Jankiness aside, eventually was able to wire up a port to the side of the calculator which lets him use his computer to send Python commands to the device when it is in its Python programming mode. After some investigation of test points, found that the Z80 and ARM chips are communicating with each other over twin serial lines using a very “janky” interface. The immediately noticeable difference is the ARM coprocessor that is not present in other graphing calculators. While he’s not sure exactly what implementation of Python the calculator is running, he did tear it apart to try and figure out as much as he could about what this machine is doing. They still run their code on a Z80 microcontroller, but found himself in possession of one which has a modern ARM coprocessor in it and thus can run Python. These testing standards are so entrenched, in fact, that TI has not had to update the hardware in these calculators since the early 90s. Texas Instruments is a world-class semiconductors company, but unfortunately what they are best known for among the general public is dated consumer-grade calculators thanks to entrenched standardized testing.
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