Apple 400K Drive
| Apple 400K Drive | |
|---|---|
 |
|
| Macintosh External Disk Drive, model M0130 (National Museum of American History) | |
| Manufacturer | Apple Computer, Inc. |
| Type | Floppy disk drive |
| Release date | 1984 |
| Interface | DB-19 external disk port |
| Compatible | Macintosh 128K, Macintosh 512K, Macintosh Plus, Macintosh SE (800K models) |
| Dimensions | 7.6 × 13.4 × 18 cm (3 × 5.3 × 7.1 in) |
| Successor | Macintosh 800K External Drive (M0131) |
| Model | M0130 |
The Macintosh External Disk Drive (model M0130) is Apple's single-sided 400 KB external 3.5-inch floppy drive for the first Macintosh models.[1] Apple's Service Source calls it the Apple 400K Drive, and Apple's hardware guide describes the drive in early Macintosh models as a single-sided version of the Apple 800K drive.[2] Bill Fernandez was Apple's engineering project manager for the drive in 1984, a fast-track project started after the Macintosh launch; Sony was the second source for the product.[3]
Disk format
[edit | edit source]A 400K Macintosh disk has 80 tracks of 512-byte sectors, 800 sectors in all. Track 0 is the outermost. The tracks are divided into five groups of 16, each read at a different rotational speed, slower at the edge of the disk, so the outer groups hold more sectors.[4]
| Tracks | Sectors per track | Sectors |
|---|---|---|
| 0-15 | 12 | 0-191 |
| 16-31 | 11 | 192-367 |
| 32-47 | 10 | 368-527 |
| 48-63 | 9 | 528-671 |
| 64-79 | 8 | 672-799 |
The disks use the Macintosh File System (MFS) and Apple GCR encoding.[5] GCR is written with NRZI encoding, in which a one is a flux transition and a zero is none; to avoid runs of three or more zeros, the disk driver formats each group of three data bytes as four 8-bit patterns.[6]
Speed control line
[edit | edit source]The 400K drive does not regulate its own motor speed. The computer sends it a pulse-width-modulated (PWM) signal on pin 10 of the external connector, generated from a disk-speed buffer in RAM: by the Analog Signal Generator (ASG) in the original Macintosh,[7] by a PAL in the Macintosh Plus and by the BBU in the Macintosh SE.[8] The 800K drive controls its own motor speed and ignores the signal. On the Macintosh SE the speed-control signal serves only the 400K external drive and is not available to internal drives. Apple warned developers never to depend on the signal, for example in copy-protection schemes, because some drives ignore it.[8]
Compatibility
[edit | edit source]The external disk connector of the Macintosh Plus and the original Macintosh SE accepts an 800K drive, the single-sided 400K drive or an Apple Hard Disk 20.[9] The Macintosh II has no external floppy connector and no disk-speed control line.[8] Apple gives the Macintosh 512K enhanced as the first model able to drive double-sided disk drives.[10]
Later machines
[edit | edit source]The disk-speed PWM signal is not provided by the Macintosh Portable, Macintosh SE FDHD, Macintosh SE/30 or the Macintosh II family, which do not use the 400K drive.[10] Those machines use the SWIM disk controller, and the SWIM does not support the single-sided drive.[11] Macintosh SE computers made after August 1989 have the SWIM and the FDHD drive and are marked FDHD on the front.[9] A 400K drive that is not recognised on one of these machines is not faulty.
Connector
[edit | edit source]| Pin | Signal | Pin | Signal |
|---|---|---|---|
| 1-4 | GND | 12 | PH1 (state-control line) |
| 5 | −12 V | 13 | PH2 (state-control line) |
| 6 | +5 V | 14 | PH3 (Register Write Strobe) |
| 7-8 | +12 V | 15 | /WRREQ (Write Data Request) |
| 9 | n.c. | 16 | SEL (state-control line) |
| 10 | PWM (motor-speed control) | 17 | /ENBL.EXT (External Drive Enable) |
| 11 | PH0 (state-control line) | 18 | RD (Read Data) |
| 19 | WR (Write Data) |
Inside Macintosh gives the same pinout for the original Macintosh, with the IWM names CA0-CA2 and LSTRB for pins 11-14.[7]
Current draw
[edit | edit source]A single-sided Macintosh floppy drive draws 500 mA at +12 V and 500 mA at +5 V; an Apple 800K drive draws 600 mA at +12 V and 360 mA at +5 V. The combined load of all devices connected to a Macintosh Plus must not exceed 600 mA at +12 V, 700 mA at +5 V and 10 mA at −12 V,[12] so a 400K drive alone uses 500 mA of the 600 mA available at +12 V. On the original Macintosh, Apple rated the disk jack for this drive's load and allowed another device on the jack to exceed it by no more than 100 mA at +12 V, 200 mA at +5 V and 10 mA at −12 V, counting all other connectors.[7]
Data rate
[edit | edit source]The IWM sends data to the drive at up to about 500 kbit/s. Because four bytes of GCR data correspond to three bytes in RAM, the effective maximum is about 375 kbit/s, before time spent seeking and on other system tasks.[13]
Daisy-chaining
[edit | edit source]With an Apple Hard Disk 20, the Hard Disk 20 plugs into the computer and the external floppy drive plugs into the DB-19 connector on the back of the Hard Disk 20, which has the same pinout as the computer's.[8]
Disassembly
[edit | edit source]Apple's procedure:[14]
- Turn the drive over, bottom facing up. Remove the six screws from the bottom cover.
- Lift the back of the cover about an inch and pull it out from the front bezel.
- Lift the back of the metal drive housing, supporting the cable connector, and remove the drive from the bottom cover.
- Remove the cable connector.
- Remove the screw that holds the grounding tab in place.
- Remove the screw on the outside of the metal drive housing and slide the drive out of the housing.
Troubleshooting
[edit | edit source]Not recognised on a later Macintosh
[edit | edit source]The SWIM machines do not support single-sided drives.[11] Test the drive on a Macintosh 128K, 512K, Plus or an SE made before the FDHD change.
Speed errors, or reads fail on outer tracks
[edit | edit source]The drive's motor speed comes from the computer on pin 10, so check the host as well as the drive.[8] A faulty PAL (Macintosh Plus), BBU (Macintosh SE) or a break in the pin 10 line affects speed control. Try the drive on a second machine before condemning the mechanism.
Intermittent or dead with other devices attached
[edit | edit source]Check the +12 V budget above: a 400K drive takes 500 mA of the Macintosh Plus's 600 mA allowance on its own.[12]
For head cleaning, lubrication and mechanism faults, see Macintosh Floppy Drive Maintenance and Floppy Drive Repair.
Documentation
[edit | edit source]- Apple 400K Drive Service Source: Apple's seven-page Service Source volume, with the take-apart procedure and an exploded view only. It has no specifications, symptom chart, adjustments, parts list or component values.[14]
- Guide to the Macintosh Family Hardware, 2nd edition, chapter 9: the interface, connector pinouts and power limits.[2]
- Inside Macintosh Volumes II and III: the disk format and the original Macintosh disk interface.[4]
See also
[edit | edit source]- Apple Floppy Disk Drive
- Apple 3.5 Drive Service Source
- Apple SuperDrive
- Apple UniDisk 3.5
- Macintosh 128K
References
[edit | edit source]- ↑ National Museum of American History, "Apple Macintosh Model M0130", object 1990.0493.01: 400K external disk drive, model number M0130 from the serial number, made 1984-1985, measured 7.6 × 13.4 × 18 cm. Retrieved 2026-09-30.
- ↑ 2.0 2.1 Apple Computer, Guide to the Macintosh Family Hardware, 2nd edition (Addison-Wesley, 1990), chapter 9, p. 329. Hosted as File:Apple Guide to the Macintosh Family Hardware 2nd Edition 1990.pdf.
- ↑ Bill Fernandez, "Macintosh External Disk Drive, Apple Computer, Inc. -- 1984", portfolio page. Retrieved 2026-09-30.
- ↑ 4.0 4.1 4.2 Apple Computer, Inside Macintosh, Volume II (1985), "The Disk Driver", p. II-211. Hosted as File:Inside Macintosh Volumes I-II-III 1985.pdf.
- ↑ Guide to the Macintosh Family Hardware, 2nd ed., Table 9-1, p. 330.
- ↑ Guide to the Macintosh Family Hardware, 2nd ed., p. 331.
- ↑ 7.0 7.1 7.2 Apple Computer, Inside Macintosh, Volume III (1985), "The Disk Interface", p. III-33. Hosted as File:Inside Macintosh Volumes I-II-III 1985.pdf.
- ↑ 8.0 8.1 8.2 8.3 8.4 Guide to the Macintosh Family Hardware, 2nd ed., "Compatibility with single-sided disk drives" and "Compatibility with the Hard Disk 20", p. 342.
- ↑ 9.0 9.1 Guide to the Macintosh Family Hardware, 2nd ed., "800 KB drive interface", p. 332.
- ↑ 10.0 10.1 Guide to the Macintosh Family Hardware, 2nd ed., "Disk-speed control", pp. 72-73.
- ↑ 11.0 11.1 Guide to the Macintosh Family Hardware, 2nd ed., "FDHD drive interface", p. 345.
- ↑ 12.0 12.1 12.2 Guide to the Macintosh Family Hardware, 2nd ed., Table 9-5 and warning, p. 338.
- ↑ Guide to the Macintosh Family Hardware, 2nd ed., p. 333.
- ↑ 14.0 14.1 Apple Computer, Apple 400K Drive (Service Source), Take Apart, "400K Mechanism", pp. 1-3. Hosted as Apple 400K Drive Service Source, file File:Apple 400k drive.pdf.