Modbus RTT Jitter at 30–80 ms: Gateway Measured 1–4 ms — the Root Cause Was the Test Bench
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.