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From: Marko_Mäkelä
Date: 2001-09-25
Subject: Reading the cassette sense line?

Hi all,

I'm working on a device that allows fast data communications between RS-232 and the cassette port. The device consists of two ICs and a few passive components. So far, I've programmed the microcontroller on the board so that I can load and save programs in the standard format of any Commodore computer when the device is hooked to a Unix-like box.
(Versions for Amiga, Mac, Windows and others should not be too hard to make. The source code will be made available under the GPL soon, and it is available upon request even now.)

The next step is to implement a faster communication protocol via the cassette interface, with a raw transfer speed of less than 200 cycles per byte, or more than 4kB/s on the plus/4 running at 886723 Hz. (I'm using
38400bps RS-232 connection with XON/XOFF handshaking, so the raw speed on the C= side won't be the bottleneck; it'll be at most 3840 bytes per second anyway.)

I could otherwise use the cassette sense line bidirectionally, but the line is unidirectional in the plus/4. In all other Commodores, including the PET, the P500 and the CBM II series, the line is bidirectional.

Since the cassette sense line is wired in a weird way in the plus/4, I'd like to ask the plus/4 experts' advice: how can the state of the line determined by the processor (sample machine code, please)?

Marko