Editing File:Apple Directly Connected Disks Specification 1985.pdf

== Summary ==
'''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 Maintenance|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.

== Source ==
bitsavers.org, <code>pdf/apple/disk/hd20/Directly_Connected_Disks_Specification_1.2a_May85.pdf</code>.

== Licensing ==
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.

[[Category:Apple Documentation]]