These sites contain some information about the adb bus:

http://developer.apple.com/legacy/mac/library/technotes/hw/hw_01.html

http://www.eetasia.com/ARTICLES/2000JUN/2000JUN02_CT_AN8.PDF

http://developer.apple.com/documentation/mac/Devices/Devices-205.html

http://developer.apple.com/documentation/Hardware/Developer_Notes/Macintosh_CPUs-G3/PowerMacintosh_G3/PowerMacG3.2b.html