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Apple Directly Connected Disks Specification

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The Rigid Disk Interface Spec is Apple's 12-page confidential engineering specification for Directly Connected Disks (DCD): hard disks that attach to a Macintosh through the DB-19 external floppy disk port. The first page carries the handwritten note "DCD Spec 1.2a 5/17/85", and each page is marked "Apple Confidential".[1] Bitsavers files it with the Apple Hard Disk 20 documents; the Hard Disk 20 connects to the same external disk connector on the Macintosh Plus and SE.[2]

Contents

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  • I. Physical interface: pinout and signal definition (p. 1)
  • II. Handshake and data transmission, with the flowthrough circuit for chaining several DCDs (pp. 1-5)
  • Command and response formats and checksum (pp. 5-7)
  • Timing diagrams for data transmission from drive to Mac and from Mac to drive (PDF pp. 8-9)
  • Command format and diagnostic commands (PDF pp. 10-12)

What it settles

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The pinout table shows how a DCD uses the DB-19 connector. Pins 5-8, the −12 V, +5 V and +12 V supplies that a floppy drive takes from the port, are marked "N/C, DCD to be self-powered!". PWM (pin 10), /WrReq (15) and HDSel (16) are not connected. PH0-PH2 carry the handshake, PH3 allows more than one DCD on the port, /ENBL2 (pin 17) enables the device, RD (pin 18) carries read data and is also the IWM sense line, and WR (pin 19) carries write data.[3]

Write data runs at 489.58 kbit/s, a 2.042 µs data cell, which the specification attributes to the Macintosh clock of 7.8333 MHz. Because the IWM needs the high bit of every transmitted byte set, the net rate is 428.38 kbit/s, and data is sent in groups of seven data bytes packed into eight transmitted bytes, the eighth carrying the seven low-order bits.[3][4] At start-up the Macintosh reads the ID states on the phase lines to tell a DCD from an external Sony floppy drive, then toggles Phase3 to reach the next device in the chain; every DCD must support the flowthrough circuit, or return "phantom" states, so that the Macintosh can find the end of the chain.[5]

What it does not contain

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It is a protocol specification. It has no schematic of a drive controller, no drive mechanics and no service procedures; for the Hard Disk 20 itself see Apple Hard Disk 20 Service Source.

Pages that cite this document

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Download: Apple Directly Connected Disks Specification 1985.pdf

References

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  1. ↑ Apple Computer, Inc., Rigid Disk Interface Spec (DCD Spec 1.2a, 17 May 1985), 12 pp. Hosted as File:Apple Directly Connected Disks Specification 1985.pdf; scan from bitsavers.org, filed under pdf/apple/disk/hd20/.
  2. ↑ Apple Computer, Guide to the Macintosh Family Hardware, 2nd edition (1990), chapter 9, p. 342. Hosted as File:Apple Guide to the Macintosh Family Hardware 2nd Edition 1990.pdf.
  3. ↑ 3.0 3.1 Rigid Disk Interface Spec, section I, p. 1.
  4. ↑ Rigid Disk Interface Spec, section II, pp. 1-2 and Figure 1.
  5. ↑ Rigid Disk Interface Spec, section II, p. 3 and Figure 2.
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