need technical information on amiga 500 disk format

Hi guys, not sure if this is in the right place, feel free to put this somewhere else.

Right, well... I have an ambitious project to create an external floppy disk controller that will read/write amiga disks and connect via parrell port.

I am having trouble finding detailed information on floppy disk (perticularly the amiga veriaty) data structures and encoding.

Basicly, what I want to know is if i were to have an amiga disk and the first 5 bytes were 01100110 01010101 00001111 10101110 11101110 (for example) and I stepped the head to the track 00 position, selected side 0, and grounded the motor enable and drive select, what would come out of the read data pin?

I know that amiga disks are 880k, double density. So, if they work the way I think they do, then they are the same as IBM disks but with 22 sectors per track instead of 18.

However, I really have no idea how the data is encoded.
I know how all of the control lines work, but I don't know much about the atchual data.

I would be very grateful of any light shone on the subject.

[Codetapper]

A 'random' stream of MFM encoded data is your answer. You have to manually know the sync pattern the Amiga is looking for (which is $4489 on a standard disk) and that marks the start point where you can start extracting the data.

Originally Posted by Codetapper

A 'random' stream of MFM encoded data is your answer. You have to manually know the sync pattern the Amiga is looking for (which is $4489 on a standard disk) and that marks the start point where you can start extracting the data.

I have just read up on Modified Freqency Modulation, and (correct me if im wrong) 10001101 would be put onto disk as 01 00 10 10 01 01 00 01.

So data 1 gets converted to 01, and 0 gets converted to 00 if there is a 1 preceding it and 10 if there is an 0 preceding it. am I right?

[Geijer]

If you have not found it already I really can recommend "Amiga Disk Drives Inside & Out" by Abacus (I put a PDF-copy in the Zone)

/geijer

Originally Posted by Geijer

If you have not found it already I really can recommend "Amiga Disk Drives Inside & Out" by Abacus (I put a PDF-copy in the Zone)

/geijer

That book was extremly helpful, but I still dont understand MFM well enough to build a FDC.

I know that for a 1 the magnetic reversal is in the middle of the bit and for a 0 the reversal is at the end of the bit. What I dont understand is how the decoder knows whether it's seeing the middle of the bit or the end.

Also, I'm not sure if the write data pin sets the polarity of the head directly (lowsate = north, highstate = south or simular) or whether you pulse the pin to cause a pole reversal.

[pandy71]

IMO USB connection can be better, check FTDI chip Vinculum II - instead parallel, all You need is sample data from FDD and decode them offline - check this site http://techtravels.org/amiga/amigablog/