Editing Apple UniDisk 3.5

{{Infobox computer peripheral
| image = [[File:Apple Unidisk 3.5.jpg]]
| caption = Apple UniDisk 3.5 (Model A2M2053)
| manufacturer = Apple Computer, Inc. (Sony mechanism)
| type = Floppy disk drive
| released = September 1985
| discontinued = 1990
| price = US$499
| interface = 19-pin D-sub (SmartPort)
| compatible = Apple IIc (ROM upgrade), Apple IIe (Liron card), Apple IIGS
| dimensions = ~200 mm × 175 mm × 50 mm
| weight = ~800 g
| predecessor = [[Disk II]]
| successor = [[Apple 3.5 Drive]]
| model = A2M2053
}}

The '''Apple UniDisk 3.5''' (model A2M2053) was Apple's first 3½-inch floppy drive for the Apple II series, released in September 1985—four months before the equivalent drive was available for the Macintosh. The UniDisk 3.5 was an "intelligent" or "smart" drive containing its own processor, RAM, and firmware, making it incompatible with the Macintosh despite using an identical Sony 800K mechanism.

== Background ==

Apple introduced the 3.5-inch format with the Macintosh in 1984, but the Apple II's 1 MHz 6502 processor was too slow to handle the higher bitrate of 3.5-inch drives using software-driven control. The UniDisk 3.5 solved this by incorporating its own controller within the drive.

== Design ==

=== Smart Drive Architecture ===
The UniDisk 3.5 contains substantial internal electronics:
* '''CPU:''' 65C02 at 2.0 MHz (GTE G65SC02P-2)
* '''RAM:''' 2 KB static RAM (Hitachi HM6116LP-2)
* '''ROM:''' 8 KB firmware (A2M2053-V1.0)
* '''Controller:''' IWM (Integrated Woz Machine) chip

This architecture allows the drive to handle disk I/O independently, communicating with the Apple II via the SmartPort protocol.

=== Physical Construction ===
The drive uses Snow White design language:
* Matching case color for Apple IIc
* Daisy-chain pass-through connector
* Manual eject button
* Sony 800K double-sided mechanism

=== SmartPort Interface ===
The 19-pin D-sub connector uses the SmartPort protocol:
* Drive handles low-level disk operations
* Host computer issues high-level commands
* Enables use with slower processors
* Supports daisy-chaining multiple devices

== Compatibility ==

=== Requires Special Hardware ===
The UniDisk 3.5 cannot use standard Disk II controllers:

'''Apple IIc:'''
* Original IIc requires ROM upgrade (256K ROM)
* Later production IIc includes compatible ROM
* IIc Plus natively compatible

'''Apple IIe:'''
* Requires Liron controller card (or later SWIM-based card)
* Standard Disk II controller not compatible

'''Apple IIGS:'''
* Native SmartPort support
* However, [[Apple 3.5 Drive]] (A9M0106) operates faster
* UniDisk 3.5 was intentionally slowed for Apple II compatibility

=== Not Macintosh Compatible ===
Despite using the same Sony mechanism as the Mac 800K drive, the UniDisk 3.5 is ''not'' compatible with any Macintosh. The internal controller circuitry can be bypassed to enable Mac compatibility, but this voids the "smart" functionality.

== Format ==

{| class="wikitable"
|-
! Parameter !! Value
|-
| Capacity || 800 KB
|-
| Sides || Double-sided
|-
| Tracks || 80 per side
|-
| Encoding || GCR variable speed
|-
| File system || ProDOS (Apple II), HFS (not recommended)
|}

The 800K format matches the Macintosh format, enabling disk exchange between platforms when properly formatted.

== Specifications ==

{| class="wikitable"
|-
! Parameter !! Value
|-
| Model number || A2M2053
|-
| Capacity || 800 KB
|-
| Media || 3.5-inch DD floppy
|-
| Interface || 19-pin D-sub (SmartPort)
|-
| Internal CPU || 65C02 @ 2.0 MHz
|-
| Internal RAM || 2 KB
|-
| Internal ROM || 8 KB
|-
| Mechanism || Sony 800K double-sided
|-
| Case color || Snow White (IIc matching)
|-
| Price (1985) || US$499
|}

== Market Reception ==

The 3.5-inch format was not widely adopted by Apple II users (except IIGS owners):
* Most commercial software remained on 5.25-inch disks
* Larger installed base of Disk II drives
* Required expensive controller card for IIe
* ROM upgrade needed for original IIc

The IIGS, with its enhanced capabilities and native SmartPort support, was the primary market for 3.5-inch drives in the Apple II ecosystem.

== Successor: Apple 3.5 Drive ==

The [[Apple 3.5 Drive]] (A9M0106, 1986) replaced the UniDisk 3.5:
* Removed internal controller (cheaper)
* Cross-platform (Mac and Apple II)
* Only compatible with IIGS and IIc Plus
* Faster operation on IIGS

The UniDisk 3.5 remained available for IIe and original IIc users who couldn't use the simpler Apple 3.5 Drive.

== Maintenance ==

=== Head Cleaning ===
Use a 3.5-inch head cleaning disk periodically.

=== Firmware ===
The internal ROM can become corrupted or fail. Replacement ROMs are scarce; non-functional smart drives are often converted to "dumb" drives for Mac use.

=== Common Issues ===

{| class="wikitable"
|-
! Problem !! Cause !! Solution
|-
| Not recognized || SmartPort not supported || Verify compatible host
|-
| Read errors || Dirty heads || Clean heads
|-
| Intermittent || Failing internal electronics || Difficult repair
|-
| Slow operation || Normal on Apple II || IIGS operates faster
|}

== Collecting ==

=== Identification ===
* Model A2M2053 on label
* Snow White case (not Platinum)
* "UniDisk 3.5" nameplate
* 19-pin D-sub connector

=== Value Factors ===
* Working internal electronics (smart functionality)
* Original packaging
* Condition of mechanism

UniDisk 3.5 drives command higher prices than Apple 3.5 Drives due to their smart drive capability and relative scarcity.

=== Testing ===
Test on appropriate system:
* Apple IIc (with ROM upgrade)
* Apple IIe (with Liron card)
* Apple IIGS

Verify SmartPort functionality, not just basic disk operations.

== See Also ==
* [[Disk II]]
* [[Apple 3.5 Drive]]
* [[Apple IIGS]]
* [[Apple IIc]]

== References ==
* Wikipedia — Macintosh External Disk Drive
* wiki.apple2.org — A2 3.5 Drives
* GitHub rigreco/UniDisk — UniDisk co-processor documentation

[[Category:Apple Vintage Accessories]]

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