Verified Bidirectional IoT Retrofitting of Legacy Industrial Temperature Controllers Through Modbus Write Readback

Abstract

Legacy industrial PID temperature controllers, such as the Autonics TN series (TNH/TNL), are widely deployed to run process cycles in small and medium-scale industrial operations but generally lack native IoT connectivity. While retrofitting such controllers for cloud-based monitoring (the read direction) is comparatively well established, writing process configuration back to the controller reliably, and verifying that the write actually took effect, remains a harder and less-explored problem. This study addresses that gap by proposing and evaluating a register-level write-then-read-back verification protocol for bidirectional deployment of process patterns (PTTN: pattern number, step count, per-step setpoint and duration, and end-state behavior) to a TNH controller over a direct RS-485 serial path (Jalur A), one of two architecturally distinct control paths in the system under study; the other, an ESP32-mediated cloud path (Jalur B), is implemented and compile-verified but not yet validated on physical hardware. Using a Modbus RTU simulator and 100 synthetic patterns ranging from 1 to 20 steps, the protocol achieved 100.00% Pattern Deployment Accuracy (SD = 0.00%), with p50, p90, and p99 end-to-end latencies of 511, 543, and 578 ms, respectively, that did not scale meaningfully with pattern complexity. A subsequent fault-injection experiment (N = 45 trials: 15 baseline plus 30 induced-fault) found that under transient device unresponsiveness, all interrupted trials were reported as client-side failures even though independent read-back confirmed the controller state had been correctly updated, revealing a false-failure risk that could cause naive retry logic to redundantly or unsafely re-send already-successful commands. These simulator-based results establish the correctness and reliability characteristics of the verified deployment logic and motivate, without yet empirically validating, the necessity of the cloud-capable Jalur B path, whose hardware validation remains future work.