Modbus

A field note on shutdown deadlocks and timeout misjudgment in a Qt multithreaded Modbus workflow, with practical fixes.

An industrial gateway (TI AM64x, Linux PREEMPT_RT) measured 30–60 ms Modbus-TCP RTT and 50–80 ms RTU jitter against a Windows simulator. After correcting the measurement semantics and running a three-step orthogonal isolation loopback (localhost → dual-NIC netns direct link → dual RS-485 crossover), the jitter is attributed to the test peer under these test conditions: board-side pure software stack 0.69 ms, physical NIC 1.02 ms, serial 4.15 ms (wire time about 70%); peer-side interventions such as the FTDI Latency Timer pinned the Windows bench’s measured lower bound at 45–50 ms, from which a reference timeout baseline is derived; a single gateway transaction consumes roughly 1%–15% of the end-to-end link budget — not a blocker.

A postmortem on CPU optimization under a high-frequency polling workload: by shrinking the UI logging path, removing high-frequency state machines, and fixing the wait model, CPU usage dropped from 3.6%~4.2% to 0.1%~0.6%.

A practical guide to a lightweight Modbus tool for embedded development and field commissioning: fast frame building, log viewing, Frame Analyzer with scaling/register annotations/JSON-CSV persistence, Link to Analyzer live linkage, and Force Parse.