You may try those links :
http://www.s-ellis.freeserve.co.uk/atari/ports/dma.html
for pins descriptions.
I also found this file on http://www.atari-history.com I suppose : adhi_3_release_notes.pdf
That is AHDI specifications (despite the wrong spelling of the name file).
This one could be usefull as well even if it is talking about SCSI :
http://www.scsimechanic.com/scsi/SCSI2-08.html