. Tests use a local
+socket server and cover normal/fragmented replies, close/reconnect, timeout,
+bad transaction IDs, bad MBAP length, Modbus exceptions, uncertain writes and
+coils. They run through the direct engine and bridge. Pool/handle tests are
+included. These tests do not compile or execute Structured Text.
+
+In CODESYS compile both libraries and start a read-only bench application.
+Verify first scan with zero registered requests, then registered requests,
+reply timeouts, priorities, data freshness, disconnect/reconnect and disposal.
+Test full download/reset and online-change lifecycle separately. Only then test
+writes with appropriate device registers. Validate embedded deployment on a
+target without the manually configured component.