Rediscovering the 3-Pin Port: A Novel RECEIVE() Protocol Discovery Enabling Low-Cost Classroom Data Logging on Casio FX-9750 and FX-9860 Series Calculators

This project documents a novel discovery in the serial communications protocol of Casio graphing calculators equipped with the 3-pin (2.5 mm) serial port, specifically the FX-9750 and FX-9860 series. The author identifies two READY-gated pause windows in the Casio RECEIVE() operation that allows external microcontrollers to send sensor data at preset time intervals (reliable up to 300 seconds) without triggering a COM ERROR. First discovered in 2008, new work in 2025 extended compatibility to newer calculator models and microcontrollers. This finding restores a data-logging capability built into calculators already sitting in classroom sets, rediscovering what the 3-pin port was always capable of long after dedicated peripherals disappeared from schools. The system supports Picaxe, Micro:bit, ESP8266, and ESP32 microcontrollers, enabling synchronous multi-sensor data logging, live graphing, optional wireless communication (200+ metre range), and web-based data sharing. A single Casio BASIC program provides universal connectivity. The design is paired with a structured pedagogical framework - Build It, Test It, Use It - in which learners construct their own sensor units, verify correct operation through guided activities, and then design authentic investigations relevant to their own questions and contexts. This approach supports progression to Relational and Extended Abstract levels on the SOLO taxonomy. The overall design prioritises equity and accessibility in STEM education, targeting resource-constrained schools and junior year levels historically excluded from authentic scientific investigation. Total hardware cost is approximately USD $6–$10. At this price point, the use of AI-generated or simulated scientific data in the classroom becomes a choice requiring active justification rather than a practical necessity; other researchers have raised legitimate concerns that AI training datasets may be compromised by the proliferation of fabricated or low-quality research in the scientific literature. Where authentic, real-world empirical data can be collected cheaply and reliably, it should be. This deposit documents the READY-gated RECEIVE() pause window discovery and single-sensor implementation only. Synchronous multi-sensor encoding is the subject of forthcoming publication. Project page: https://mikefentonnz.github.io/projects/casio-calculator-data-logger-hack.html