Although the documentation says it will work on System 3.2, I couldn't get it to work on anything lower than 4.1. In the video, I mistakenly said I was using System 4.2, but it's really 4.1.

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That's odd. I wonder if it's a limitation of the 128K ROM. There's a number of applications that require an SE and won't run on a Plus. Could this be one of them? Do you have an SE to try it on? Though the SE came with System 4.0, so it doesn't make sense it would include 3.2 and require an SE. Perhaps it is a RAM limitation? But then why would it work with System 4.1? Very odd. The 512Ke is essentially identical to a Plus otherwise, right? Some changes in the serial port pinouts, but would that be enough to limit it to the Plus? And how would System 4.1 compensate for those differences? Odd.