Apple Directly Connected Disks Specification
The Rigid Disk Interface Specification / Software Protocol for Directly Connected Disks, version 1.2a (Apple Computer, Inc., May 1985), is Apple's internal engineering specification for the DCD interface - the protocol the Apple Hard Disk 20 uses to reach a Macintosh through the floppy disk port.
What is in it
[edit | edit source]- The DB-19 pinout as a DCD device uses it: PH0 to PH2 for handshaking, PH3 to allow more than one DCD device, IENBL2 as the enable, RD as read data and sense, WR as write data. HDSel, PWM and WrReq are not connected.
- The DCD device is self-powered. Pins 5, 6, 7 and 8 - the −12 V, +5 V and +12 V supplies the port offers a floppy drive - are marked "N/C ... DCD to be self-powered". This is why an HD20 has its own mains supply and cooling fan and draws nothing from the computer.
- Serial data at 489.58 kbit/s (a 2.042 microsecond data cell), set by the Macintosh's 7.8333 MHz clock rather than a full 8 MHz, for a net data rate of 428.38 kbit/s.
- The seven-data-bytes-to-eight-transmitted-bytes grouping forced by the IWM's requirement that the most significant bit of every transmitted byte be set.
- The command, status and handshake sequences, and the checksum.
Download: Apple Directly Connected Disks Specification 1985.pdf