File:Apple Directly Connected Disks Specification 1985.pdf

Rigid Disk Interface Specification / Software Protocol for Directly Connected Disks, version 1.2a (Apple Computer, Inc., May 1985).

Apple's internal engineering specification for the Directly Connected Disk (DCD) interface - the protocol the Apple Hard Disk 20 uses to talk to a Macintosh through the floppy disk port. It is the source for:

• The DB-19 pinout as used by a DCD device: PH0-PH2 for handshaking, PH3 to allow multiple DCD devices, IENBL2 as enable, RD as read data and sense, WR as write data.
• The DCD device is self-powered. The -12 V, +5 V and +12 V pins of the disk port are marked "N/C - DCD to be self-powered", which is why an HD20 has its own mains supply and fan.
• Serial data transmission at 489.58 kbit/s (2.042 microsecond data cell), constrained by the Macintosh's 7.8333 MHz clock rather than a full 8 MHz, and the seven-data-bytes-to-eight-transmitted-bytes grouping the IWM's always-set MSB requires.

bitsavers.org, pdf/apple/disk/hd20/Directly_Connected_Disks_Specification_1.2a_May85.pdf.

Copyright Apple Computer, Inc., 1985. Internal engineering documentation, never placed under a free licence. Hosted here for reference and repair use only. Not public domain.