The Strange World Of Ohio Scientific Floppy Disk Controllers

I am currently trying to decipher various OSI disk images and failing badly but one fascinating part of this was discovering how the floppy hardware worked. It’s not quite Wozniak grade but it is close.

The OSI disk controller is a 6850 ACIA and 6820 PIA. The PIA side is kind of as you would expect. Lines in and out and a load of pullups and other gunk for the signal levels on the 8" drives.

The ACIA is used for the data stream. The receive side is a clock recovery circuit which outputs a clock and data both of which are fed to the 6850. The send side is a 1 or 2MHz clock according to density/media type that clocks the ACIA and some glue to combine the clock and the data bit.

As it’s an ACIA you get start and stop bits, and parity (which it seems it uses) plus the data.

Schematic for the curious.

Needless to say this makes the disk format very strange

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Floppy disc controlers I think were just out about $50 each. That was a bit cheaper.

Very interesting - I think I see from the schematics that the clock recovery and data separation is not part of the 420 board. From a bit of reading it seems that 8inch drives would include these functions, but only a minority of 5¼ drives would. However, the conservative encoding means that these functions are easy, apparently:

since the OSI system never misses inserting the clock pulse before each data pulse that is written on the disk, the Data Separator would be easy and simple to design. Some of the disk writing systems withhold some of the clock pulses written on the disk. This makes the data separator much more difficult. The data separator I came up with, and found to work very well, is shown in figure 3.

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Elektor magazine copied the OSI design and adapted OS65D driver to the hardware addresses changed for their Elektor Junior computer.

Floppy disk interface – Retro Computing (hansotten.nl)

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